Creating Operators

When assembling a C++ application, two types of operators can be used:

1. Native C++ operators: custom operators defined in C++ without using the GXF API, by creating a subclass of <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8OperatorE">holoscan::Operator</a>. These C++ operators can pass arbitrary C++ objects around between operators.

2. GXF Operators: operators defined in the underlying C++ library by inheriting from the <a href="api/cpp/classholoscan_1_1ops_1_1GXFOperator.html#_CPPv4N8holoscan3ops11GXFOperatorE">holoscan::ops::GXFOperator</a> class. These operators wrap GXF codelets from GXF extensions. Examples are <a href="api/cpp/classholoscan_1_1ops_1_1VideoStreamReplayerOp.html#_CPPv4N8holoscan3ops21VideoStreamReplayerOpE">VideoStreamReplayerOp</a> for replaying video files, <a href="api/cpp/classholoscan_1_1ops_1_1FormatConverterOp.html#_CPPv4N8holoscan3ops17FormatConverterOpE">FormatConverterOp</a> for format conversions, and <a href="api/cpp/classholoscan_1_1ops_1_1HolovizOp.html#_CPPv4N8holoscan3ops9HolovizOpE">HolovizOp</a> for visualization.

Native C++ Operators

Operator Lifecycle (C++)

The lifecycle of a <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8OperatorE">holoscan::Operator</a> is made up of three stages:

• <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator5startEv">start()</a> is called once when the operator starts, and is used for initializing heavy tasks such as allocating memory resources and using parameters.

• <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> is called when the operator is triggered, which can occur any number of times throughout the operator lifecycle between start() and stop().

• <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator4stopEv">stop()</a> is called once when the operator is stopped, and is used for deinitializing heavy tasks such as deallocating resources that were previously assigned in start().

All operators on the workflow are scheduled for execution. When an operator is first executed, the start() method is called, followed by the compute() method. When the operator is stopped, the stop() method is called. The compute() method is called multiple times between start() and stop().

If any of the scheduling conditions specified by Conditions are not met (for example, the <a href="api/cpp/classholoscan_1_1CountCondition.html#_CPPv4N8holoscan14CountConditionE">CountCondition</a> would cause the scheduling condition to not be met if the operator has been executed a certain number of times), the operator is stopped and the stop() method is called.

We will cover how to use Conditions in the Specifying operator inputs and outputs (C++) section of the user guide.

Typically, the start() and the stop() functions are only called once during the application’s lifecycle. However, if the scheduling conditions are met again, the operator can be scheduled for execution, and the start() method will be called again.

Fig. 15 The sequence of method calls in the lifecycle of a Holoscan Operator

We can override the default behavior of the operator by implementing the above methods. The following example shows how to implement a custom operator that overrides start, stop and compute methods.

Listing 2 The basic structure of a Holoscan Operator (C++)

            

            
#include "holoscan/holoscan.hpp"

using holoscan::Operator;
using holoscan::OperatorSpec;
using holoscan::InputContext;
using holoscan::OutputContext;
using holoscan::ExecutionContext;
using holoscan::Arg;
using holoscan::ArgList;

class MyOp : public Operator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS(MyOp)

  MyOp() = default;

  void setup(OperatorSpec& spec) override {
  }

  void start() override {
    HOLOSCAN_LOG_TRACE("MyOp::start()");
  }

  void compute(InputContext&, OutputContext& op_output, ExecutionContext&) override {
    HOLOSCAN_LOG_TRACE("MyOp::compute()");
  };

  void stop() override {
    HOLOSCAN_LOG_TRACE("MyOp::stop()");
  }
};
        

Creating a custom operator (C++)

To create a custom operator in C++ it is necessary to create a subclass of <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8OperatorE">holoscan::Operator</a>. The following example demonstrates how to use native operators (the operators that do not have an underlying, pre-compiled GXF Codelet).

Code Snippet: examples/ping_multi_port/cpp/ping_multi_port.cpp

Listing 3 examples/ping_multi_port/cpp/ping_multi_port.cpp

            

            
#include "holoscan/holoscan.hpp"

class ValueData {
 public:
  ValueData() = default;
  explicit ValueData(int value) : data_(value) {
    HOLOSCAN_LOG_TRACE("ValueData::ValueData(): {}", data_);
  }
  ~ValueData() {
    HOLOSCAN_LOG_TRACE("ValueData::~ValueData(): {}", data_);
  }

  void data(int value) { data_ = value; }

  int data() const { return data_; }

 private:
  int data_;
};

namespace holoscan::ops {

class PingTxOp : public Operator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS(PingTxOp)

  PingTxOp() = default;

  void setup(OperatorSpec& spec) override {
    spec.output<std::shared_ptr<ValueData>>("out1");
    spec.output<std::shared_ptr<ValueData>>("out2");
  }

  void compute(InputContext&, OutputContext& op_output, ExecutionContext&) override {
    auto value1 = std::make_shared<ValueData>(index_++);
    op_output.emit(value1, "out1");

    auto value2 = std::make_shared<ValueData>(index_++);
    op_output.emit(value2, "out2");
  };
  int index_ = 0;
};

class PingMiddleOp : public Operator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS(PingMiddleOp)

  PingMiddleOp() = default;

  void setup(OperatorSpec& spec) override {
    spec.input<std::shared_ptr<ValueData>>("in1");
    spec.input<std::shared_ptr<ValueData>>("in2");
    spec.output<std::shared_ptr<ValueData>>("out1");
    spec.output<std::shared_ptr<ValueData>>("out2");
    spec.param(multiplier_, "multiplier", "Multiplier", "Multiply the input by this value", 2);
  }

  void compute(InputContext& op_input, OutputContext& op_output, ExecutionContext&) override {
    auto value1 = op_input.receive<std::shared_ptr<ValueData>>("in1").value();
    auto value2 = op_input.receive<std::shared_ptr<ValueData>>("in2").value();

    HOLOSCAN_LOG_INFO("Middle message received (count: {})", count_++);

    HOLOSCAN_LOG_INFO("Middle message value1: {}", value1->data());
    HOLOSCAN_LOG_INFO("Middle message value2: {}", value2->data());

    // Multiply the values by the multiplier parameter
    value1->data(value1->data() * multiplier_);
    value2->data(value2->data() * multiplier_);

    op_output.emit(value1, "out1");
    op_output.emit(value2, "out2");
  };

 private:
  int count_ = 1;
  Parameter<int> multiplier_;
};

class PingRxOp : public Operator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS(PingRxOp)

  PingRxOp() = default;

  void setup(OperatorSpec& spec) override {
    spec.param(receivers_, "receivers", "Input Receivers", "List of input receivers.", {});
  }

  void compute(InputContext& op_input, OutputContext&, ExecutionContext&) override {
    auto value_vector =
        op_input.receive<std::vector<std::shared_ptr<ValueData>>>("receivers").value();

    HOLOSCAN_LOG_INFO("Rx message received (count: {}, size: {})", count_++, value_vector.size());

    HOLOSCAN_LOG_INFO("Rx message value1: {}", value_vector[0]->data());
    HOLOSCAN_LOG_INFO("Rx message value2: {}", value_vector[1]->data());
  };

 private:
  Parameter<std::vector<IOSpec*>> receivers_;
  int count_ = 1;
};

}  // namespace holoscan::ops

class App : public holoscan::Application {
 public:
  void compose() override {
    using namespace holoscan;

    auto tx = make_operator<ops::PingTxOp>("tx", make_condition<CountCondition>(10));
    auto mx = make_operator<ops::PingMiddleOp>("mx", Arg("multiplier", 3));
    auto rx = make_operator<ops::PingRxOp>("rx");

    add_flow(tx, mx, {{"out1", "in1"}, {"out2", "in2"}});
    add_flow(mx, rx, {{"out1", "receivers"}, {"out2", "receivers"}});
  }
};

int main(int argc, char** argv) {
  auto app = holoscan::make_application<MyPingApp>();
  app->run();

  return 0;
}
        

Code Snippet: examples/native_operator/cpp/app_config.yaml

In this application, three operators are created: PingTxOp, PingMxOp, and PingRxOp

1. The PingTxOp operator is a source operator that emits two values every time it is invoked. The values are emitted on two different output ports, out1 (for even integers) and out2 (for odd integers).

2. The PingMxOp operator is a middle operator that receives two values from the PingTxOp operator and emits two values on two different output ports. The values are multiplied by the multiplier parameter.

3. The PingRxOp operator is a sink operator that receives two values from the PingMxOp operator. The values are received on a single input, receivers, which is a vector of input ports. The PingRxOp operator receives the values in the order they are emitted by the PingMxOp operator.

As covered in more detail below, the inputs to each operator are specified in the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator5setupER12OperatorSpec">setup()</a> method of the operator. Then inputs are received within the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> method via <a href="api/cpp/classholoscan_1_1InputContext.html#_CPPv4I0EN8holoscan12InputContext7receiveEN8holoscan8expectedI5DataTN8holoscan12RuntimeErrorEEEPKc">op_input.receive()</a> and outputs are emitted via <a href="api/cpp/classholoscan_1_1OutputContext.html#_CPPv4I00EN8holoscan13OutputContext4emitEvRNSt10shared_ptrI5DataTEEPKc">op_output.emit()</a>.

Note that for native C++ operators as defined here, any object including a shared pointer can be emitted or received. For large objects such as tensors it may be preferable from a performance standpoint to transmit a shared pointer to the object rather than making a copy. When shared pointers are used and the same tensor is sent to more than one downstream operator, one should avoid in-place operations on the tensor or race conditions between operators may occur.

Specifying operator parameters (C++)

In the example holoscan::ops::PingMxOp operator above, we have a parameter multiplier that is declared as part of the class as a private member using the <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4N8holoscan12OperatorSpec5paramER9ParameterIPN8holoscan6IOSpecEEPKcPKcPKc13ParameterFlag">param()</a> templated type:

            

            
Parameter<int> multiplier_;
        

It is then added to the <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4N8holoscan12OperatorSpecE">OperatorSpec</a> attribute of the operator in its <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator5setupER12OperatorSpec">setup()</a> method, where an associated string key must be provided. Other properties can also be mentioned such as description and default value:

            

            
// Provide key, and optionally other information
spec.param(multiplier_, "multiplier", "Multiplier", "Multiply the input by this value", 2);
        

See the Configuring operator parameters section to learn how an application can set these parameters.

Specifying operator inputs and outputs (C++)

To configure the input(s) and output(s) of C++ native operators, call the <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec5inputER6IOSpecv">spec.input()</a> and <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec6outputER6IOSpecv">spec.output()</a> methods within the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator5setupER12OperatorSpec">setup()</a> method of the operator.

The <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec5inputER6IOSpecv">spec.input()</a> and <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec6outputER6IOSpecv">spec.output()</a> methods should be called once for each input and output to be added. The <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4N8holoscan12OperatorSpecE">OperatorSpec</a> object and the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator5setupER12OperatorSpec">setup()</a> method will be initialized and called automatically by the <a href="api/cpp/classholoscan_1_1Application.html#_CPPv4N8holoscan11ApplicationE">Application</a> class when its <a href="api/cpp/classholoscan_1_1Application.html#_CPPv4N8holoscan11Application3runEv">run()</a> method is called.

These methods (<a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec5inputER6IOSpecv">spec.input()</a> and <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec6outputER6IOSpecv">spec.output()</a>) return an <a href="api/cpp/classholoscan_1_1IOSpec.html#_CPPv4N8holoscan6IOSpecE">IOSpec</a> object that can be used to configure the input/output port.

By default, the <a href="api/cpp/classholoscan_1_1MessageAvailableCondition.html#_CPPv4N8holoscan25MessageAvailableConditionE">holoscan::MessageAvailableCondition</a> and <a href="api/cpp/classholoscan_1_1DownstreamMessageAffordableCondition.html#_CPPv4N8holoscan36DownstreamMessageAffordableConditionE">holoscan::DownstreamMessageAffordableCondition</a> conditions are applied (with a min_size of 1) to the input/output ports. This means that the operator’s <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> method will not be invoked until a message is available on the input port and the downstream operator’s input port (queue) has enough capacity to receive the message.

            

            
  void setup(OperatorSpec& spec) override {
    spec.input<std::shared_ptr<ValueData>>("in");
    // Above statement is equivalent to:
    // spec.input<std::shared_ptr<ValueData>>("in")
    // .condition(ConditionType::kMessageAvailable, Arg("min_size") = 1);

    spec.output<std::shared_ptr<ValueData>>("out");
    // Above statement is equivalent to:
    // spec.output<std::shared_ptr<ValueData>>("out")
    // .condition(ConditionType::kDownstreamMessageAffordable, Arg("min_size") = 1);
    ...
  }
        

In the above example, the <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec5inputER6IOSpecv">spec.input()</a> method is used to configure the input port to have the <a href="api/cpp/classholoscan_1_1MessageAvailableCondition.html#_CPPv4N8holoscan25MessageAvailableConditionE">holoscan::MessageAvailableCondition</a> with a minimum size of 1. This means that the operator’s <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> method will not be invoked until a message is available on the input port of the operator. Similarly, the <a href="api/cpp/classholoscan_1_1OperatorSpec.html#_CPPv4I0EN8holoscan12OperatorSpec6outputER6IOSpecv">spec.output()</a> method is used to configure the output port to have the <a href="api/cpp/classholoscan_1_1DownstreamMessageAffordableCondition.html#_CPPv4N8holoscan36DownstreamMessageAffordableConditionE">holoscan::DownstreamMessageAffordableCondition</a> with a minimum size of 1. This means that the operator’s <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> method will not be invoked until the downstream operator’s input port has enough capacity to receive the message.

If you want to change this behavior, use the <a href="api/cpp/classholoscan_1_1IOSpec.html#_CPPv4IDpEN8holoscan6IOSpec9conditionER6IOSpec13ConditionTypeDpRR5ArgsT">IOSpec::condition()</a> method to configure the conditions. For example, to configure the input and output ports to have no conditions, you can use the following code:

            

            
  void setup(OperatorSpec& spec) override {
    spec.input<std::shared_ptr<ValueData>>("in")
        .condition(ConditionType::kNone);

    spec.output<std::shared_ptr<ValueData>>("out")
        .condition(ConditionType::kNone);
    // ...
  }
        

The example code in the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator5setupER12OperatorSpec">setup()</a> method configures the input port to have no conditions, which means that the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> method will be called as soon as the operator is ready to compute. Since there is no guarantee that the input port will have a message available, the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> method should check if there is a message available on the input port before attempting to read it.

The <a href="api/cpp/classholoscan_1_1InputContext.html#_CPPv4I0EN8holoscan12InputContext7receiveEN8holoscan8expectedI5DataTN8holoscan12RuntimeErrorEEEPKc">receive()</a> method of the <a href="api/cpp/classholoscan_1_1InputContext.html#_CPPv4N8holoscan12InputContextE">InputContext</a> object can be used to access different types of input data within the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator7computeER12InputContextR13OutputContextR16ExecutionContext">compute()</a> method of your operator class, where its template argument (DataT) is the data type of the input. This method takes the name of the input port as an argument (which can be omitted if your operator has a single input port), and returns the input data. If input data is not available, the method returns an object of the holoscan::RuntimeError class which contains an error message describing the reason for the failure. The holoscan::RuntimeError class is a derived class of std::runtime_error and supports accessing more error information, for example, with what() method.

In the example code fragment below, the PingRxOp operator receives input on a port called “in” with data type ValueData. The <a href="api/cpp/classholoscan_1_1InputContext.html#_CPPv4I0EN8holoscan12InputContext7receiveEN8holoscan8expectedI5DataTN8holoscan12RuntimeErrorEEEPKc">receive()</a> method is used to access the input data. The value is checked to be valid or not with the if condition. If value is of holoscan::RuntimeError type, then if condition will be false. Otherwise, the data() method of the ValueData class is called to get the value of the input data.

            

            
// ...

class PingRxOp : public holoscan::ops::GXFOperator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS_SUPER(PingRxOp, holoscan::ops::GXFOperator)
  PingRxOp() = default;
  void setup(OperatorSpec& spec) override {
    spec.input<ValueData>("in");
  }
  void compute(InputContext& op_input, OutputContext&, ExecutionContext&) override {
    // The type of `value` is `ValueData`
    auto value = op_input.receive<ValueData>("in");
    if (value){
      HOLOSCAN_LOG_INFO("Message received (value: {})", value.data());
    }
  }
};
        

For GXF Entity objects (<a href="api/cpp/classholoscan_1_1gxf_1_1Entity.html#_CPPv4N8holoscan3gxf6EntityE">holoscan::gxf::Entity</a> wraps underlying GXF nvidia::gxf::Entity class), the <a href="api/cpp/classholoscan_1_1InputContext.html#_CPPv4I0EN8holoscan12InputContext7receiveEN8holoscan8expectedI5DataTN8holoscan12RuntimeErrorEEEPKc">receive()</a> method will return the GXF Entity object for the input of the specified name. In the example below, the PingRxOp operator receives input on a port called “in” with data type <a href="api/cpp/classholoscan_1_1gxf_1_1Entity.html#_CPPv4N8holoscan3gxf6EntityE">holoscan::gxf::Entity</a>.

            

            
// ...

class PingRxOp : public holoscan::ops::GXFOperator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS_SUPER(PingRxOp, holoscan::ops::GXFOperator)
  PingRxOp() = default;
  void setup(OperatorSpec& spec) override {
    spec.input<holoscan::gxf::Entity>("in");
  }
  void compute(InputContext& op_input, OutputContext&, ExecutionContext&) override {
    // The type of `in_entity` is 'holoscan::gxf::Entity'.
    auto in_entity = op_input.receive<holoscan::gxf::Entity>("in");
    if (in_entity) {
      // Process with `in_entity`.
      // ...
    }
  }
};
        

For objects of type std::any, the <a href="api/cpp/classholoscan_1_1InputContext.html#_CPPv4I0EN8holoscan12InputContext7receiveEN8holoscan8expectedI5DataTN8holoscan12RuntimeErrorEEEPKc">receive()</a> method will return a std::any object containing the input of the specified name. In the example below, the PingRxOp operator receives input on a port called “in” with data type std::any. The type() method of the std::any object is used to determine the actual type of the input data, and the std::any_cast<T>() function is used to retrieve the value of the input data.

            

            
// ...

class PingRxOp : public holoscan::ops::GXFOperator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS_SUPER(PingRxOp, holoscan::ops::GXFOperator)
  PingRxOp() = default;
  void setup(OperatorSpec& spec) override {
    spec.input<std::any>("in");
  }
  void compute(InputContext& op_input, OutputContext&, ExecutionContext&) override {
    // The type of `in_any` is 'std::any'.
    auto in_any = op_input.receive<std::any>("in");
    auto& in_any_type = in_any.type();

    if (in_any_type == typeid(holoscan::gxf::Entity)) {
      auto in_entity = std::any_cast<holoscan::gxf::Entity>(in_any);
      // Process with `in_entity`.
      // ...
    } else if (in_any_type == typeid(std::shared_ptr<ValueData>)) {
      auto in_message = std::any_cast<std::shared_ptr<ValueData>>(in_any);
      // Process with `in_message`.
      // ...
    } else if (in_any_type == typeid(nullptr_t)) {
      // No message is available.
    } else {
      HOLOSCAN_LOG_ERROR("Invalid message type: {}", in_any_type.name());
      return;
    }
  }
};
        

The Holoscan SDK provides built-in data types called Domain Objects, defined in the include/holoscan/core/domain directory. For example, the <a href="api/cpp/classholoscan_1_1Tensor.html#_CPPv4N8holoscan6TensorE">holoscan::Tensor</a> is a Domain Object class that is used to represent a multi-dimensional array of data, which can be used directly by OperatorSpec, InputContext, and OutputContext.

Receiving any number of inputs (C++)

Instead of assigning a specific number of input ports, it may be desired to have the ability to receive any number of objects on a port in certain situations. This can be done by defining Parameter with std::vector<IOSpec*>> (Parameter<std::vector<IOSpec*>> receivers_) and calling spec.param(receivers_, "receivers", "Input Receivers", "List of input receivers.", {}); as done for PingRxOp in the native operator ping example.

Listing 4 examples/ping_multi_port/cpp/ping_multi_port.cpp

            

            
class PingRxOp : public Operator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS(PingRxOp)

  PingRxOp() = default;

  void setup(OperatorSpec& spec) override {
    spec.param(receivers_, "receivers", "Input Receivers", "List of input receivers.", {});
  }

  void compute(InputContext& op_input, OutputContext&, ExecutionContext&) override {
    auto value_vector = op_input.receive<std::vector<ValueData>>("receivers");

    HOLOSCAN_LOG_INFO("Rx message received (count: {}, size: {})", count_++, value_vector.size());

    HOLOSCAN_LOG_INFO("Rx message value1: {}", value_vector[0]->data());
    HOLOSCAN_LOG_INFO("Rx message value2: {}", value_vector[1]->data());
  };

 private:
  Parameter<std::vector<IOSpec*>> receivers_;
  int count_ = 1;
};

}  // namespace holoscan::ops

class App : public holoscan::Application {
 public:
  void compose() override {
    using namespace holoscan;

    auto tx = make_operator<ops::PingTxOp>("tx", make_condition<CountCondition>(10));
    auto mx = make_operator<ops::PingMiddleOp>("mx", Arg("multiplier", 3));
    auto rx = make_operator<ops::PingRxOp>("rx");

    add_flow(tx, mx, {{"out1", "in1"}, {"out2", "in2"}});
    add_flow(mx, rx, {{"out1", "receivers"}, {"out2", "receivers"}});
  }
};
        

Then, once the following configuration is provided in the compose() method, the PingRxOp will receive two inputs on the receivers port.

            

            
134: add_flow(mx, rx, {{"out1", "receivers"}, {"out2", "receivers"}});
        

By using a parameter (receivers) with std::vector<holoscan::IOSpec*> type, the framework creates input ports (receivers:0 and receivers:1) implicitly and connects them (and adds the references of the input ports to the receivers vector).

Building your C++ operator

You can build your C++ operator using CMake, by calling find_package(holoscan) in your CMakeLists.txt to load the SDK libraries. Your operator will need to link against holoscan::core:

Listing 5 /CMakeLists.txt

            

            
# Your CMake project
cmake_minimum_required(VERSION 3.20)
project(my_project CXX)

# Finds the holoscan SDK
find_package(holoscan REQUIRED CONFIG PATHS "/opt/nvidia/holoscan")

# Create a library for your operator
add_library(my_operator SHARED my_operator.cpp)

# Link your operator against holoscan::core
target_link_libraries(my_operator
    PUBLIC holoscan::core
)
        

Once your CMakeLists.txt is ready in <src_dir>, you can build in <build_dir> with the command line below. You can optionally pass Holoscan_ROOT if the SDK installation you’d like to use differs from the PATHS given to find_package(holoscan) above.

            

            
# Configure
cmake -S <src_dir> -B <build_dir> -D Holoscan_ROOT="/opt/nvidia/holoscan"
# Build
cmake --build <build_dir> -j
        

Using your C++ Operator in an Application

• If the application is configured in the same CMake project as the operator, you can simply add the operator CMake target library name under the application executable target_link_libraries call, as the operator CMake target is already defined.

  Copy

  Copied!

              
  
              
  # operator
  add_library(my_op my_op.cpp)
  target_link_libraries(my_operator PUBLIC holoscan::core)
  
  # application
  add_executable(my_app main.cpp)
  target_link_libraries(my_operator
    PRIVATE
    holoscan::core
    my_op
  )
          

• If the application is configured in a separate project as the operator, you need to export the operator in its own CMake project, and import it in the application CMake project, before being able to list it under target_link_libraries also. This is the same as what is done for the SDK built-in operators, available under the holoscan::ops namespace.

You can then include the headers to your C++ operator in your application code.

GXF Operators

With the Holoscan C++ API, we can also wrap GXF Codelets from GXF extensions as Holoscan Operators.

Given an existing GXF extension, we can create a simple “identity” application consisting of a replayer, which reads contents from a file on disk, and our recorder from the last section, which will store the output of the replayer exactly in the same format. This allows us to see whether the output of the recorder matches the original input files.

The MyRecorderOp Holoscan Operator implementation below will wrap the MyRecorder GXF Codelet shown here.

Operator definition

Listing 6 my_recorder_op.hpp

            

            
#ifndef APPS_MY_RECORDER_APP_MY_RECORDER_OP_HPP
#define APPS_MY_RECORDER_APP_MY_RECORDER_OP_HPP

#include "holoscan/core/gxf/gxf_operator.hpp"

namespace holoscan::ops {

class MyRecorderOp : public holoscan::ops::GXFOperator {
 public:
  HOLOSCAN_OPERATOR_FORWARD_ARGS_SUPER(MyRecorderOp, holoscan::ops::GXFOperator)

  MyRecorderOp() = default;

  const char* gxf_typename() const override { return "MyRecorder"; }

  void setup(OperatorSpec& spec) override;

  void initialize() override;

 private:
  Parameter<holoscan::IOSpec*> receiver_;
  Parameter<std::shared_ptr<holoscan::Resource>> my_serializer_;
  Parameter<std::string> directory_;
  Parameter<std::string> basename_;
  Parameter<bool> flush_on_tick_;
};

}  // namespace holoscan::ops

#endif/* APPS_MY_RECORDER_APP_MY_RECORDER_OP_HPP */
        

The holoscan::ops::MyRecorderOp class wraps a MyRecorder GXF Codelet by inheriting from the <a href="api/cpp/classholoscan_1_1ops_1_1GXFOperator.html#_CPPv4N8holoscan3ops11GXFOperatorE">holoscan::ops::GXFOperator</a> class. The HOLOSCAN_OPERATOR_FORWARD_ARGS_SUPER macro is used to forward the arguments of the constructor to the base class.

We first need to define the fields of the MyRecorderOp class. You can see that fields with the same names are defined in both the MyRecorderOp class and the MyRecorder GXF codelet .

Listing 7 Parameter declarations in gxf_extensions/my_recorder/my_recorder.hpp

            

            
  nvidia::gxf::Parameter<nvidia::gxf::Handle<nvidia::gxf::Receiver>> receiver_;
  nvidia::gxf::Parameter<nvidia::gxf::Handle<nvidia::gxf::EntitySerializer>> my_serializer_;
  nvidia::gxf::Parameter<std::string> directory_;
  nvidia::gxf::Parameter<std::string> basename_;
  nvidia::gxf::Parameter<bool> flush_on_tick_;
        

Comparing the MyRecorderOp holoscan parameter to the MyRecorder gxf codelet:

We then need to implement the following functions:

• const char* gxf_typename() const override: return the GXF type name of the Codelet. The fully-qualified class name (MyRecorder) for the GXF Codelet is specified.

• void setup(OperatorSpec& spec) override: setup the OperatorSpec with the inputs/outputs and parameters of the Operator.

• void initialize() override: initialize the Operator.

Setting up parameter specifications

The implementation of the setup(OperatorSpec& spec) function is as follows:

Listing 8 my_recorder_op.cpp

            

            
#include "./my_recorder_op.hpp"

#include "holoscan/core/fragment.hpp"
#include "holoscan/core/gxf/entity.hpp"
#include "holoscan/core/operator_spec.hpp"

#include "holoscan/core/resources/gxf/video_stream_serializer.hpp"

namespace holoscan::ops {

void MyRecorderOp::setup(OperatorSpec& spec) {
  auto& input = spec.input<holoscan::gxf::Entity>("input");
  // Above is same with the following two lines (a default condition is assigned to the input port if not specified):
  //
  // auto& input = spec.input<holoscan::gxf::Entity>("input")
  // .condition(ConditionType::kMessageAvailable, Arg("min_size") = 1);

  spec.param(receiver_, "receiver", "Entity receiver", "Receiver channel to log", &input);
  spec.param(my_serializer_,
             "serializer",
             "Entity serializer",
             "Serializer for serializing input data");
  spec.param(directory_, "out_directory", "Output directory path", "Directory path to store received output");
  spec.param(basename_, "basename", "File base name", "User specified file name without extension");
  spec.param(flush_on_tick_,
             "flush_on_tick",
             "Boolean to flush on tick",
             "Flushes output buffer on every `tick` when true",
             false);
}

void MyRecorderOp::initialize() {...}

}  // namespace holoscan::ops
        

Here, we set up the inputs/outputs and parameters of the Operator. Note how the content of this function is very similar to the MyRecorder GXF codelet’s registerInterface function.

• In the C++ API, GXF Receiver and Transmitter components (such as DoubleBufferReceiver and DoubleBufferTransmitter) are considered as input and output ports of the Operator so we register the inputs/outputs of the Operator with input<T> and output<T> functions (where T is the data type of the port).

• Compared to the pure GXF application that does the same job, the SchedulingTerm of an Entity in the GXF Application YAML are specified as <a href="api/cpp/classholoscan_1_1Condition.html#_CPPv4N8holoscan9ConditionE">Condition</a>s on the input/output ports (e.g., <a href="api/cpp/classholoscan_1_1MessageAvailableCondition.html#_CPPv4N8holoscan25MessageAvailableConditionE">holoscan::MessageAvailableCondition</a> and <a href="api/cpp/classholoscan_1_1DownstreamMessageAffordableCondition.html#_CPPv4N8holoscan36DownstreamMessageAffordableConditionE">holoscan::DownstreamMessageAffordableCondition</a>).

The highlighted lines in MyRecorderOp::setup above match the following highlighted statements of GXF Application YAML:

Listing 9 A part of apps/my_recorder_app_gxf/my_recorder_gxf.yaml

            

            
name: recorder
components:
 - name: input
   type: nvidia::gxf::DoubleBufferReceiver
 - name: allocator
   type: nvidia::gxf::UnboundedAllocator
 - name: component_serializer
   type: nvidia::gxf::StdComponentSerializer
   parameters:
     allocator: allocator
 - name: entity_serializer
   type: nvidia::holoscan::stream_playback::VideoStreamSerializer   # inheriting from nvidia::gxf::EntitySerializer
   parameters:
     component_serializers: [component_serializer]
 - type: MyRecorder
   parameters:
     receiver: input
     serializer: entity_serializer
     out_directory: "/tmp"
     basename: "tensor_out"
 - type: nvidia::gxf::MessageAvailableSchedulingTerm
   parameters:
     receiver: input
     min_size: 1

        

In the same way, if we had a Transmitter GXF component, we would have the following statements (Please see available constants for <a href="api/cpp/enum_condition_8hpp_1a5dc906177a4609bd59caa475ba7cdb30.html#_CPPv4N8holoscan13ConditionTypeE">holoscan::ConditionType</a>):

            

            
  auto& output = spec.output<holoscan::gxf::Entity>("output");
  // Above is same with the following two lines (a default condition is assigned to the output port if not specified):
  //
  // auto& output = spec.output<holoscan::gxf::Entity>("output")
  // .condition(ConditionType::kDownstreamMessageAffordable, Arg("min_size") = 1);
        

Initializing the operator

Next, the implementation of the initialize() function is as follows:

Listing 10 my_recorder_op.cpp

            

            
#include "./my_recorder_op.hpp"

#include "holoscan/core/fragment.hpp"
#include "holoscan/core/gxf/entity.hpp"
#include "holoscan/core/operator_spec.hpp"

#include "holoscan/core/resources/gxf/video_stream_serializer.hpp"

namespace holoscan::ops {

void MyRecorderOp::setup(OperatorSpec& spec) {...}

void MyRecorderOp::initialize() {
  // Set up prerequisite parameters before calling GXFOperator::initialize()
  auto frag = fragment();
  auto serializer =
      frag->make_resource<holoscan::VideoStreamSerializer>("serializer");
  add_arg(Arg("serializer") = serializer);

  GXFOperator::initialize();
}

}  // namespace holoscan::ops
        

Here we set up the pre-defined parameters such as the serializer. The highlighted lines above matches the highlighted statements of GXF Application YAML:

Listing 11 Another part of apps/my_recorder_app_gxf/my_recorder_gxf.yaml

            

            
name: recorder
components:
 - name: input
   type: nvidia::gxf::DoubleBufferReceiver
 - name: allocator
   type: nvidia::gxf::UnboundedAllocator
 - name: component_serializer
   type: nvidia::gxf::StdComponentSerializer
   parameters:
     allocator: allocator
 - name: entity_serializer
   type: nvidia::holoscan::stream_playback::VideoStreamSerializer   # inheriting from nvidia::gxf::EntitySerializer
   parameters:
     component_serializers: [component_serializer]
 - type: MyRecorder
   parameters:
     receiver: input
     serializer: entity_serializer
     out_directory: "/tmp"
     basename: "tensor_out"
 - type: nvidia::gxf::MessageAvailableSchedulingTerm
   parameters:
     receiver: input
     min_size: 1
        

Building your GXF operator

There are no differences in CMake between building a GXF operator and building a native C++ operator, since the GXF codelet is actually loaded through a GXF extension as a plugin, and does not need to be added to target_link_libraries(my_operator ...).

Using your GXF Operator in an Application

There are no differences in CMake between using a GXF operator and using a native C++ operator in an application. However, the application will need to load the GXF extension library which holds the wrapped GXF codelet symbols, so the application needs to be configured to find the extension library in its yaml configuration file, as documented here.

Interoperability between GXF and native C++ operators

To support sending or receiving tensors to and from operators (both GXF and native C++ operators), the Holoscan SDK provides the C++ classes below:

• A class template called holoscan::MyMap which inherits from std::unordered_map<std::string, std::shared_ptr<T>>. The template parameter T can be any type, and it is used to specify the type of the std::shared_ptr objects stored in the map.

A <a href="api/cpp/classholoscan_1_1TensorMap.html#_CPPv4N8holoscan9TensorMapE">holoscan::TensorMap</a> class defined as a specialization of holoscan::Map for the <a href="api/cpp/classholoscan_1_1Tensor.html#_CPPv4N8holoscan6TensorE">holoscan::Tensor</a> type.

Fig. 16 Supporting Tensor Interoperability

Consider the following example, where GXFSendTensorOp and GXFReceiveTensorOp are GXF operators, and where ProcessTensorOp is a C++ native operator:

Fig. 17 The tensor interoperability between C++ native operator and GXF operator

The following code shows how to implement ProcessTensorOp’s compute() method as a C++ native operator communicating with GXF operators. Focus on the use of the holoscan::gxf::Entity:

Listing 12 examples/tensor_interop/cpp/tensor_interop.cpp

            

            
void compute(InputContext& op_input, OutputContext& op_output,
               ExecutionContext& context) override {
    // The type of `in_message` is 'holoscan::TensorMap'.
    auto in_message = op_input.receive<holoscan::TensorMap>("in").value();
    // the type of out_message is TensorMap
    TensorMap out_message;

    for (auto& [key, tensor] : in_message) {  // Process with 'tensor' here.
      cudaError_t cuda_status;
      size_t data_size = tensor->nbytes();
      std::vector<uint8_t> in_data(data_size);
      CUDA_TRY(cudaMemcpy(in_data.data(), tensor->data(), data_size, cudaMemcpyDeviceToHost));
      HOLOSCAN_LOG_INFO("ProcessTensorOp Before key: '{}', shape: ({}), data: [{}]",
                        key,
                        fmt::join(tensor->shape(), ","),
                        fmt::join(in_data, ","));
      for (size_t i = 0; i < data_size; i++) { in_data[i] *= 2; }
      HOLOSCAN_LOG_INFO("ProcessTensorOp After key: '{}', shape: ({}), data: [{}]",
                        key,
                        fmt::join(tensor->shape(), ","),
                        fmt::join(in_data, ","));
      CUDA_TRY(cudaMemcpy(tensor->data(), in_data.data(), data_size, cudaMemcpyHostToDevice));
      out_message.insert({key, tensor});
    }
    // Send the processed message.
    op_output.emit(out_message);
  };
        

• The input message is of type <a href="api/cpp/classholoscan_1_1TensorMap.html#_CPPv4N8holoscan9TensorMapE">holoscan::TensorMap</a> object.

• Every <a href="api/cpp/classholoscan_1_1Tensor.html#_CPPv4N8holoscan6TensorE">holoscan::Tensor</a>in the TensorMap object is copied on the host as in_data.

• The data is processed (values multiplied by 2)

• The data is moved back to the <a href="api/cpp/classholoscan_1_1Tensor.html#_CPPv4N8holoscan6TensorE">holoscan::Tensor</a> object on the GPU.

• A new <a href="api/cpp/classholoscan_1_1TensorMap.html#_CPPv4N8holoscan9TensorMapE">holoscan::TensorMap</a> object out_messageis created to be sent to the next operator with <a href="api/cpp/classholoscan_1_1OutputContext.html#_CPPv4I00EN8holoscan13OutputContext4emitEvRNSt10shared_ptrI5DataTEEPKc">op_output.emit()</a>.

When assembling a Python application, two types of operators can be used:

1. Native Python operators: custom operators defined in Python, by creating a subclass of <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator">holoscan.core.Operator</a>. These Python operators can pass arbitrary Python objects around between operators and are not restricted to the stricter parameter typing used for C++ API operators.

2. Python wrappings of C++ Operators: operators defined in the underlying C++ library by inheriting from the <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8OperatorE">holoscan::Operator</a> class. These operators have Python bindings available within the <a href="api/python/holoscan_python_api_operators.html#module-holoscan.operators">holoscan.operators</a> module. Examples are <a href="api/python/holoscan_python_api_operators.html#holoscan.operators.VideoStreamReplayerOp">VideoStreamReplayerOp</a> for replaying video files, <a href="api/python/holoscan_python_api_operators.html#holoscan.operators.FormatConverterOp">FormatConverterOp</a> for format conversions, and <a href="api/python/holoscan_python_api_operators.html#holoscan.operators.HolovizOp">HolovizOp</a> for visualization.

Native Python Operator

Operator Lifecycle (Python)

The lifecycle of a <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator">holoscan.core.Operator</a> is made up of three stages:

• <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.start">start()</a> is called once when the operator starts, and is used for initializing heavy tasks such as allocating memory resources and using parameters.

• <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute()</a> is called when the operator is triggered, which can occur any number of times throughout the operator lifecycle between start() and stop().

• <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.stop">stop()</a> is called once when the operator is stopped, and is used for deinitializing heavy tasks such as deallocating resources that were previously assigned in start().

All operators on the workflow are scheduled for execution. When an operator is first executed, the start() method is called, followed by the compute() method. When the operator is stopped, the stop() method is called. The compute() method is called multiple times between start() and stop().

If any of the scheduling conditions specified by Conditions are not met (for example, the <a href="api/python/holoscan_python_api_conditions.html#holoscan.conditions.CountCondition">CountCondition</a> would cause the scheduling condition to not be met if the operator has been executed a certain number of times), the operator is stopped and the stop() method is called.

We will cover how to use <a href="api/python/holoscan_python_api_conditions.html#module-holoscan.conditions">Conditions</a> in the Specifying operator inputs and outputs (Python) section of the user guide.

Typically, the start() and the stop() functions are only called once during the application’s lifecycle. However, if the scheduling conditions are met again, the operator can be scheduled for execution, and the start() method will be called again.

Fig. 18 The sequence of method calls in the lifecycle of a Holoscan Operator

We can override the default behavior of the operator by implementing the above methods. The following example shows how to implement a custom operator that overrides start, stop and compute methods.

Listing 13 The basic structure of a Holoscan Operator (Python)

            

            
from holoscan.core import (
    ExecutionContext,
    InputContext,
    Operator,
    OperatorSpec,
    OutputContext,
)


class MyOp(Operator):

    def __init__(self, fragment, *args, **kwargs):
        super().__init__(fragment, *args, **kwargs)

    def setup(self, spec: OperatorSpec):
        pass

    def start(self):
        pass

    def compute(self, op_input: InputContext, op_output: OutputContext, context: ExecutionContext):
        pass

    def stop(self):
        pass
        

setup method vs initialize vs __init__

The <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup</a> method aims to get the “operator’s spec” by providing <a href="api/python/holoscan_python_api_core.html#holoscan.core.OperatorSpec">OperatorSpec</a> object as a spec param. When <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.__init__">__init__</a> is called, it calls C++’s <a href="api/cpp/classholoscan_1_1Operator.html#_CPPv4N8holoscan8Operator4specERKNSt10shared_ptrI12OperatorSpecEE">Operator::spec</a> method (and also sets <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.spec">self.spec</a> class member), and calls <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup</a> method so that Operator’s <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.spec">spec</a> property holds the operator’s specification. (See the source code for more details.)

Since the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup</a> method can be called multiple times with other <a href="api/python/holoscan_python_api_core.html#holoscan.core.OperatorSpec">OperatorSpec</a> object (e.g., to enumerate the operator’s description), in the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup</a> method, a user shouldn’t initialize something in the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator">Operator</a> object. Such initialization needs to be done in <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.initialize">initialize</a> method. The <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.__init__">__init__</a> method is for creating the Operator object and it can be used for initializing the operator object itself by passing miscellaneous arguments. Still, it doesn’t ‘initialize’ the corresponding GXF entity object.

Creating a custom operator (Python)

To create a custom operator in Python it is necessary to create a subclass of <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator">holoscan.core.Operator</a>. A simple example of an operator that takes a time-varying 1D input array named “signal” and applies convolution with a boxcar (i.e. rect) kernel.

For simplicity, this operator assumes that the “signal” that will be received on the input is already a numpy.ndarray or is something that can be cast to one via (np.asarray). We will see more details in a later section on how we can interoperate with various tensor classes, including the GXF Tensor objects used by some of the C++-based operators.

Code Snippet: examples/numpy_native/convolve.py

Listing 14 examples/numpy_native/convolve.py

            

            
import os

from holoscan.conditions import CountCondition
from holoscan.core import Application, Operator, OperatorSpec
from holoscan.logger import LogLevel, set_log_level

import numpy as np


class SignalGeneratorOp(Operator):
    """Generate a time-varying impulse.

Transmits an array of zeros with a single non-zero entry of a
specified `height`. The position of the non-zero entry shifts
to the right (in a periodic fashion) each time `compute` is
called.

Parameters
----------
fragment : holoscan.core.Fragment
The Fragment (or Application) the operator belongs to.
height : number
The height of the signal impulse.
size : number
The total number of samples in the generated 1d signal.
dtype : numpy.dtype or str
The data type of the generated signal.
"""

    def __init__(self, fragment, *args, height=1, size=10, dtype=np.int32, **kwargs):
        self.count = 0
        self.height = height
        self.dtype = dtype
        self.size = size
        super().__init__(fragment, *args, **kwargs)

    def setup(self, spec: OperatorSpec):
        spec.output("signal")

    def compute(self, op_input, op_output, context):

        # single sample wide impulse at a time-varying position
        signal = np.zeros((self.size,), dtype=self.dtype)
        signal[self.count % signal.size] = self.height
        self.count += 1

        op_output.emit(signal, "signal")


class ConvolveOp(Operator):
    """Apply convolution to a tensor.

Convolves an input signal with a "boxcar" (i.e. "rect") kernel.

Parameters
----------
fragment : holoscan.core.Fragment
The Fragment (or Application) the operator belongs to.
width : number
The width of the boxcar kernel used in the convolution.
unit_area : bool, optional
Whether or not to normalize the convolution kernel to unit area.
If False, all samples have implitude one and the dtype of the
kernel will match that of the signal. When True the sum over
the kernel is one and a 32-bit floating point data type is used
for the kernel.
"""

    def __init__(self, fragment, *args, width=4, unit_area=False, **kwargs):
        self.count = 0
        self.width = width
        self.unit_area = unit_area
        super().__init__(fragment, *args, **kwargs)

    def setup(self, spec: OperatorSpec):
        spec.input("signal_in")
        spec.output("signal_out")

    def compute(self, op_input, op_output, context):

        signal = op_input.receive("signal_in")
        assert isinstance(signal, np.ndarray)

        if self.unit_area:
            kernel = np.full((self.width,), 1/self.width, dtype=np.float32)
        else:
            kernel = np.ones((self.width,), dtype=signal.dtype)

        convolved = np.convolve(signal, kernel, mode='same')

        op_output.emit(convolved, "signal_out")


class PrintSignalOp(Operator):
    """Print the received signal to the terminal."""

    def setup(self, spec: OperatorSpec):
        spec.input("signal")

    def compute(self, op_input, op_output, context):
        signal = op_input.receive("signal")
        print(signal)


class ConvolveApp(Application):
    """Minimal signal processing application.

Generates a time-varying impulse, convolves it with a boxcar kernel, and
prints the result to the terminal.

A `CountCondition` is applied to the generate to terminate execution
after a specific number of steps.
"""

    def compose(self):
        signal_generator = SignalGeneratorOp(
            self,
            CountCondition(self, count=24),
            name="generator",
            **self.kwargs("generator"),
        )
        convolver = ConvolveOp(self, name="conv", **self.kwargs("convolve"))
        printer = PrintSignalOp(self, name="printer")
        self.add_flow(signal_generator, convolver)
        self.add_flow(convolver, printer)


def main(config_file):
    app = ConvolveApp()
    # if the --config command line argument was provided, it will override this config_file`
    app.config(config_file)
    app.run()


if __name__ == "__main__":
    config_file = os.path.join(os.path.dirname(__file__), 'convolve.yaml')
    main(config_file=config_file)
        

Code Snippet: examples/numpy_native/convolve.yaml

Listing 15 examples/numpy_native/convolve.yaml

            

            
signal_generator:
  height: 1
  size: 20
  dtype: int32

convolve:
  width: 4
  unit_area: false
        

In this application, three native Python operators are created: SignalGeneratorOp, ConvolveOp and PrintSignalOp. The SignalGeneratorOp generates a synthetic signal such as [0, 0, 1, 0, 0, 0] where the position of the non-zero entry varies each time it is called. ConvolveOp performs a 1D convolution with a boxcar (i.e. rect) function of a specified width. PrintSignalOp just prints the received signal to the terminal.

As covered in more detail below, the inputs to each operator are specified in the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup()</a> method of the operator. Then inputs are received within the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute</a> method via <a href="api/python/holoscan_python_api_core.html#holoscan.core.InputContext.receive">op_input.receive()</a> and outputs are emitted via <a href="api/python/holoscan_python_api_core.html#holoscan.core.OutputContext.emit">op_output.emit()</a>.

Note that for native Python operators as defined here, any Python object can be emitted or received. When transmitting between operators, a shared pointer to the object is transmitted rather than a copy. In some cases, such as sending the same tensor to more than one downstream operator, it may be necessary to avoid in-place operations on the tensor in order to avoid any potential race conditions between operators.

Specifying operator parameters (Python)

In the example SignalGeneratorOp operator above, we added three keyword arguments in the operator’s __init__ method, used inside the compose() method of the operator to adjust its behavior:

            

            
def __init__(self, fragment, *args, width=4, unit_area=False, **kwargs):
    # Internal counter for the time-dependent signal generation
    self.count = 0

    # Parameters
    self.width = width
    self.unit_area = unit_area

    # To forward remaining arguments to any underlying C++ Operator class
    super().__init__(fragment, *args, **kwargs)
        

            

            
def setup(self, spec: OperatorSpec):
    spec.param("width", 4)
    spec.param("unit_area", False)
        

See the Configuring operator parameters section to learn how an application can set these parameters.

Specifying operator inputs and outputs (Python)

To configure the input(s) and output(s) of Python native operators, call the spec.input() and spec.output() methods within the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup()</a> method of the operator.

The spec.input() and spec.output() methods should be called once for each input and output to be added. The <a href="api/python/holoscan_python_api_core.html#holoscan.core.OperatorSpec">holoscan.core.OperatorSpec</a> object and the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup()</a> method will be initialized and called automatically by the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Application">Application</a> class when its <a href="api/python/holoscan_python_api_core.html#holoscan.core.Application.run">run()</a> method is called.

These methods (spec.input() and spec.output()) return an <a href="api/python/holoscan_python_api_core.html#holoscan.core.IOSpec">IOSpec</a> object that can be used to configure the input/output port.

By default, the <a href="api/python/holoscan_python_api_conditions.html#holoscan.conditions.MessageAvailableCondition">holoscan.conditions.MessageAvailableCondition</a> and <a href="api/python/holoscan_python_api_conditions.html#holoscan.conditions.DownstreamMessageAffordableCondition">holoscan.conditions.DownstreamMessageAffordableCondition</a> conditions are applied (with a min_size of 1) to the input/output ports. This means that the operator’s <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute()</a> method will not be invoked until a message is available on the input port and the downstream operator’s input port (queue) has enough capacity to receive the message.

            

            
def setup(self, spec: OperatorSpec):
        spec.input("in")
        # Above statement is equivalent to:
        # spec.input("in")
        # .condition(ConditionType.MESSAGE_AVAILABLE, min_size = 1)
        spec.output("out")
        # Above statement is equivalent to:
        # spec.output("out")
        # .condition(ConditionType.DOWNSTREAM_MESSAGE_AFFORDABLE, min_size = 1)
    
        

In the above example, the spec.input() method is used to configure the input port to have the <a href="api/python/holoscan_python_api_conditions.html#holoscan.conditions.MessageAvailableCondition">holoscan.conditions.MessageAvailableCondition</a> with a minimum size of 1. This means that the operator’s <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute()</a> method will not be invoked until a message is available on the input port of the operator. Similarly, the spec.output() method is used to configure the output port to have a <a href="api/python/holoscan_python_api_conditions.html#holoscan.conditions.DownstreamMessageAffordableCondition">holoscan.conditions.DownstreamMessageAffordableCondition</a> with a minimum size of 1. This means that the operator’s <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute()</a> method will not be invoked until the downstream operator’s input port has enough capacity to receive the message.

If you want to change this behavior, use the <a href="api/python/holoscan_python_api_core.html#holoscan.core.IOSpec.condition">IOSpec.condition()</a> method to configure the conditions. For example, to configure the input and output ports to have no conditions, you can use the following code:

            

            
from holoscan.core import ConditionType, OperatorSpec
# ...
    def setup(self, spec: OperatorSpec):
        spec.input("in").condition(ConditionType.NONE)
        spec.output("out").condition(ConditionType.NONE)
        

The example code in the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.setup">setup()</a> method configures the input port to have no conditions, which means that the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute()</a> method will be called as soon as the operator is ready to compute. Since there is no guarantee that the input port will have a message available, the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute()</a> method should check if there is a message available on the input port before attempting to read it.

The <a href="api/python/holoscan_python_api_core.html#holoscan.core.InputContext.receive">receive()</a> method of the <a href="api/python/holoscan_python_api_core.html#holoscan.core.InputContext">InputContext</a> object can be used to access different types of input data within the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Operator.compute">compute()</a> method of your operator class. This method takes the name of the input port as an argument (which can be omitted if your operator has a single input port).

For standard Python objects, <a href="api/python/holoscan_python_api_core.html#holoscan.core.InputContext.receive">receive()</a> will directly return the Python object for input of the specified name.

The Holoscan SDK also provides built-in data types called Domain Objects, defined in the include/holoscan/core/domain directory. For example, the <a href="api/python/holoscan_python_api_core.html#holoscan.core.Tensor">Tensor</a> is a Domain Object class that is used to represent a multi-dimensional array of data, which can be used directly by OperatorSpec, InputContext, and OutputContext.

In both cases, it will return None if there is no message available on the input port:

            

            
# ...
    def compute(self, op_input, op_output, context):
        msg = op_input.receive("in")
        if msg:
            # Do something with msg
        

Receiving any number of inputs (Python)

Instead of assigning a specific number of input ports, it may be desired to have the ability to receive any number of objects on a port in certain situations. This can be done by calling spec.param(port_name, kind='receivers') as done for PingRxOp in the native operator ping example located at examples/native_operator/python/ping.py:

Code Snippet: examples/native_operator/python/ping.py

Listing 16 examples/native_operator/python/ping.py

            

            
class PingRxOp(Operator):
    """Simple receiver operator.

This operator has:
input: "receivers"

This is an example of a native operator that can dynamically have any
number of inputs connected to is "receivers" port.
"""

    def __init__(self, fragment, *args, **kwargs):
        self.count = 1
        # Need to call the base class constructor last
        super().__init__(fragment, *args, **kwargs)

    def setup(self, spec: OperatorSpec):
        spec.param("receivers", kind="receivers")

    def compute(self, op_input, op_output, context):
        values = op_input.receive("receivers")
        print(f"Rx message received (count:{self.count}, size:{len(values)})")
        self.count += 1
        print(f"Rx message value1:{values[0].data}")
        print(f"Rx message value2:{values[1].data}")
        

and in the compose method of the application, two parameters are connected to this “receivers” port:

            

            
        self.add_flow(mx, rx, {("out1", "receivers"), ("out2", "receivers")})
        

This line connects both the out1 and out2 ports of operator mx to the receivers port of operator rx.

Here, values as returned by op_input.receive("receivers") will be a tuple of python objects.

Python wrapping of a C++ operator

For convenience while maintaining highest performance, operators written in C++ can be wrapped in Python. In the Holoscan SDK, we’ve used pybind11 to wrap all the built-in operators in <a href="https://github.com/nvidia-holoscan/holoscan-sdk/tree/v0.6.0/python/holoscan/operators">python/holoscan/operators</a>. We’ll highlight the main components below:

Trampoline classes for handling Python kwargs

In a C++ file (my_op_pybind.cpp in our skeleton code below), create a subclass of the C++ Operator class to wrap. In the subclass, define a new constructor which takes a Fragment, an explicit list of parameters with potential default values (argA, argB below…), and an operator name to fully initialize the operator similar to what is done in <a href="https://github.com/nvidia-holoscan/holoscan-sdk/blob/v0.5.0/include/holoscan/core/fragment.hpp#L207">Fragment::make_operator</a>:

Listing 17 my_op_python/my_op_pybind.cpp

            

            
#include <holoscan/core/fragment.hpp>
#include <holoscan/core/operator.hpp>
#include <holoscan/core/operator_spec.hpp>

#include "my_op.hpp"

class PyMyOp : public MyOp {
 public:
  using MyOp::MyOp;

  PyMyOp(
    Fragment* fragment,
    TypeA argA, TypeB argB = 0, ...,
    const std::string& name = "my_op"
  ) : MyOp(ArgList{
            Arg{"argA", argA},
            Arg{"argB", argB},
            ...
          }) {
    # If you have arguments you can't pass directly to the `MyOp` constructor as an `Arg`, do
    # the conversion and pass the result to `this->add_arg` before setting up the spec below.
    name_ = name;
    fragment_ = fragment;
    spec_ = std::make_shared<OperatorSpec>(fragment);
    setup(*spec_.get());
  }
}
        

Example: Look at the implementation of PyLSTMTensorRTInferenceOp on HoloHub for a specific example, or any of the Py*Op classes used for the SDK built-in operators here. In the latter, you can find examples of add_arg used for less straightforward arguments.

Documentation strings

Prepare documentation strings (const char*) for your python class and its parameters, which we’ll use in the next step.

Listing 18 my_op_python/my_op_pybind_docs.hpp

            

            
#include "../macros.hpp"

namespace doc::MyOp {

  PYDOC(cls, R"doc(
My operator.
)doc")

  PYDOC(constructor, R"doc(
Create the operator.

Parameters
----------
fragment : holoscan.core.Fragment
The fragment that the operator belongs to.
argA : TypeA
argA description
argB : TypeB, optional
argB description
name : str, optional
The name of the operator.
)doc")

  PYDOC(initialize, R"doc(
Initialize the operator.

This method is called only once when the operator is created for the first time,
and uses a light-weight initialization.
)doc")

  PYDOC(setup, R"doc(
Define the operator specification.

Parameters
----------
spec : holoscan.core.OperatorSpec
The operator specification.
)doc")

}
        

Examples: Continuing with the LSTMTensorRTInferenceOp example on HoloHub, the documentation strings are defined in lstm_tensor_rt_inference_pydoc.hpp. The documentation strings for the SDK built-in operators are located in operators_pydoc.hpp.

Writing glue code

In the same C++ file as the first section, call py::class_ within PYBIND11_MODULE to define your operator python class.

Listing 19 my_op_python/my_op_pybind.cpp (continued)

            

            
#include <pybind11/pybind11.h>

#include "my_op_pybind_docs.hpp"

using pybind11::literals::operator""_a;
namespace py = pybind11;

#define STRINGIFY(x) #x
#define MACRO_STRINGIFY(x) STRINGIFY(x)

// See notes above, value of `<module_name>` is important
PYBIND11_MODULE(<module_name>, m) {
  m.doc() = R"pbdoc(
My Module Python Bindings
---------------------------------------
.. currentmodule:: <module_name>
.. autosummary::
:toctree: _generate
add
subtract
)pbdoc";

#ifdef VERSION_INFO
  m.attr("__version__") = MACRO_STRINGIFY(VERSION_INFO);
#else
  m.attr("__version__") = "dev";
#endif

  py::class_<MyOp, PyMyOp, Operator, std::shared_ptr<MyOp>>(
    m, "MyOp", doc::MyOp::doc_cls)
  .def(py::init<Fragment*, TypeA, TypeB, ..., const std::string&>(),
      "fragment"_a,
      "argA"_a,
      "argB"_a = 0,
      ...,
      "name"_a = "my_op",
      doc::MyOp::doc_constructor)
  .def("initialize",
      &MyOp::initialize,
      doc::MyOp::doc_initialize)
  .def("setup",
      &MyOp::setup,
      "spec"_a,
      doc::MyOp::doc_setup);
}
        

Examples: Like the trampoline class, the PYBIND11_MODULE implementation of the LSTMTensorRTInferenceOp example on HoloHub is located in lstm_tensor_rt_inference.cpp. For the SDK built-in operators, their class bindings are all implemented within a single PYBIND11_MODULE in operators.cpp.

Configuring with CMake

We use CMake to configure pybind11 and build the bindings for the C++ operator you wish to wrap. There are two approaches detailed below, one for HoloHub (recommended), one for standalone CMake projects.

            

            
include(pybind11_add_holohub_module)
pybind11_add_holohub_module(
    CPP_CMAKE_TARGET my_op
    CLASS_NAME "MyOp"
    SOURCES my_op_pybind.cpp
)
        

            

            
pybind11_add_module(my_python_module my_op_pybind.cpp)
target_link_libraries(my_python_module
  PRIVATE holoscan::core
  PUBLIC my_op
)
        

Importing the class in Python

            

            
from holohub.my_op import MyOp
        

            

            
import holoscan.core
import holoscan.gxf # if your c++ operator uses gxf extensions

from <build_or_install_dir>.my_op_python import MyOp
        

Interoperability between wrapped and native Python operators

As described in the Interoperability between GXF and native C++ operators section, <a href="api/cpp/classholoscan_1_1Tensor.html#_CPPv4N8holoscan6TensorE">holoscan::Tensor</a> objects can be passed to GXF operators using a <a href="api/cpp/classholoscan_1_1TensorMap.html#_CPPv4N8holoscan9TensorMapE">holoscan::TensorMap</a> message that holds the tensor(s). In Python, this is done by sending dict type objects that have tensor names as the keys and holoscan Tensor or array-like objects as the values. Similarly, when a wrapped C++ operator that transmits a single <a href="api/cpp/classholoscan_1_1Tensor.html#_CPPv4N8holoscan6TensorE">holoscan::Tensor</a> is connected to the input port of a Python native operator, calling op_input.receive() on that port will return a Python dict containing a single item. That item’s key is the tensor name and its value is the corresponding <a href="api/python/holoscan_python_api_core.html#holoscan.core.Tensor">holoscan.core.Tensor</a>.

Consider the following example, where VideoStreamReplayerOp and HolovizOp are Python wrapped C++ operators, and where ImageProcessingOp is a Python native operator:

Fig. 19 The tensor interoperability between Python native operator and C++-based Python GXF operator

The following code shows how to implement ImageProcessingOp’s compute() method as a Python native operator communicating with C++ operators:

Listing 22 examples/tensor_interop/python/tensor_interop.py

            

            
    def compute(self, op_input, op_output, context):
        # in_message is of dict
        in_message = op_input.receive("input_tensor")

        # smooth along first two axes, but not the color channels
        sigma = (self.sigma, self.sigma, 0)

        # out_message is of dict
        out_message = dict()

        for key, value in in_message.items():
            print(f"message received (count:{self.count})")
            self.count += 1

            cp_array = cp.asarray(value)

            # process cp_array
            cp_array = ndi.gaussian_filter(cp_array, sigma)

            out_message[key] = cp_array

        op_output.emit(out_message, "output_tensor")
        

• The <a href="api/python/holoscan_python_api_core.html#holoscan.core.InputContext.receive">op_input.receive()</a> method call returns a dict object.

• The <a href="api/python/holoscan_python_api_core.html#holoscan.core.Tensor">holoscan.core.Tensor</a> object is converted to a CuPy array by using cupy.asarray() method call.

• The CuPy array is used as an input to the ndi.gaussian_filter() function call with a parameter sigma. The result of the <a href="https://docs.scipy.org/doc/scipy/reference/generated/scipy.ndimage.gaussian_filter.html">ndi.gaussian_filter()</a> function call is a CuPy array.

• Finally, a new dict object is created ,out_message, to be sent to the next operator with <a href="api/python/holoscan_python_api_core.html#holoscan.core.OutputContext.emit">op_output.emit()</a>. The CuPy array, cp_array, is added to it where the key is the tensor name. CuPy arrays do not have to explicitly be converted to a holocan.core.Tensor object first since they implement a DLPack (and __cuda__array_interface__) interface.

You can add multiple tensors to a single dict object , as in the example below:

Operator sending a message:

            

            
out_message = {
            "video": output_array,
            "labels": labels,
            "bbox_coords": bbox_coords,
        }

        # emit the tensors
        op_output.emit(out_message, "outputs")
           
        

Operator receiving the message, assuming the outputs port above is connected to the inputs port below with <a href="api/python/holoscan_python_api_core.html#holoscan.core.Fragment.add_flow">add_flow()</a>has the corresponding tensors:

            

            
in_message = op_input.receive("inputs")
    # Tensors and tensor names
    video_tensor = in_message["video"]
    labels_tensor = in_message["labels"]
    bbox_coords_tensor = in_message["bbox_coords"]