artifacts on decoding PNG file with alpha

[Stefano Sabatini]

Summary of the bug:
How to reproduce:

fmpeg -i  http://ffmpeg.org/ffmpeg-logo.png out.bmp
ffmpeg version N-50039-g9cfe7a3 Copyright (c) 2000-2013 the FFmpeg developers
  built on Feb 17 2013 13:21:08 with gcc 4.6 (Ubuntu/Linaro 4.6.3-1ubuntu5)
  configuration: --enable-pic --enable-fontconfig --enable-libschroedinger --enable-libass --enable-version3 --prefix=/home/stefano --enable-libx264 --enable-libfaac --disable-shared --enable-static --enable-debug=3 --enable-pthreads --enable-libvorbis --enable-gpl --enable-nonfree --enable-libmp3lame --enable-libtheora --enable-gpl --enable-x11grab --enable-frei0r --enable-libspeex --enable-libcaca --enable-libflite --enable-libfreetype --enable-libopencv --enable-libopencore-amrnb --enable-libopencore-amrnb --disable-optimizations
  libavutil      52. 17.101 / 52. 17.101
  libavcodec     54. 92.100 / 54. 92.100
  libavformat    54. 63.100 / 54. 63.100
  libavdevice    54.  3.103 / 54.  3.103
  libavfilter     3. 38.103 /  3. 38.103
  libswscale      2.  2.100 /  2.  2.100
  libswresample   0. 17.102 /  0. 17.102
  libpostproc    52.  2.100 / 52.  2.100
Input #0, image2, from 'http://ffmpeg.org/ffmpeg-logo.png':
  Duration: 00:00:00.04, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: png, rgba, 396x100, 25 tbr, 25 tbn, 25 tbc
Output #0, image2, to 'out.bmp':
  Metadata:
    encoder         : Lavf54.63.100
    Stream #0:0: Video: bmp, bgra, 396x100, q=2-31, 200 kb/s, 90k tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (png -> bmp)
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.04 bitrate=N/A    
video:155kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.013884%

or alternatively:

ffplay http://ffmpeg.org/ffmpeg-logo.png

The output is garbled.

[Carl Eugen Hoyos]

Is there really an issue?
ImageMagick decodes exactly the same (invisible) artefacts as FFmpeg.

[Stefano Sabatini]

Replying to cehoyos:

Is there really an issue?
ImageMagick decodes exactly the same (invisible) artefacts as FFmpeg.

Indeed there isn't.

Check with:

ffplay -f lavfi "color=s=400x300:c=white[bg]; movie=ffmpeg-logo.png, [bg] overlay"

or

ffplay ffmpeg-logo.png -vf alphaextract

libswscale is stripping the alpha plane when converting from RGBA to YUV, and the display is the natural result of this operation, the only sane path would be to make SDL support alpha, but this is unrelated to FFmpeg.

Sorry for the noise.

[Carl Eugen Hoyos]

I tested with several web-browsers and they all show a white background as gimp does, so maybe there is the same bug present in ImageMagick and FFmpeg.

[Stefano Sabatini]

Replying to cehoyos:

I tested with several web-browsers and they all show a white background as gimp does, so maybe there is the same bug present in ImageMagick and FFmpeg.

As I explained this is not a bug, as there is no "correct" way to drop the alpha channel and preserve information.