x264 is an excellent video codec implementing H.264 video compression standard. One source of its ability to greatly compress video is allowing a compressed frame to refer to several frames not only in the past but also in the future: refer to frames following the current one. Such frames are called B-frames (bidirectional), and even previous generation of video codecs, MPEG4 ASP (like XviD), had this feature, so it's not particularly new. However AVI file format and Video-for-Windows (VfW) subsystem predate those, and are designed to work strictly sequentially: frames are compressed and decomressed in strict order one by one, so a frame can only use some previous frames to refer to. Such frames are called P-frames (predicted). For this reason AVI format does not really suit MPEG4 and H.264 video. However with some tricks (or should I say hacks) it is still possible to write MPEG4 or H.264 stream to AVI file, and this is what VfW codecs like XviD and x264vfw do. They buffer some frames and then output compressed frames with some delay, sometimes combining P and B frames in single chunks. Appropriate decoder knows this trick and restores proper frame order so everything works fine.

Some time ago we received a complaint from a user of Video Enhancer about x264vfw: video compressed with this codec to AVI file showed black screen when started playing and only after jumping forward a few seconds it started to play, but with sound out of sync. To investigate the issue I made a graph in GraphEditPlus where some video was compressed with x264vfw and then written to an AVI file. But I also inserted a Sample Grabber right after the compressor to see what it outputs. Here's what I got:

click to enlarge

It turned out for some reason x264vfw spitted ~50 empty frames, 1 byte in size each, all marked as delta (P) frames. After those ~50 frames came first key frame and then normal P frames of sane sizes followed. Those 50 empty frames which were not started with a key frame (like it is expected to be in AVI file) created the effect: black screen when just started playing and audio sync issue. I don't know in what version of x264vfw this behavior started but it's really problematic for writing AVI files. Fortunately, the solution is very simple. If you open configuration dialog of x264vfw you can find "Zero Latency" check box. Check it, as well as "VirtualDub hack" and then it will work properly: will start sending sane data from the first frame, and resulting AVI file will play fine.

tags: video_enhancer directshow