使用DirectShow捕获AVI视频

Question

I'm trying to capture an AVI video, using DirectShow AVIMux and FileWriter Filters. When I connect SampleGrabber filter instead of the AVIMux , I can clearly see that the stream is 30 fps, however upon capturing the video, each frame is duplicated 4 time and I get a 120 frames instead of 30. The movie is 4 times slower than it should be and only the first frame in the set of 4 is a Key Frame. I tried the same experiment with 8 fps and for each image I received, I had 15 frames in the video. And in case of 15 fps, I got each frame 8 times.

I tried both writing the code in C++ and testing it with Graph Edit Plus. Is there any way I can control it? Maybe some restrictions on the AVIMux filter?

Answer 1

You don't specify your capture format which could have some bearing on the problem, but generally it sounds like the graph when writing to file has some bottleneck which prevents the stream from continuing to flow at 30fps. The camera is attempting to produce frames at 30fps, and it will do so as long as buffers are recycled for it to fill.

But here the buffers aren't available because the file writer is busy getting them onto the disk. The capture filter is starved and in this situation it increments the "dropped frame" counter which travels with each captured frame. AVIMux uses this count to insert an indicator into the AVI file which says in effect "a frame should have been available here to write to file, but isn't; at playback time repeat the last frame". So the file should have placeholders for 30 frames per second - some filled with actual frames, and some "dropped frames".

Also, you don't mention whether you're muxing in audio, which would be acting as a reference clock for the graph to maintain audio-video sync. When capture completes if also using an audio stream, AVIMux alters the framerate of the video stream to make the duration of the two streams equal. You can check whether AVIMux has altered the framerate of the video stream by dumping the AVI file header (or maybe right click on the file in explorer and look at the properties).

If I had to hazard a guess as to the root of the problem, I'd wager the capture driver has a bug in calculating the dropped frame count which is in turn messing up AVIMux. Does this happen with a different camera?