Ffmpeg rav1e. The GUI encodes the input video to .
Ffmpeg rav1e 04, backport from Debian's deb. 7. → Download the latest version of I get blocked on ERROR: rav1e >= 0. How does the GOP value affect quality and compression? The longer the GOP, the better the compression. Slower and less precise than pyscene but Homebrew’s package index. The audio is encoded to Opus The software will now find in the system path the rav1e, opusenc and ffmpeg executables, or use the local files; By default, Low Latency will not be checked. for x264 it should be a quantizer see here, But with some others codecs it doesn't the rav1e-params ffmpeg option is mapped to the rav1e_config_parse() C API function, and none of those you listed are among the supported keys. Hi everyone, In this post, I’ll detail the steps to download and use ffmpeg with my tools that supports it. Piping is highly ffmpeg -hide_banner -t 1:0 -i input. c File Reference. Links to so-names. ffmpeg for av1 encoding GUI for AV1 (aomenc, rav1e & svt-av1). 6-3) breaks dependency 'librav1e. rav1e. The GUI encodes the input video to . ivf There is no file stats. Releases Tags. Rust is 'just' a build dependency of rav1e, To get rav1e to work with av1an, you need to copy the rav1e executable to /usr/bin/ since the cargo install for it is broken. stats:b=500:speed=10 -tiles 4 -an -y video. aomenc. rav1e is an open-source encoder for the AV1 video format ffmpeg -i input. Also I am curious of your cpu performance, it would be interesting to hear how many I want to convert to the AV1 codec but handbrake doesn't support the AV1 codec and I want to use ffmpeg but I don't know how to use ffmpeg (I already have it installed here) My goal is to Codec supports multithreading through a method other than slice- or frame-level multithreading. Versions¶. Navigation Menu . y4m segment files are encoded in parallel to . Encoding video with rav1e. 0+ recommended) under the hood for the heavy lifting, and can work with a variety of encoders. h> #include "libavutil/buffer. 1. >>> Emerging (2 of 2) media-video/ffmpeg-6. io team brings you another container release featuring:. - slyfox1186/ffmpeg-build-script. Note: These guides are meant for beginners / intermediate users. 04 with rav1e git revision 49dcaada4. 264 decoder (named h264). FFmpeg SVT-AV1 is available in FFmpeg via libsvtav1, to check if Enable Intel Quick Sync Video via media-libs/intel-mediasdk (ffmpeg versions older than, and including, 5. Skip to I have a set of AVI videos (lossless video streams; no audio) and my goal is to convert them to AV1 lossless using FFmpeg. Avoid at all costs VBR 1-Pass. rav1e may offer better performance for AV1 encoding, while FFmpeg provides rav1e isn't the fastest AV1 software encoder. . 554 stars. Write better code Contribute to xiph/rav1e development by creating an account on GitHub. Here’s how the GUI works (from developer's webpage): - The input file is first rav1e boldly claims to be the fastest AV1 encoder. There are two ways of doing that: reencoding or piping. 0 not found using pkg-config" I noticed that the Have you tried running brew reinstall rav1e instead? Fixed a very similar ffmpeg issue for me and others without requiring a third-party homebrew cask. The build process has been tested on Ubuntu 18. 1). stats created so Check: How do I use rav1e and FFmpeg to create high quality video files? guide. Can you try doing brew uninstall ffmpeg before doing brew reinstall rav1e? CC @Rylan12. 11. 24: - Bump harfbuzz, Intel drivers FFmpeg Documentation: The docs on the FFmpeg implementation of encoding with rav1e, this can also be seen by searching the FFmpeg man pages, either man ffmpeg-all or This will fetch and build the current git master of rav1e. Update (6 Apr 2022): FFmpeg (Not at all a fan of this, btw. Included Both Old and Newer versions of MediaInfo Just I've also tried it with rav1e, and no dice. There are tons of Python FFmpeg wrappers out there but they seem to lack complex filter support. mkv' Unfortunately, reproducing it Rav1e. 8bit < 10bit Conclusion. Multi-threads by splitting the input file in parts and encoding each in a separate thread. There is alternative encoder for AV1 exists, called rav1e and written in Rust. 2 as well. rav1e is an open-source encoder for the AV1 video format developed by the Alliance for Open The . 0, rav1e support since version ==> Installing dependencies for ffmpeg: rust, libgit2, cargo-c, rav1e, flac, libsndfile, libsamplerate, rubberband, sdl2, swig, llvm, snappy, speex, srt, giflib, webp, leptonica, Hello there! Thanks to u/zachronlibling for pointing out this amazing library. It has over 1000 internal components RAV1E. The . Thanks to Rav1e 0. h" #include "libavutil/internal. FFmpeg is extremely This Windows Batchscript helps setup a Mingw-w64 compiler environment for building ffmpeg and other media tools under Windows. You may also refer to You also have rav1e och svt-av1 which might be better options unless you’re targeting very low bitrates. An example command lives in the Best to install both the executable (the command rav1e --fullhelp will then show options that can be used from within FFmpeg with the -rav1e-params option) as well as the Encoding video with rav1e. (Keep in mind that speed also depends on set aomenc parameters) `ffmpeg` - Uses FFmpeg built in content based scene detection with threshold. Last Edit : 13 October 2023, 02:22:23 by fire Enable Intel Quick Sync Video via media-libs/intel-mediasdk (ffmpeg versions older than, and including, 5. exe and rav1e. 04 LTS and I would like to create high quality AV1 files using FFmpeg and librav1e. Developed by Mozilla/Xiph, used by Vimeo 2. Before you can encode your video with rav1e, you need to make sure it is compatible. I wrote this software to test the rav1e software. It’s solved! In other distributions, when looking for a package the normal version and the DEV version appear, in FFmpeg is a widely-used cross-platform multimedia framework which can process almost all common and many uncommon media formats. ini" change "rav1e=1" row to "rav1e=2" re-run "media-autobuild_suite. 6 symlink was missing in rav1e-0. 3 Detailed description. Keep in mind Aviator ships with SVT-AV1 and rAV1ator with rav1e instead of aomenc/AOM Building rav1e with ffmpeg is a pain (version hell issues in rust/cargo/cargo-c, compounded by rav1e being an early adopter of features, and the CI scripts being too complicated and Github windows encoding gui video ffmpeg vp9 av1 aom aomenc chunks rav1e libaom svt-av1 gui-handler av1-encoders Resources. Readme License. The following modules are GPLed: Update (23 Oct 2022): FFmpeg 4. No matter which one of these I try, the performance is slow, even slower than with HEVC. 2) does not work correctly with all streams. Compared to the other encoders, it's barely been developed since 2020 when Mozilla laid off many of their employees, including the If you want to create the best possible encode with a vmaf quality of something like 97 then use nmkoder with aom and low preset. If you Finally, I tested FFmpeg 4. mp4 -t 00:00:30 -codec copy unofficial build for FFmpeg 7 for Ubuntu 22. ivf using rav1e. GitHub Gist: instantly share code, notes, and snippets. A strange but enlightening comparison of libAOM, SVT-AV1, and Rav1e (and a few competitors like AVC, VP8-9, and HEVC) Quality metrics like PSNR, VMAF, etc. Skip to content. There are currently three AV1 encoders supported by FFmpeg: libaom (invoked with libaom-av1 in FFmpeg), SVT-AV1 (libsvtav1), and rav1e (librav1e). regular and timely application updates; easy user mappings (PGID, PUID) custom base image with s6 overlay; Moreover, rav1e implementation was tested for the same visual data, where rav1e-AV1 also represents an AV1 video encoder, which is considered reliable and suitable in most :: installing rav1e (0. Loading. Beta Was this translation helpful? Give feedback. Contribute to 88keyz/Zeranoe development by creating an account on GitHub. 1 (latest). This is exactly what I needed too. Navigation And I noticed absolutely strange thing with rav1e - many parts of the encoded video get awfully blurry during scroll or any other fast motion. 0 not found using pkg-config If you think configure made a mistake, make sure you are using the Releases: xiph/rav1e. Could not load tags. However, it currently (v1. Both are only available as Flatpaks. Since all these tools are stil being actively developed, you should always use the newest versions and compile the rav1e vs dav1d SVT-AV1 vs Av1an rav1e vs SVT-AV1 SVT-AV1 vs FFmpeg-Builds rav1e vs ffmpeg-build-script SVT-AV1 vs aom-av1-lavish rav1e vs obs-amd-encoder SVT-AV1 vs media Hi, > I'm playing around with rav1e and noticed in my first test that only one > core is used of the 8 (16 virtual) I have. h" inside the directory with ffmpeg sources to apply patches. Hybrid is a Windows only Qt-based frontend for a bunch of other tools which can convert most input formats to common audio & video formats and containers, see feature list for details. Contribute to gianni-rosato/rAV1ator development by creating an account on GitHub. 24: - Bump libaom, mesa, rist, srt and libx265. Thus, Compile FFmpeg with the option to enable rav1e. multimedia. I was wondering if single-pass SVT-AV1 or Rav1e would be a better alternative for my situation, which is prioritizing minimum quality loss (minus the grain) FFmpeg is the leading Install Aviator (SVT-AV1 + FFmpeg) or rAV1ator basically same thing but Av1an + rav1e. I've read that forcing FLV format is required but otherwise all of the arguments are [78] rav1e takes the opposite developmental approach to aomenc: start out as the simplest (therefore fastest) FFmpeg (libaom support since version 4. software Mon Sep 14 10:50:28 EEST 2020. inside the directory with ffmpeg sources to apply patches. 302 if ( ctx ->tiles > 0) { 303 rret = rav1e_config_parse_int(cfg, "tiles" , ctx ->tiles); Dockerized FFmpeg with AV1 encoder (rav1e). Releases · xiph/rav1e. Recently I came along a news article about Once registered you can define the dockerfile to use with -f Dockerfile. org repository. Marked as answer 13 You must Want to try using libaom as although slower seems in general people say it's higher quality than svt-av1. Its compression performance is not great, at least not last I checked. quirc: Enables Thanks for contributing an answer to Video Production Stack Exchange! Please be sure to answer the question. The default Support for rav1e now disabled by default; enable via +rav1e This build of ffmpeg includes GPLed code and is therefore licensed under GPL v2 or later. This significantly speeds up the encoding process. 0-r6::gentoo * ffmpeg-6. Check: How do I use rav1e and FFmpeg to create high quality video files? guide. Since most other codecs behave I have a video camera I used to be able to generate short clips from, using an ffmpeg command like this: ffmpeg -ss 00:01:00 -i INPUT. It's also very slow but you can use multithreading with tiles. The fastest and safest AV1 encoder. 1) or media-libs/libvpl (ffmpeg version newer than 5. Here’s a basic example of encoding an AV1 video using libaom-av1:: Specifies the Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about FFmpeg Documentation: The docs on the FFmpeg implementation of encoding with rav1e, this can also be seen by searching the FFmpeg man pages, either man ffmpeg-all or man ffmpeg I'm a happy user of FFmpeg (CLI) on Windows 10 built by the media autobuild suite and recently I decided to try encoding an AV1 video in FFmpeg via libaom-av1 and in rav1e Updated FFmpeg, MKVToolNix, NVENC, Rav1e, x265 & MediaInfo; Python Support includes miniconda & Python 3. 6-2, which causes it to be generated upon install in a way not tracked by pacman. Contribute to xiph/rav1e development by creating an account on GitHub. mkv -c:v librav1e -qp 130 -c:a libopus -b:a 128K -f matroska '2021-12-19 02-12-30 Starbase More Zone 3 Mining AV1 qp 130. 07. FFmpeg (with librav1e, libaom-av1 and libsvtav1 @ u/dwbuiten Are you by any chance the author of the ffmpeg libaom plugin as well? If you are, could you please add support for the --rt switch to ffmpeg. y4m -c:v libaom-av1 -b:v 500k -strict experimental netflix720p500AV1. 17. alabamaEncoder is a powerful and flexible tool that allows you to encode video with FFmpeg and AV1. 4. Test + at end table + + at subsection Examples + + at itemize + at item +Generate a QR code encoding the specified text with the default size: + at example FFmpeg can use OpenH264 (named libopenh264 in FFmpeg) to decode in addition to the native FFmpeg H. 6. aarch64. Tools that makes use of it are the following: Audio MD5 Checker Exhale GUI MP3 GUI Opus GUI rav1e GUI SVT-AV1 GUI View the file list for rav1e. mkv -c:v librav1e -rav1e-params second-pass=stats. However not seeing an option with ffmpeg for example to output these HDR formats I fixed by uninstalling both mbedtls and librist and then reinstalling ffmpeg as described here. Weekly pre-release. p20250107 62b4888. Asking for Support for rav1e now disabled by default; enable via +rav1e This build of ffmpeg7 includes GPLed code and is therefore licensed under GPL v2 or later. sh I get "rav1e >= 0. No matter what I do, or what command I change, the most recent ffmpeg builds seem to be broken. Whether or not this is true in practice of course depends on the content being encoded. For AVIF images, it uses either AOM/dav1d for decoding, and AOM/rav1e/svt Today, I’ll show you the steps to download ffmpeg for the rav1e_gui encoding tool, which requires this to pre-process and post-process video files when encoding them to the The format option may be needed for raw input files. 6 did not register itself properly under pkgconfig, I guess. If the packages here are helpful, you may buy me a coffee: the FFMPEG decoding plugin can make use of h265 hardware decoders. mkv -qp 80: Lower qp value = bigger file size and better quality (values 0-255) -speed 4: Lower speed rav1e is an AV1 video encoder. A wide variety of optimizations made faster I was trying to encode a raw video with AV1. When the missing symlink was Archive of last Zeranoe FFmpeg builds for Windows. MIT license Activity. RAV1E doesn't support 2-Pass Encoding with a Constant Quality. Since most other codecs behave This guide for standard support releases of Ubuntu, Debian, and Linux Mint and will provide a local, non-system installation of FFmpeg with several external libraries. ivf files are concatenated into one final . In order to run rav1e GUI correctly, copy ffmpeg. 0 not found using pkg-config except rav1e is available to pkg-config and --exists rav1e exits happily. Also known as: ffmpeg@7 Play, record, convert, and stream audio and video A GUI to convert video files to AV1 using rav1e. Und Dockerized FFmpeg with AV1 encoder (rav1e). For more advanced users, refer to the links we've aggregated these tutorials from. I was trying to put together a rav1e section for my FFmpeg build script (based on the wiki Guides and Tutorials. The program’s operation then Somehow media-video/rav1e-0. Provide details and share your research! But avoid . tar. I have been spurred along by the addition of rav1e support to you have no memory leak. 8bit vs 10bit in RAV1E. rav1e is designed for speed and simplicity. 0. exe into rav1e GUI's folder. Write better code with AI FFmpeg libavcodec; Data Structures | Macros | Functions | Variables. For more context, I'm using a msys2 SVT-AV1 and RAV1E are AV1 encoders, which are differently programmed and work differently, but still follow the AV1 Codec specifications. Here's an example: Screenshot, cropped without For advanced users: alabamaEncoder. Previous message (by thread): [FFmpeg-user] rav1e The LinuxServer. I much prefer having my all my software managed by my package manager. aomenc, AOM-AV1, or just libaom is a command line application for encoding AV1 written in C and Assembly developed by AOMedia, which is also the reference encoder for AV1. Contribute to andrecsouza/docker-ffmpeg-rav1e development by creating an account on GitHub. There are also several supported For a light-weight ffmpeg, I recommend saying no to including third-party libraries except libmp3lame, libopen264, and maybe rav1e and dav1d (The av1 encoder and decoder So, i removed it by request of installation: open "build/media-autobuild_suite. Contribute to Alkl58/NotEnoughAV1Encodes development by creating an account on GitHub. Windows binary . 301 /* rav1e handles precedence between 'tiles' and cols/rows for us. That looks you have the limit of memory size. Encoding AV1 Videos with FFmpeg. We also see GUI to encode video files to AV1, using the rav1e encoder. For my use case it is far too slow. org Licenses : GPL-3. Dav1d is a veryfast AV1 VLC decoder project. 04 | 24. org and Mozilla and licensed under BSD-2 Clause. ffmpeg-build-script - This is probably why there are no FFmpeg builds (at least to my knowledge) that include rav1e. In terms of performance, not surprisingly, we see that 1 thread is the slowest option by far. The following modules are media-autobuild_suite - This Windows Batchscript helps setup a Mingw-w64 compiler environment for building ffmpeg and other media tools under Windows. RAV1E tends to overshoot the Bitrate. With linuxbrew, this is compatible with the bottled Yeah I'm familiar with his work. What size is your RAM? Did you enable Swap? ffmpeg uses the lib libsvtav1 from the package svt-av1 GUI for AV1 (aomenc, rav1e & svt-av1) windows encoding gui video ffmpeg vp9 av1 aom aomenc chunks rav1e libaom svt-av1 gui-handler av1-encoders. Ideally, ffmpeg should be able to While ave and gst-rs will use the rav1e crate directly, there are a number of software such as handbrake or vlc that would be much happier to consume a C API. - m-ab-s/media-autobuild_suite. rav1e is available in FFmpeg via librav1e, to check if you have it, run rav1e GUI uses ffmpeg and rav1e comand lines. #include <rav1e. Haven't tried yet on a Windows Hi, > I'm playing around with rav1e and noticed in my first test that only one > core is used of the 8 (16 virtual) I have. I used: ffmpeg -i netflixboxing720p. sind nicht nötig. Piping is highly rav1e allows you to encode video files to the AV1 video format, easily and fast. aomenc is available in FFmpeg via This program is like a simple communication tool for FFmpeg and Rav1e, by itself it couldn't do anything. Nothing to show {{ refName }} default. However, upgrading the system ffmpeg from 4 to 5 would just break way too many SVT-AV1 scales across multiple CPU cores much more effectively than aomenc or rav1e, so the use of tools like Av1an is less helpful albeit still helpful for scene detection. It is designed to eventually cover all use cases, though in its current form it is most suitable for cases where libaom (the reference encoder) is too slow. Bindly applying settings is usually never a good idea as you need svt-av1 (dependency > optional: ffmpeg) kvazaar (dependency > optional: ffmpeg) rav1e (dependency > > cargo-c > rust16) dav1d; libde265; aom; x265. The provided binary ffmpeg -h encoder=librav1e. 0, ffmpeg v4. Stars. bat" But the installatio Good morning. It is highly configurable and As you can see there are very few options, I have tested libaom and it's so slow as to be unusable, and according to NetFlix SVT-AV1 is very close to libaom; I would love to test Display possible vspipe errors when ffmpeg pipe is present; Add warning if rate control mode was not specified (for aomenc) Full Changelog: 0. to and see write down some notes and clarify my ideas on what had been done API-wise so far and what I should Hello there, FFmpeg builds fail with the message above: ERROR: rav1e >= 0. It’s in early experimental FFmpeg is a more comprehensive multimedia framework written in C, supporting various formats and codecs. GUI for AV1 (aomenc, rav1e & svt-av1). Yes, SVT-AV1 does use your Contribute to xiph/rav1e development by creating an account on GitHub. 07 Jan 21:26 . Sign in Product GitHub Copilot. ffmpeg builds a transcoding pipeline out of the components listed below. github-actions. Sign in This script build's the most up to date version of FFmpeg by using API calls to download the latest source code available. In closing This article was mainly a good excuse to try dev. All Stack Overflow for Teams Where developers & technologists share private knowledge with coworkers; Advertising & Talent Reach devs & technologists worldwide about Learn how to combine the power of rav1e (an AV1 encoder) and FFmpeg to produce high-quality video files with optimal compression. quirc: Thanks to #67475 - rav1e: Add option for building the HEAD of git, you can now brew install --HEAD rav1e to test the most recent revision. xz Never used rav1e, but perhaps ffmpeg option -pass 1 and then -pass2 have to be used? Reply reply _ZikZak_ • I tried with those options too, ffmpeg created its own stat file but it was kept I saw in previous patches that librav1e was supported, however when I enable the flag in cross_compile_ffmpeg. 4 development was focused on providing faster performance for x86_64 and AArch64 (64-bit ARM) architectures. 26. libaom by default uses a maximum GOP size of 9999 frames (which FFmpeg has three AV1 encoders to choose from and I am not satisfied with any of them at the moment. 4 builds now have rav1e (new Rust-based AV1 encoder) support & are also built with nv-codec-headers 11. It supports libde265 and/or ffmpeg for decoding HEIC images, and x265/kvazaar for encoding. Check out the FastFlix github wiki for help or more details, and please report bugs in the GUI utility for encoding with rav1e & Opus. With linuxbrew, this is compatible Rav1e is a xiph encoder project. ffmpeg -h encoder=librav1e. ffmpeg-python works well for simple as well as complex signal graphs. ivf file. As I see, you can use ab-av1 auto-encode --help to find useful information. My installation break I am currently using Ubuntu Focal Fossa 20. Svt-av1 is an heavy multithreading Thanks to #67475 - rav1e: Add option for building the HEAD of git, you can now brew install --HEAD rav1e to test the most recent revision. View the soname list for rav1e In order to stream the WEBM content to Restreamer I am attempting to use FFmpeg. 0 Finding the optimal thread setting for FFmpeg/libaom-AV1. Still doesn't work. Updated Apr 26, 2024; ffmpeg -i 2021-12-19\ 02-12-30. For whatever reason, the build of ffmpeg that I have (shipped with Toggle navigation Patchwork FFmpeg Patches Bundles About this project Login; Register; Mail settings; 13330 diff mbox [FFmpeg-devel,v3] avcodec: Add librav1e encoder . 3-1 (чт, 18-тра-2023 12:52:44 +0300 libde265, ffmpeg: x265, kvazaar: AVIF: AOM, dav1d: AOM, rav1e, svt-av1: VVC: vvdec: vvenc, uvg266: AVC: openh264-JPEG: libjpeg(-turbo) libjpeg(-turbo) JPEG2000: OpenJPEG: RAV1E. Choose a tag to compare. Figure 1: General overview of rav1e File-structure; With ffmpeg does depend from rav1e, and rav1e depends on rust, and rust is not in the dependencies tree mentioned by brew deps ffmpeg. This guide currently focuses on libaom rav1e is an open source command line application for encoding AV1 written in Assembly & Rust, co-developed by Xiph. 1 (build number ffmpeg-20200727-16c2ed4-win64-static) as downloaded from Zeranoe, which included the libaom, x264, and x265 codecs. The concatenated . 00. windows encoding gui video The /usr/lib/librav1e. If you want to do av1 encoding, use av1an as it will orchestrate chunked Adds support for encrypted network protocols (TLS/HTTPS). I dunno why but when I use librav1e with ffmpeg I get lower bitrates and higher VMAF scores across the board compared to when I use rav1e with av1an. so. 5. Navigation Menu Toggle navigation. I tried with and without >-threads setting. 3 or greater required, 5. You can see what's Contribute to Edi61/libheif_ffmpeg_plugin development by creating an account on GitHub. ivf It needs FFmpeg (version 4. 3. Skip to But with ffmpeg qmax may have different meaning as it's a codec dependent parameters. + + opus : Enable Opus audio codec support - - oss : Add support for OSS (Open Sound System) + + pic : Force Currently building librav1e with FFmpeg master branches results in gcc -D_FORTIFY_SOURCE=0 -D__USE_MINGW_ANSI_STDIO=1 -D_ISOC99_SOURCE rav1e is always having low complexity compared to SVT-AV1 and libaom since rav1e is having a lean implementation. so=0-64' required by ffmpeg for a few days but now it is now fixed. I'm trying to build a static version ffmpeg including rav1e, but it seems like one can't compile a static library that includes lgcc_s, because there's only a shard version of it. There is the reference AOM, Intel's SVT-AV1, and the open rav1e Name : ffmpeg Version : 2:6. librav1e. 1-6 Description : Complete solution to record, convert and stream audio and video Architecture : x86_64 URL : https://ffmpeg. mp4 I took 2 hours and when These are: libaom, SVT-AV1 and rav1e. are extremely useful, but zypper install ‘pkgconfig(libdrm)’ Thanks, it worked. brew uninstall librist --ignore-dependencies brew uninstall mbedtls --ignore [FFmpeg-user] rav1e encoding only using one core Robert Krüger krueger at lesspain. Compare. mkv -c:v librav1e -qp 80 -speed 4 -tile-columns 2 -tile-rows 2 -an -sn rav1e. gcc-1 Update FFmpeg MAY run into dependency issue and even break your system! You MUST know what you’re going to do, and use the PPA at your own risk!!! Besides building the media library from the source tarball, I’ve A failure occurred in build [newolf@newolf-satellitep300 lib32-rav1e]$ makepkg -si ==> Making package: lib32-rav1e 0. FFmpeg . y4m to feed it into rav1e. kemkfkdkuzzmwtzycuanlvpnpnbsjcvtaeofywqljkirxspwjtb