Given a set of objects in contiguous frames, the challenge of object tracking is to devise an algorithm that can accurately track each identified object from frame to frame. A typical object tracking algorithm achieves this tracking by assigning a unique ID to each detected object and returning the position of each object in successive frames, as long as the objects are within the frame. While there are several techniques to achieve this tracking, this document describes an efficient and highly accurate object tracking algorithm based on the NVIDIA® Optical Flow hardware accelerator.
What is Object Tracking?
A Classic Object Tracking Solution
Figure 1 represents a classic object tracking solution, in a typical intelligent video analytics scenario. Field cameras generate multiple streams that are fed to a video decoder (NVDEC in this case), and the decoded frames are passed through an object detector and a feature extractor/processor. The tracker, with the combined knowledge of object boundaries and object features, tracks the objects of interest across frames.
Figure 1. Classical Object Tracking Solution
Optical Flow based Object Tracking Solution
NVIDIA Turing™ and later GPUs have a dedicated hardware accelerator to calculate the optical flow between frames. Figure 2 represents our proposed solution where the feature tracker/extractor part is replaced with an algorithm that is based on the optical flow vectors that are returned by the GPU’s optical flow engine. In classic techniques, the feature extractor/processors runs on CPU or in some cases on GPU (for example, by using CUDA). In this solution, this step is done on the Optical Flow Engine, which frees the CPU/GPU from a major computationally intensive operation and leaves the CPU/GPU free for other tasks.
Figure 2. Optical Flow-Based Object Tracking Solution
Object Tracker in Optical Flow SDK
The NVIDIA Optical Flow SDK contains an end-to-end object tracking application and a library that can be easily integrated into your custom application. For ease of integration, an API is provided in the NvOFTracker.h file. The source code for the application and the library is also available in the SDK.
The application uses the GPU’s NVDEC to decode the input video bitstream, a yolov3-based object detector that runs on the GPU to detect objects of interest, an optical flow hardware accelerator to compute the optical flow between successive frames, and a combination of various algorithms to track the objects through the successive video frames.
- Decode the incoming bitstream into frames.
- Feed the decoded frames to an object detector. We use a yolov3-based object detector in the application, but this object can be replaced with any object detector you choose.
- This step detects Regions of Interest (ROI) and their respective classes.
- The object detector runs every Kth frame where 0 ≤ K ≤ 4.
- Feed decoded frames to the Optical Flow Engine.
- This step provides flow vectors between the frames, for example, frame (P-1) and frame P.
- The tracker completes the following tasks:
- Maintains an ROI store. This store is constantly updated with detector ROIs.
- Calculates a Representative Flow for each of the ROIs.
- Warps the ROIs that are present in ROI store with the representative flow.
- Finds the best match for the warped ROI among the current frame ROIs, based on following algorithm:
- Calculate the centroid distance between the current frame ROI and the warped ROI.
- Calculate the Intersection Over Union (IOU) of the current frame ROI and the warped ROI.
- Use the weighted average of the above as the cost in the Hungarian Algorithm to find the ROI matching between frames.
- Add the ROIs to output list.
- If there is no match, the tracker warps the ROI in ROI store and adds it to output list with the existing ID.
- ROIs in current frame that do not find a match are given new IDs and added to output list
- The ROI store is subsequently updated accommodating the current frame ROIs
- Repeat Steps 1-4 for as many frames as there are in the bitstream.
Figure 3. High-Level Overview of Sample Tracking Pipeline with NVOFTRACKER
Deriving the ROI Representative Flow
As mentioned in step 4b on on page 4, NvOFTracker computes the Representative Flow for each ROI in the ROI store. The flow that is generated by NVIDIA Optical Flow engine needs to be converted to a sparse single flow vector for each of the ROIs. NvOFTracker segments the ROI in the flow vector domain into regions of similar flow based on certain thresholds and uses the median of the dominating region as the ROI representative flow. For more information, refer to the CConnectedRegionGenerator class in the CConnectedRegionGenerator.h file.
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