I am trying to find h264 frames in a mp4 container of a segmented file (styp). For segmentation, I currently use the MP4Box dash. I used the MP4Box parser, and I noticed that in each keyframe (IDR) the sample size is different from the size of the same frame in the source file. I calculated the difference and always 37 bytes for the same file in other segments.
What do these bytes represent? They are displayed only in segmented mp4 file types before each keyframe. I tested with other files, and the number of these bytes varied a bit (the other file was 39 bytes) and are not the same data (although they have several byte patterns). Is this something from the mp4 or H264 standard, or even the MP4Box? I have no clue.