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Andreas Rheinhardt c638d1d126 avcodec/utvideodec: Avoid implicit qsort when creating Huffman tables 
The Huffman trees used by Ut Video have two important characteristics:
(i) Longer codes are on the left of the tree and (ii) for codes of the
same length, the symbol is descending from left to right in the tree.
Therefore all the information that needs to be transmitted is how long
the code corresponding to a given symbol is; and this is also all that
is transmitted.

Before 341914495e, the decoder used qsort
to sort the (length, symbol) pairs by ascending length and for equal
lengths by ascending symbol. Since said commit, the decoder uses
a first pass over the lengths table to count how many symbols of each
length there are; with (i) one can then easily calculate the code of
the left-most code with a given length in the tree and from there one
can calculate the codes for all entries, using one running counter for
each possible length. This eliminated the explicit qsort in
build_huff().

Yet ff_init_vlc_sparse() sorts the table itself as it has to ensure that
all the entries that will be placed in the same subtable are contiguous.
The tables created now are non-contiguous (they are ordered by symbol
and codes of different length aren't ordered at all; only codes of the
same length are ordered according to (ii)).

This commit therefore modifies the algorithm used to automatically create
tables whose codes are sorted from left to right in the tree. The key to
do so is the observation that the counts obtained in the first pass can
be used to contain the range of the codes of each length in the second
pass: If counts[i] is the count of codes with length i, then the first
counts[32] codes are of length 32, the next counts[31] codes are of
length 31 etc. So one knows the index of the lowest symbol whose code
has length 32 (if any): It is counts[32] - 1 due to (ii), whereas the
index of the lowest symbol whose code has length 31 (if any) is
counts[32] + counts[31] - 1; the index of the second-to-lowest symbol of
length 32 (if existing) is counts[32] - 2 etc.

If one follows the algorithm outlined above, one can switch to
ff_init_vlc_from_lengths() which has no implicit qsort; it also means
that one can offload the computation of the codes.

This turned out to be beneficial for performance: For the sample from
ticket #4044 it decreased the decicycles spent on one call to
build_huff() from 508480 to 340688 (GCC 9.3, looping 10 times over the
file to get enough runs and then repeating this ten times); for another
sample (YUV420p, natural content, 5500 frames, also ten iterations)
the time went down from 382346 to 275533 decicycles.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
2020-12-08 17:51:47 +01:00
compat avfilter/scale_cuda: add lanczos algorithm 2020-11-04 01:43:21 +01:00
doc hlsenc: expand hls_fmp4_init_filename with strftime() 2020-12-08 21:02:23 +08:00
ffbuild ffbuild: Refine MIPS handling 2020-07-23 16:30:02 +02:00
fftools fftools/ffprobe: add support for HDR10+ metadata 2020-12-05 19:20:11 -03:00
libavcodec avcodec/utvideodec: Avoid implicit qsort when creating Huffman tables 2020-12-08 17:51:47 +01:00
libavdevice avdevice/decklink: remove the duplicated warning message 2020-12-05 09:00:53 +08:00
libavfilter avfilter/aeval: add timeline support for aeval 2020-12-08 15:34:14 +01:00
libavformat hlsenc: expand hls_fmp4_init_filename with strftime() 2020-12-08 21:02:23 +08:00
libavresample avresample: remove deprecated attribute from the AVAudioResampleContext struct 2018-01-09 10:56:53 -03:00
libavutil hwcontext_vulkan: wait and signal semaphores when transferring to CUDA 2020-12-05 23:53:23 +01:00
libpostproc Bump minor versions after branching 4.3 2020-06-08 22:49:04 +02:00
libswresample swresample/audioconvert: Fix left shift of negative value 2020-09-30 10:50:45 +02:00
libswscale lsws/x86/yuv2rgb: Fix compilation with mmxext or ssse3 disabled. 2020-11-14 15:37:57 +01:00
presets
tests avfilter/af_earwax: fix filter behavior 2020-12-07 21:09:08 +01:00
tools tools/target_bsf_fuzzer: set bitstream filter options 2020-11-20 19:20:33 -03:00
.gitattributes fate: add SCC test 2017-01-27 17:06:42 +01:00
.gitignore tools/python: add script to convert TensorFlow model (.pb) to native model (.model) 2019-07-01 10:23:47 -03:00
.mailmap mailmap: add entry for myself 2020-07-13 11:24:04 +08:00
.travis.yml Merge commit '899ee03088d55152a48830df0899887f055da1de' 2019-03-14 15:53:16 -03:00
Changelog avformat/mspdec: Microsoft Paint (MSP) demuxer 2020-12-06 20:03:47 +11:00
configure avcodec/mss4: Don't duplicate standard JPEG tables 2020-12-08 17:51:46 +01:00
CONTRIBUTING.md Add CONTRIBUTING.md 2016-09-18 10:02:13 +01:00
COPYING.GPLv2
COPYING.GPLv3
COPYING.LGPLv2.1
COPYING.LGPLv3
CREDITS
INSTALL.md INSTALL.md: Fix Markdown formatting 2019-01-31 10:29:16 -09:00
LICENSE.md avfilter/vf_geq: Relicense to LGPL 2019-12-28 11:20:48 +01:00
MAINTAINERS avformat: add amv muxer 2020-11-09 14:58:38 +10:00
Makefile tools/enum_options: fix build and add to Makefile 2020-11-20 15:20:24 +01:00
README.md Remove the ffserver program 2018-01-06 18:31:37 +00:00
RELEASE RELEASE: We are after the 4.3 branch point, update for that 2020-06-10 00:20:24 +02:00

README.md

FFmpeg README

FFmpeg is a collection of libraries and tools to process multimedia content such as audio, video, subtitles and related metadata.

Libraries

  • libavcodec provides implementation of a wider range of codecs.
  • libavformat implements streaming protocols, container formats and basic I/O access.
  • libavutil includes hashers, decompressors and miscellaneous utility functions.
  • libavfilter provides a mean to alter decoded Audio and Video through chain of filters.
  • libavdevice provides an abstraction to access capture and playback devices.
  • libswresample implements audio mixing and resampling routines.
  • libswscale implements 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, ismindex and qt-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.