Class V4L2VideoCaptureOp

• Defined in File v4l2_video_capture.hpp

Nested Types

• Struct V4L2VideoCaptureOp::Buffer

Base Type

• public holoscan::Operator (Class Operator)

class V4L2VideoCaptureOp : public holoscan::Operator

Operator class to get the video stream from V4L2.

https://www.kernel.org/doc/html/v4.9/media/uapi/v4l/v4l2.html

Inputs a video stream from a V4L2 node, including USB cameras and HDMI IN.

• Input stream is on host. If no pixel format is specified in the yaml configuration file, the pixel format will be automatically selected. However, only AB24, YUYV, and MJPG are then supported. If a pixel format is specified in the yaml file, then this format will be used. However, note that the operator then expects that this format can be encoded as RGBA32. If not, the behavior is undefined.

• Output stream is on host. Always RGBA32 at this time.

Use holoscan::ops::FormatConverterOp to move data from the host to a GPU device.

==Named Outputs==

• signal : nvidia::gxf::VideoBuffer

  • A message containing a video buffer on the host with format GXF_VIDEO_FORMAT_RGBA.

==Parameters==

• allocator: Memory allocator to use for the output.

• device: The device to target (e.g. “/dev/video0” for device 0). Default value is "/dev/video0".

• width: Width of the video stream. Optional (default: 0).

• height: Height of the video stream. Optional (default: 0).

• num_buffers: Number of V4L2 buffers to use. Optional (default: 4).

• pixel_format: Video stream pixel format (little endian four character code (fourcc)). Default value is "auto".

• exposure_time: Exposure time of the camera sensor in multiples of 100 μs (e.g. setting exposure_time to 100 is 10 ms). Optional (default: auto exposure, or camera sensor default). Use v4l2-ctl -d /dev/<your_device> -L for a range of values supported by your device.

  • When not set by the user, V4L2_CID_EXPOSURE_AUTO is set to V4L2_EXPOSURE_AUTO, or to V4L2_EXPOSURE_APERTURE_PRIORITY if the former is not supported.

  • When set by the user, V4L2_CID_EXPOSURE_AUTO is set to V4L2_EXPOSURE_SHUTTER_PRIORITY, or to V4L2_EXPOSURE_MANUAL if the former is not supported. The provided value is then used to set V4L2_CID_EXPOSURE_ABSOLUTE.

• gain: Gain of the camera sensor. Optional (default: auto gain, or camera sensor default). Use v4l2-ctl -d /dev/<your_device> -L for a range of values supported by your device.

  • When not set by the user, V4L2_CID_AUTOGAIN is set to false (if supported).

  • When set by the user, V4L2_CID_AUTOGAIN is set to true (if supported). The provided value is then used to set V4L2_CID_GAIN.

==Device Memory Requirements==

When using this operator with a BlockMemoryPool, a single device memory block is needed (storage_type = 1). The size of this memory block can be determined by rounding the width and height up to the nearest even size and then padding the rows as needed so that the row stride is a multiple of 256 bytes. C++ code to calculate the block size is as follows:

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#include <cstdint>

int64_t get_block_size(int32_t height, int32_t width) {
  int32_t height_even = height + (height & 1);
  int32_t width_even = width + (width & 1);
  int64_t row_bytes = width_even * 4;  // 4 bytes per pixel for 8-bit RGBA
  int64_t row_stride = (row_bytes % 256 == 0) ? row_bytes : ((row_bytes / 256 + 1) * 256);
  return height_even * row_stride;
}
        

Public Functions

HOLOSCAN_OPERATOR_FORWARD_ARGS (V4L2VideoCaptureOp) V4L2VideoCaptureOp()=default

virtual void setup(OperatorSpec &spec) override

Define the operator specification.

Parameters

spec – The reference to the operator specification.

virtual void start() override

Implement the startup logic of the operator.

This method is called multiple times over the lifecycle of the operator according to the order defined in the lifecycle, and used for heavy initialization tasks such as allocating memory resources.

virtual void initialize() override

Initialize the operator.

This function is called when the fragment is initialized by Executor::initialize_fragment().

virtual void compute(InputContext &op_input, OutputContext &op_output, ExecutionContext &context) override

Implement the compute method.

This method is called by the runtime multiple times. The runtime calls this method until the operator is stopped.

Parameters

• op_input – The input context of the operator.

• op_output – The output context of the operator.

• context – The execution context of the operator.

virtual void stop() override

Implement the shutdown logic of the operator.

This method is called multiple times over the lifecycle of the operator according to the order defined in the lifecycle, and used for heavy deinitialization tasks such as deallocation of all resources previously assigned in start.