带有B帧的mp4文件需要CTTS框吗？

Question

I'm trying to play an mp4 file which does not contain ctts box which contains the presentation time offset from the decoding time. I know that we use ctts offsets to decide the presentation order in case of b frames. In the mp4 ISO document, it is mentioned that ctts is not mandatory and it is only required when the decoding and presentation time are different. My mp4 file conatains b frames and ctts information is neccessary for me.

my question is given that the mp4 file does not contain ctts information, is there a way to decide the presentation order? like manually create the ctts offset from GOP size and key frame interval.

Answer 1

是的，ctts框是100％必需的。

Answer 2

CTTS is only required if your frames were reorder. In most cases frames are reordered but not always. So first check if your frames are stored reordered. There is no general solution to recreate the presentation time. But most AVC stream follow the same GOP reordering pattern. So on disk you see

I P1 B1 B2 P2 B3 which normally presents as
I B1 B2 P1 B3

So you can calculate your presentation time like this:

     I  P1  B1  B2  P2  B3
DTS: 1  2   3   4   5   6
PTS: 1  4   2   3   7   5

The presentation time for each P frame is N + [number of trailing consecutive B-Frames] + 1. This probably works for most AVC streams but it is not guaranteed to work for all.