225de53c9d
DCT-related FFmpeg functions often add an unsigned 8-bit sample to a
signed 16-bit coefficient, then clip the result back to an unsigned
8-bit value. RISC-V has no signed 16-bit to unsigned 8-bit clip, so
instead our most common sequence is:
VWADDU.WV
set SEW to 16 bits
VMAX.VV zero # clip negative values to 0
set SEW to 8 bits
VNCLIPU.WI # clip values over 255 to 255 and narrow
Here we use a different sequence which does not require toggling the
vector type. This assumes that the wide addend vector is biased by
-128:
VWADDU.WV
VNCLIP.WI # clip values to signed 8-bit and narrow
VXOR.VX 0x80 # flip sign bit (convert signed to unsigned)
Also the VMAX is effectively replaced by a VXOR of half-width. In this
function, this comes for free as we anyway add a constant to the wide
vector in the prologue.
On C908, this has no observable effects. On X60, this improves
microbenchmarks by about 20%.
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.