[FFmpeg-trac] #10516(ffmpeg:new): SSA/ASS PlayResX/PlayResY not used

[FFmpeg]

#10516: SSA/ASS PlayResX/PlayResY not used
------------------------------------+----------------------------------
             Reporter:  nanone      |                     Type:  defect
               Status:  new         |                 Priority:  normal
            Component:  ffmpeg      |                  Version:  5.1.3
             Keywords:  libass ass  |               Blocked By:
             Blocking:              |  Reproduced by developer:  0
Analyzed by developer:  0           |
------------------------------------+----------------------------------
 == Summary of the bug

 When hardcoding SSA/ASS subtitles into video, FFmpeg does not seem to use
 PlayResX/PlayResY properties, but instead seems to use the video
 resolution, resulting in an incorrectly scaled subtitle.

 To rule out that the underlying issue could be libass itself, I've tested
 it with VLC and hardcoding it with Handbrake, both giving the correct
 result (correct meaning before hardcoding as displayed in VLC/Aegisub;
 also see attachment {{{mkv-ffmpeg-comparison.png}}} where {{{MKV}}} is the
 correct one).

 The hypothesis that FFmpeg is scaling subtitles based on the video
 resolution instead of PlayResX/Y can be further tested by importing the
 MKV file into Aegisub, load the subtitle file, and resample to 1920x1080.
 This gives the same incorrect result as FFmpeg does and therefore
 strenghten my beliefs that FFmpeg is scaling these subtitles without
 taking into account the PlayRes properties in the subtitle file.

 == Reproduction

 Attachments include a MKV file with embedded subtitle, having PlayResX/Y
 set to 1280x720, with the video stream dimensions 1920x1080.

 {{{
 $ ffmpeg -loglevel debug -i test-video.mkv -map 0:0 -filter_complex
 subtitles='test.mkv':si=0 -c:v libx264 -crf 18 -movflags faststart test-
 video.mp4
 }}}

 Check output of {{{test-video.mp4}}} which corresponds to "FFmpeg MP4" in
 the screenshot {{{mkv-ffmpeg-comparison.png}}}.

 == FFmpeg version

 {{{
 $ ffmpeg version n5.1.3-15-g25cd95a9dc-20230810 Copyright (c) 2000-2022
 the FFmpeg developers
   built with gcc 13.1.0 (crosstool-NG 1.25.0.196_227d99d)
   configuration: --prefix=/ffbuild/prefix --pkg-config-flags=--static
 --pkg-config=pkg-config --cross-prefix=x86_64-w64-mingw32- --arch=x86_64
 --target-os=mingw32 --enable-gpl --enable-version3 --disable-debug
 --disable-w32threads --enable-pthreads --enable-iconv --enable-libxml2
 --enable-zlib --enable-libfreetype --enable-libfribidi --enable-gmp
 --enable-lzma --enable-fontconfig --enable-libvorbis --enable-opencl
 --disable-libpulse --enable-libvmaf --disable-libxcb --disable-xlib
 --enable-amf --enable-libaom --enable-libaribb24 --enable-avisynth
 --disable-chromaprint --enable-libdav1d --enable-libdavs2 --disable-
 libfdk-aac --enable-ffnvcodec --enable-cuda-llvm --enable-frei0r --enable-
 libgme --enable-libkvazaar --enable-libass --enable-libbluray --disable-
 libjxl --enable-libmp3lame --enable-libopus --enable-librist --enable-
 libssh --enable-libtheora --enable-libvpx --enable-libwebp --enable-lv2
 --disable-openal --enable-libopencore-amrnb --enable-libopencore-amrwb
 --enable-libopenh264 --enable-libopenjpeg --enable-libopenmpt --enable-
 librav1e --enable-librubberband --enable-schannel --enable-sdl2 --enable-
 libsoxr --enable-libsrt --enable-libsvtav1 --enable-libtwolame --enable-
 libuavs3d --disable-libdrm --disable-vaapi --enable-libvidstab --enable-
 vulkan --enable-libshaderc --disable-libplacebo --enable-libx264 --enable-
 libx265 --enable-libxavs2 --enable-libxvid --enable-libzimg --enable-
 libzvbi --extra-cflags=-DLIBTWOLAME_STATIC --extra-cxxflags= --extra-
 ldflags=-pthread --extra-ldexeflags= --extra-libs=-lgomp --extra-
 version=20230810
   libavutil      57. 28.100 / 57. 28.100
   libavcodec     59. 37.100 / 59. 37.100
   libavformat    59. 27.100 / 59. 27.100
   libavdevice    59.  7.100 / 59.  7.100
   libavfilter     8. 44.100 /  8. 44.100
   libswscale      6.  7.100 /  6.  7.100
   libswresample   4.  7.100 /  4.  7.100
   libpostproc    56.  6.100 / 56.  6.100
 }}}

 Also tested with versions 4.4.4, 5.1.3, 6.0 giving same results (libass
 enabled; [https://github.com/BtbN/FFmpeg-
 Builds/releases/tag/autobuild-2023-08-10-12-50 releases])
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/10516>
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