darknet_video.py off by 1 frame

[mazatov]

Running darknet_video.py on the video makes a video where the detections and the video are off by one frame.

The amount of frames it's off by seems to depend on max_size of the darknet_image_queue

[mazatov]

Also, not clear why max_size =1 for the queues. Larger max_size would speed things up. Seems like the author suspected that to be a problem.

[mazatov]

Update. The darknet_image is overwritten in video_capture every time it's being created because it sits in the same memory so effectively the darknet_image_queue is not working properly

Proposed fix here:

First time doing PR here @AlexeyAB , so let me know if I need to do more. Seems like darknet detector demo has the same error though so possibly it's in C++ code too but I don't know enough to make changes there.

[Tacokeet]

Hey @mazatov, I also found this problem, thanks for the solution. The delay within the darknet detector demo can be fixed by adding the flag -avgframes 1 found here #5806.

[mazatov]

Thanks didn't know about this flag -avgframes 1 option.

Still, it's a bit of hackish fix as it assumes that cv2.videoreader is going to work faster than darknet.detect_image and the frame in the queue will always be overwritten by the next frame. While it's true for most computer hardware, I can imagine a situation where detect_image outperformed cv2.videoreader and you would suddenly not have that 1 shift in frames.

My proposed fix gets rid of the problem without any sacrifice in speed(at least on my computer)

[Tacokeet]

Okay I just tried it out on my computer.
It fixes the 1 frame problem but there is memory problem. It keeps ramping up the memory usage until it crashes.

[mazatov]

Good point, if you have a long video and GPU can't keep up with the loading it would happen. I should add maxsize to the queues so they don't load too much into memory. I put maxsize to 250 for all queues to avoid this problem.

[mazatov]

@Tacokeet , I was still getting a memory problem so I worked on it a bit more. I did a hacky fix to the issue, but it works. You can check the updated PR.

[Tacokeet]

Thanks for the update! Seems you forgot to push the correct version because 'darknet_images' is not defined. But after adding it seems to run nice. I can now finish a ~9 min video without any memory issues.

[mazatov]

Oh, right! Thanks for pointing it out!