video preprocessor output is storage-inefficient avi, compared to mp4 input

[ruck94301]

This item is transplanted here from issue #314.

Lezio asks,
regarding the video preprocessing module, i.e., the tool we use to increase contrast, crop/trim videos, etc.
Preprocessed videos are generally much larger than the raw input videos.
I often see 800 MB mp4 videos being preprocessed into 6 GB avi videos with no apparent change in resolution.
Potential improvement for the preprocessing stage?

[yujiahu415]

This is probably related to how opencv reads and decodes the frames. I think it uses ffmepg codec, but you can double check. Lezio's video must be under lossless compression during recording since the size is very small considering their resolution and duration. I have no idea how the compression was done by the recording software, but it is very efficient. From LabGym side, there isn't much I can do for this.