> On Mar 9, 2015, at 4:05 PM, Carl Eugen Hoyos <[email protected]> wrote: > > Dave Rice <dave <at> dericed.com> writes: > >>>> As far as I tested I can say (for the >>>> ffv1-supported pixelformats "gbrp10le" and >>>> "gbrp12le") it works lossless. All FrameMD5s >>>> are equal: >>> >>> (This is not the first time that I am slightly >>> surprised people use FFmpeg to prove that an >>> encoder of FFmpeg is lossless.) >> >> Why is this slightly surprising? > > Assuming this is done to rule out bugs > in FFmpeg: > I find it unlikely that ffv1 is not lossless > and that you will find out like this (as > opposed to feed random data to the codec).
Using framemd5 verification after ffv1 encoding, I’ve found about 3 frames that weren’t lossless. This is out of probably millions of frames, but computer problems happen. In all of the cases the a local computer was using an input and an output that were accessed over a network and re-running the process a second time produced an error-free output. The flac utility has a --verify option for this level of paranoia so that the file written is assessed. I added an enhancement ticket for a similar approach in ffmpeg here: https://trac.ffmpeg.org/ticket/2866 <https://trac.ffmpeg.org/ticket/2866>. The issue is hard to reproduce as reproduction usually involves huge amounts of encoding. > I find it much more likely that a bug in > the dpx decoder exists: Since you use the > same decoder twice in above test you > would not find such a bug. This is true with the input but not with this output. This is exactly how we found the libopenjpeg bug referenced here: https://trac.ffmpeg.org/ticket/3745 <https://trac.ffmpeg.org/ticket/3745>. The framemd5 of the original file represents the properly decoded frame but the corresponding framemd5 of the improperly decoded jpeg2000 frame gives a different framemd5 value. > A bug in both gm and FFmpeg that leads to > identical output seems less likely to me. Certainly true. dave > Of course I do believe that framemd5 is > useful to compare frames. > > Carl Eugen > > _______________________________________________ > ffmpeg-user mailing list > [email protected] > http://ffmpeg.org/mailman/listinfo/ffmpeg-user _______________________________________________ ffmpeg-user mailing list [email protected] http://ffmpeg.org/mailman/listinfo/ffmpeg-user
