GIF encodes with wrong frame rate about libavif

[wantehchang]

@xgretsch Thank you for the bug report.

Vignesh, could you take a look at this? Thanks.

[vigneshvg]

I can reproduce this issue. Let me take a look.

[vigneshvg]

For some reason, ffmpeg is not preserving the frame rate when converting the GIF to Y4M. If you look at the ffmpeg output:

Input #0, gif, from 'bugreport.gif':
  Stream #0:0: Video: gif, bgra, 400x400, 0.50 fps, 100 tbr, 100 tbn

Output #0, yuv4mpegpipe, to 'bugreport.y4m':
  Stream #0:0: Video: wrapped_avframe, yuv420p(tv, progressive), 400x400, q=2-31, 200 kb/s, 100 fps, 100 tbn

If i force the frame rate of the y4m conversion with -r 1/2, then the avif file's rate matches that of the GIF:

ffmpeg -i bugreport.gif -pix_fmt yuv420p -f yuv4mpegpipe -r 1/2 bugreport.y4m

I will take a look and see why ffmpeg is not respecting the GIF's frame rate. In the meantime this is the workaround.

[vigneshvg]

Also, note that you can use ffmpeg to convert the GIF to AVIF directly while respecting the frame rate like so:

ffmpeg -i input.gif -fps_mode passthrough output.avif

[xgretsch]

I can confirm that your workaround does the job. Many thanks! I assume I should leave the issue open, even though this will fix it for my purposes - but feel free to say otherwise.

[vigneshvg]

Yeah, please leave it open. I will close it once i confirm why ffmpeg does not do this automatically.

[xgretsch]

Just to keep me on my toes, I now have a different file on which the "passthrough" mode does not work, but for which my original script which went via a y4m intermediate is fine.

![bugreport2](https://github.com/AOMediaCodec/libavif/assets/898362/5f1d44d7-b74f-4978-a822-2829cd8f5bc4)

[xgretsch]

Here's the output, which seems to have been lost from the previous post:

# ffmpeg -i bugreport2.gif -fps_mode passthrough bugreport2.avif
ffmpeg version 6.1.1-3ubuntu5 Copyright (c) 2000-2023 the FFmpeg developers
  built with gcc 13 (Ubuntu 13.2.0-23ubuntu3)
  configuration: --prefix=/usr --extra-version=3ubuntu5 --toolchain=hardened --libdir=/usr/lib/aarch64-linux-gnu --incdir=/usr/include/aarch64-linux-gnu --arch=arm64 --enable-gpl --disable-stripping --disable-omx --enable-gnutls --enable-libaom --enable-libass --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libcodec2 --enable-libdav1d --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libglslang --enable-libgme --enable-libgsm --enable-libharfbuzz --enable-libmp3lame --enable-libmysofa --enable-libopenjpeg --enable-libopenmpt --enable-libopus --enable-librubberband --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx265 --enable-libxml2 --enable-libxvid --enable-libzimg --enable-openal --enable-opencl --enable-opengl --disable-sndio --enable-libdc1394 --enable-libdrm --enable-libiec61883 --enable-chromaprint --enable-frei0r --enable-ladspa --enable-libbluray --enable-libjack --enable-libpulse --enable-librabbitmq --enable-librist --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libx264 --enable-libzmq --enable-libzvbi --enable-lv2 --enable-sdl2 --enable-libplacebo --enable-librav1e --enable-pocketsphinx --enable-librsvg --enable-libjxl --enable-shared
  libavutil      58. 29.100 / 58. 29.100
  libavcodec     60. 31.102 / 60. 31.102
  libavformat    60. 16.100 / 60. 16.100
  libavdevice    60.  3.100 / 60.  3.100
  libavfilter     9. 12.100 /  9. 12.100
  libswscale      7.  5.100 /  7.  5.100
  libswresample   4. 12.100 /  4. 12.100
  libpostproc    57.  3.100 / 57.  3.100
Input #0, gif, from 'bugreport2.gif':
  Metadata:
    comment         : GIF optimized with https://ezgif.com/optimize
  Duration: 00:00:10.00, start: 0.000000, bitrate: 160 kb/s
  Stream #0:0: Video: gif, bgra, 400x400, 0.50 fps, 0.50 tbr, 100 tbn
Stream mapping:
  Stream #0:0 -> #0:0 (gif (native) -> av1 (libaom-av1))
Press [q] to stop, [?] for help
[libaom-av1 @ 0xaaaae58383d0] v3.8.2
[libaom-av1 @ 0xaaaae58383d0] Neither bitrate nor constrained quality specified, using default CRF of 32
[libaom-av1 @ 0xaaaae58383d0] aom_codec_enc_cfg
[libaom-av1 @ 0xaaaae58383d0] generic settings
  g_usage:                      0
  g_threads:                    10
  g_profile:                    1
  g_w:                          400
  g_h:                          400
  g_bit_depth:                  8
  g_input_bit_depth:            8
  g_timebase:                   {2/1}
  g_error_resilient:            0
  g_pass:                       0
  g_lag_in_frames:              35
[libaom-av1 @ 0xaaaae58383d0] rate control settings
  rc_dropframe_thresh:          0
  rc_end_usage:                 3
  rc_twopass_stats_in:          (nil)(0)
  rc_target_bitrate:            256
[libaom-av1 @ 0xaaaae58383d0] quantizer settings
  rc_min_quantizer:             0
  rc_max_quantizer:             63
[libaom-av1 @ 0xaaaae58383d0] bitrate tolerance
  rc_undershoot_pct:            25
  rc_overshoot_pct:             25
[libaom-av1 @ 0xaaaae58383d0] decoder buffer model
  rc_buf_sz:                    6000
  rc_buf_initial_sz:            4000
  rc_buf_optimal_sz:            5000
[libaom-av1 @ 0xaaaae58383d0] 2 pass rate control settings
  rc_2pass_vbr_bias_pct:        50
  rc_2pass_vbr_minsection_pct:  0
  rc_2pass_vbr_maxsection_pct:  2000
[libaom-av1 @ 0xaaaae58383d0] keyframing settings
  kf_mode:                      1
  kf_min_dist:                  0
  kf_max_dist:                  9999
[libaom-av1 @ 0xaaaae58383d0] tile settings
  tile_width_count:             0
  tile_height_count:            0
[libaom-av1 @ 0xaaaae58383d0] 
[libaom-av1 @ 0xaaaae58383d0] Failed to initialize encoder: Invalid parameter
[libaom-av1 @ 0xaaaae58383d0]   Additional information: g_timebase.num out of range [1..cfg->g_timebase.den]
[vost#0:0/libaom-av1 @ 0xaaaae5838090] Error while opening encoder - maybe incorrect parameters such as bit_rate, rate, width or height.
Error while filtering: Invalid argument
[out#0/avif @ 0xaaaae5832c00] Nothing was written into output file, because at least one of its streams received no packets.
frame=    0 fps=0.0 q=0.0 Lsize=       0kB time=N/A bitrate=N/A speed=N/A    
Conversion failed!
#

[vigneshvg]

Thanks for the detailed bug report. I was able to reproduce it. This was a bug in libaom and i have sent a patch to fix it here: https://aomedia-review.googlesource.com/c/aom/+/190883

libaom currently doesn't allow an fps of < 1 and this input file has an fps of 0.5. You would have to wait for the patch to land and then update libaom to tip of tree for this to work.

[vigneshvg]

In the meantime, the workaround is to use a different AV1 encoder like rav1e with ffmpeg. This command works for me:

ffmpeg -i bugreport2.gif -fps_mode passthrough -c:v librav1e output.avif

This requires that your ffmpeg must be built with rav1e enabled as a codec.

However, this does come with quality implications. Files produced by rav1e may or may not be as good as libaom.

[wantehchang]

@xgretsch Thanks for the bug report. Vignesh's bug fix will be in the upcoming libaom v3.9.1 patch release. I will let you know when libaom v3.9.1 is available.

You are using libaom v3.8.2. Will you be able to upgrade to libaom v3.9.1 (when it is available)? Or do you need this bug fixed in a libaom v3.8.3 patch release?