选择特定的框架快速OpenCV Python

Question

I am trying to rapidly select and process different frames from a video using OpenCV Python. To select a frame, I have used the 'CAP_PROP_POS_FRAMES' (or cap.set(2, frame_no)). However when using this I noticed a delay of about 200 ms to decode the selected frame. My script will be jumping in between frames a lot (not necessarily chronological) which means this will cause a big delay between each iteration. I suspected OpenCV is buffering the upcoming frames after I set the frame number. Therefore I tried pre-decoding of the video by basically putting the entire video as a list so it can be accessed from RAM. This worked fantastic except bigger videos completely eat up my memory. I was hoping someone knows a way to either set the frame number without this 200ms delay or to decode the video without using all of my memory space. Any suggestions are also welcome!

Answer 1

I don't know how to avoid that 200ms delay, but I have a suggestion on how you could decode the video first even if its size is greater than your RAM. You could use numpy's memmap :

https://docs.scipy.org/doc/numpy-1.13.0/reference/generated/numpy.memmap.html .

In practice you could have a function that initializes this memory-mapped matrix and then iterate over each frame of the video using VideoCapture and then store each frame in this matrix. After that you will be able to jump between each frame just by accessing this memory-mapped matrix.