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603334a043
The previous formula was introduced without justification in 6e713841e8,
and the only thing Paul had to say about it over IRC was that it was copied
from an unspecified source on the internet.
I decided to do some testing and came to the conclusion that this term not
only produces "illegal" files, but also lowers PSNR score, over the naive
implementation without this extra term.
Here are the results of a round-trip test, using allrgb/allyuv (respectively)
as the input, and fade=alpha=yes:n=256 to cycle through every possible alpha
value, comparing the round-trip output against the input:
Before patch:
PSNR r:26.677431 g:26.677431 b:26.677431 a:inf average:27.926818 min:6.012093 max:55.400791
PSNR y:26.677431 u:21.101981 v:21.101981 a:inf average:23.548981 min:9.013835 max:53.182303 (full)
PSNR y:27.348055 u:21.101981 v:21.101981 a:inf average:23.625238 min:9.554991 max:45.652221 (limited)
After patch:
PSNR r:27.321996 g:27.321996 b:27.321996 a:inf average:28.571384 min:6.012093 max:52.424553
PSNR y:27.321996 u:23.187879 v:23.187879 a:inf average:25.431773 min:9.013835 max:50.199232 (full)
PSNR y:27.868544 u:23.187879 v:23.187879 a:inf average:25.515660 min:9.554991 max:45.078298 (limited)
It's worth pointing out that previous version sometimes artificially inflates
PSNR by producing values that are too high (i.e. RGB > A), such as for the
input pair (R = 255, A = 2) which should give R = 2, but actually gives R = 3
under the old logic.
As a second evaluation without this shortcoming, here is a comparison against
the reference value computed with a floating point format:
Before patch:
PSNR r:53.600599 g:53.957833 b:53.540948 a:inf average:54.945316 min:50.508901 max:inf (premul only)
PSNR r:30.734183 g:30.734183 b:30.734183 a:inf average:31.983570 min:12.058264 max:inf (round-trip)
After patch:
PSNR r:61.751104 g:65.239091 b:61.339191 a:inf average:63.710714 min:55.441130 max:inf (premul only)
PSNR r:32.611851 g:32.611851 b:32.611851 a:inf average:33.861238 min:12.058264 max:inf (round-trip)
FFmpeg README
FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.
Libraries
libavcodecprovides implementation of a wider range of codecs.libavformatimplements streaming protocols, container formats and basic I/O access.libavutilincludes hashers, decompressors and miscellaneous utility functions.libavfilterprovides means to alter decoded audio and video through a directed graph of connected filters.libavdeviceprovides an abstraction to access capture and playback devices.libswresampleimplements audio mixing and resampling routines.libswscaleimplements color conversion and scaling routines.
Tools
- ffmpeg is a command line toolbox to manipulate, convert and stream multimedia content.
- ffplay is a minimalistic multimedia player.
- ffprobe is a simple analysis tool to inspect multimedia content.
- Additional small tools such as
aviocat,ismindexandqt-faststart.
Documentation
The offline documentation is available in the doc/ directory.
The online documentation is available in the main website and in the wiki.
Examples
Coding examples are available in the doc/examples directory.
License
FFmpeg codebase is mainly LGPL-licensed with optional components licensed under GPL. Please refer to the LICENSE file for detailed information.
Contributing
Patches should be submitted to the ffmpeg-devel mailing list using
git format-patch or git send-email. Github pull requests should be
avoided because they are not part of our review process and will be ignored.