In message <bc31bba9-06eb-579a-8904-fc9a4ed1a...@sky.com>, "Sven C. Dack" <sven.c.d...@sky.com> wrote:
>this is only possible when the input is composed of absolute frames, i.e. >with >Motion JPEG. Your video will probably have been compressed with one of the >more >advanced algorithms and these purposely do not treat frames as absolute, but >use >their differences for their encoding. As such can you not drop every second >frame, >because you don't have full frames and dropping some of the intermediate >frame information means you lose vital information for reconstructing the >frames. I accept your answer, fully and without reservation, but it sort-of makes me think that I have misunderstood, in a rather fundamental way, how video compression works, specifically with respect to MPEG4/AVC. (I should have mentioned this earlier, but that is the specific type of video I'm dealing with.) I though that for, e.g., MPEG4/AVC, most frames are expressed in terms of their differences from some slightly earlier complete reference frame. If so, and if care was taken so that all of the reference frames were preserved in full, wouldn't it be possible to drop, say, all odd numbered frames in the following sequence of frames, up until the next reference frame? As I say, I am already resigned to accepting that, for now, I'll have just go ahead and reencode down to my desired lower frame rate, but I'm still somewhat interested in understanding why what I've described above wouldn't work. I may learn something! And that's never a bad thing. Is each frame within, say, an MPEG4/AVC video dependent upon -both- the nearest preceeding reference frame -and- also the other non-reference preceeding frames ? I guess so. If so, then I guess that also explains why some folks have written that MPEG4/AVC is harder to edit than, say, Motion JPEG... yes? _______________________________________________ ffmpeg-user mailing list ffmpeg-user@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-user To unsubscribe, visit link above, or email ffmpeg-user-requ...@ffmpeg.org with subject "unsubscribe".
