October 2025 - ffmpeg-user - lists.ffmpeg.org

Transcode DTS-WAV 6-channel files to MP3 2-channel
by Ulf Zibis 08 Oct '25

08 Oct '25

Hi, yes, i can simply transcode such files to MP3 with FFMPEG, but my question is, how this is done in deep. Does FFMPEG simply extract the left and right front channels or is there some calculation made from all 6 channels? I can imagine, that e.g. the low frequencies are under-represented in the DTS left and right front channels in favour to the sub woofer channel, so there may be a loss when only using them for transcoding to MP3. Also I have the subjective impression, that after normal transcoding, the resulting MP3 has slight less quality, even when I use -q1 quality. I'm wondering that I can hear the difference with my 65 years old ears, because I don't recognize a quality constraint even with some 128 kb/s CBR MP3 files. Are there some settings to optimize this transcoding? -Ulf

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zscale
by Mark Filipak 08 Oct '25

08 Oct '25

Does zscale actually do color conversions or does it just change the color tags? Thanks -- Mark.

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Issue with variable FPS input over image2pipe – zero-sized output in one approach
by Palaash Jain 03 Oct '25

03 Oct '25

Hi, I’m seeing inconsistent behavior in FFmpeg when feeding frames over image2pipe from a sender that transmits raw JPEG bytes at variable FPS (frames are only sent when changes occur, so the input rate is not constant). I’ve tried two different approaches on the receiver side: --------------------------------------------------------------- Approach 1 (works) On the receiver, I maintain a buffer and duplicate the last frame to enforce a constant FPS before passing frames to FFmpeg. Command line: > -f image2pipe -framerate {targetFPS} -i - \ > -vf fps={targetFPS} -vsync cfr \ > -c:v libx264 -pix_fmt yuv420p -crf 32 -preset ultrafast -tune zerolatency \ > -g {targetFPS*2} -b:v 2M -maxrate 2M -bufsize 4M \ > -movflags frag_keyframe+empty_moov+default_base_moof \ > -f mp4 -flush_packets 1 pipe:1 This works fine: - The receiver outputs data immediately over HTTP and writes to disk. - Files written on exit are valid. --------------------------------------------------------------- Approach 2 (fails) Here I pass frames directly as they arrive (variable FPS, no duplication logic). Command line: > -f image2pipe -vcodec mjpeg -framerate {TARGET_FPS} -i - \ > -vcodec libx264 -pix_fmt yuv420p \ > -preset ultrafast -crf 23 \ > -r {TARGET_FPS} \-y {outputFile} In this case: - On process exit, the file is created but is 0 KB in size. > -f image2pipe -vcodec mjpeg -framerate {TARGET_FPS} -i - \ > -vcodec libx264 -pix_fmt yuv420p \ > -preset ultrafast -tune zerolatency -crf 23 \ > -r {TARGET_FPS} \ > -g {TARGET_FPS*2} -sc_threshold 0 \ > -movflags frag_keyframe+empty_moov+default_base_moof \ > -f mp4 pipe:1 - Using above config to stream over HTTP, my C# code creates a new FFmpeg process, passes the frames and sends the final output over HTTP to receiver. --------------------------------------------------------------- Question Why does the second approach fail to produce any output when the input arrives at variable FPS? Is there a recommended way to handle sparse/irregular frame arrival over image2pipe without manually duplicating frames in my application? > AIM: Sender sends raw JPEG bytes to my receiver, my receiver must just do the work of passing them to ffmpeg, ffmpeg must do all the processing, keeping the video output natural at constant FPS and then stream the final video bytes that ffmpeg produces over a network (e.g., using HTTP POST). What are the correct command line options for this? Thanks, Palaash Jain

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Why does ffmpeg break into a subtitle line?
by Jason Cho 02 Oct '25

02 Oct '25

See the console session. I have 1.srt and ask ffmpeg to extract -ss 0:00:0 -to 0:00:30. The tricky part is that, the subtitle file has a line spanning 00:00:00,400 --> 00:01:15,300. I expect ffmpeg to give me a subtitle 00:00:00,400 --> 0:00:30. But it actually gives the original full subtitle. Is it expected or a bug? Can ffmpeg break into a subtitle line? ``` $ cat 1.srt 00:00:00,400 --> 00:01:15,300 This is an example of a subtitle. $ ffmpeg.exe -i 1.srt -ss 0:00:0 -to 0:00:30 -c srt output.srtffmpeg version 2025-10-01-git-1a02412170-full_build-www.gyan.dev Copyright (c) 2000-2025 the FFmpeg developers built with gcc 15.2.0 (Rev8, Built by MSYS2 project) configuration: --enable-gpl --enable-version3 --enable-static --disable-w32threads --disable-autodetect --enable-fontconfig --enable-iconv --enable-gnutls --enable-lcms2 --enable-libxml2 --enable-gmp --enable-bzlib --enable-lzma --enable-libsnappy --enable-zlib --enable-librist --enable-libsrt --enable-libssh --enable-libzmq --enable-avisynth --enable-libbluray --enable-libcaca --enable-libdvdnav --enable-libdvdread --enable-sdl2 --enable-libaribb24 --enable-libaribcaption --enable-libdav1d --enable-libdavs2 --enable-libopenjpeg --enable-libquirc --enable-libuavs3d --enable-libxevd --enable-libzvbi --enable-liboapv --enable-libqrencode --enable-librav1e --enable-libsvtav1 --enable-libvvenc --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs2 --enable-libxeve --enable-libxvid --enable-libaom --enable-libjxl --enable-libvpx --enable-mediafoundation --enable-libass --enable-frei0r --enable-libfreetype --enable-libfribidi --enable-libharfbuzz --enable-liblensfun --enable-libvidstab --enable-libvmaf --enable-libzimg --enable-amf --enable-cuda-llvm --enable-cuvid --enable-dxva2 --enable-d3d11va --enable-d3d12va --enable-ffnvcodec --enable-libvpl --enable-nvdec --enable-nvenc --enable-vaapi --enable-libshaderc --enable-vulkan --enable-libplacebo --enable-opencl --enable-libcdio --enable-openal --enable-libgme --enable-libmodplug --enable-libopenmpt --enable-libopencore-amrwb --enable-libmp3lame --enable-libshine --enable-libtheora --enable-libtwolame --enable-libvo-amrwbenc --enable-libcodec2 --enable-libilbc --enable-libgsm --enable-liblc3 --enable-libopencore-amrnb --enable-libopus --enable-libspeex --enable-libvorbis --enable-ladspa --enable-libbs2b --enable-libflite --enable-libmysofa --enable-librubberband --enable-libsoxr --enable-chromaprint --enable-whisper libavutil 60. 13.100 / 60. 13.100 libavcodec 62. 16.100 / 62. 16.100 libavformat 62. 6.100 / 62. 6.100 libavdevice 62. 2.100 / 62. 2.100 libavfilter 11. 9.100 / 11. 9.100 libswscale 9. 3.100 / 9. 3.100 libswresample 6. 2.100 / 6. 2.100 Input #0, srt, from '1.srt': Duration: N/A, bitrate: N/A Stream #0:0: Subtitle: subrip (srt) Stream mapping: Stream #0:0 -> #0:0 (subrip (srt) -> subrip (srt)) Press [q] to stop, [?] for help Output #0, srt, to 'output.srt': Metadata: encoder : Lavf62.6.100 Stream #0:0: Subtitle: subrip Metadata: encoder : Lavc62.16.100 srt [out#0/srt @ 0000027a4ed681c0] video:0KiB audio:0KiB subtitle:0KiB other streams:0KiB global headers:0KiB muxing overhead: 103.030303% size= 0KiB time=00:01:15.30 bitrate= 0.0kbits/s speed=1.04e+04x elapsed=0:00:00.00 $ cat output.srt1 00:00:00,400 --> 00:01:15,300 This is an example of a subtitle. ```

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