Automatically enable alpha channel pixel format on fade with alpha=1

[slhck]

When you fade in, say, an image as an overlay, and that image has no alpha channel, it will start fading in from black, even when you set alpha=1 in the fade filter.

$ ffmpeg -i big_buck_bunny_480p_surround-fix.avi -loop 1 -i logo.png \
-filter_complex "[1:v] fade=t=in:st=3:alpha=1 [logo]; \
[0:v][logo] overlay [outv]" \
-map "[outv]" -map 0:a -pix_fmt yuv420p -f matroska - | ffplay -

ffmpeg version 2.5.3 Copyright (c) 2000-2015 the FFmpeg developers
  built on Feb 12 2015 19:55:48 with Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/2.5.3 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libfontconfig --enable-libfreetype --enable-libtheora --enable-libvorbis --enable-libvpx --enable-librtmp --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libfaac --enable-libass --enable-ffplay --enable-libspeex --enable-libschroedinger --enable-libfdk-aac --enable-openssl --enable-libopus --enable-frei0r --enable-libcaca --enable-libsoxr --enable-libquvi --enable-libvidstab --enable-libx265 --enable-libopenjpeg --disable-decoder=jpeg2000 --extra-cflags='-I/usr/local/Cellar/openjpeg/1.5.1_1/include/openjpeg-1.5 ' --enable-nonfree --enable-vda
  libavutil      54. 15.100 / 54. 15.100
  libavcodec     56. 13.100 / 56. 13.100
  libavformat    56. 15.102 / 56. 15.102
  libavdevice    56.  3.100 / 56.  3.100
  libavfilter     5.  2.103 /  5.  2.103
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
Input #0, avi, from 'big_buck_bunny_480p_surround-fix.avi':
  Duration: 00:09:56.46, start: 0.000000, bitrate: 2957 kb/s
    Stream #0:0: Video: mpeg4 (Simple Profile) (FMP4 / 0x34504D46), yuv420p, 854x480 [SAR 1:1 DAR 427:240], 2500 kb/s, 24 fps, 24 tbr, 24 tbn, 24 tbc
    Stream #0:1: Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, 5.1(side), fltp, 448 kb/s
ffplay version 2.5.3 Copyright (c) 2003-2015 the FFmpeg developers
  built on Feb 12 2015 19:55:48 with Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM 3.5svn)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/2.5.3 --enable-shared --enable-pthreads --enable-gpl --enable-version3 --enable-hardcoded-tables --enable-avresample --cc=clang --host-cflags= --host-ldflags= --enable-libx264 --enable-libmp3lame --enable-libvo-aacenc --enable-libxvid --enable-libfontconfig --enable-libfreetype --enable-libtheora --enable-libvorbis --enable-libvpx --enable-librtmp --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libfaac --enable-libass --enable-ffplay --enable-libspeex --enable-libschroedinger --enable-libfdk-aac --enable-openssl --enable-libopus --enable-frei0r --enable-libcaca --enable-libsoxr --enable-libquvi --enable-libvidstab --enable-libx265 --enable-libopenjpeg --disable-decoder=jpeg2000 --extra-cflags='-I/usr/local/Cellar/openjpeg/1.5.1_1/include/openjpeg-1.5 ' --enable-nonfree --enable-vda
Input #1, png_pipe, from 'logo.png':
  Duration: N/A, bitrate: N/A
    Stream #1:0: Video: png, rgb24, 400x300 [SAR 11811:11811 DAR 4:3],   libavutil      54. 15.100 / 54. 15.100
25 fps,   libavcodec     56. 13.100 / 56. 13.100
25 tbr, 25 tbn,   libavformat    56. 15.102 / 56. 15.102
25 tbc
  libavdevice    56.  3.100 / 56.  3.100
  libavfilter     5.  2.103 /  5.  2.103
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  1.100 /  1.  1.100
  libpostproc    53.  3.100 / 53.  3.100
[libx264 @ 0x7fa83d009200] using SAR=1/1
[libx264 @ 0x7fa83d009200] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX
[libx264 @ 0x7fa83d009200] profile High, level 3.0
[libx264 @ 0x7fa83d009200] 264 - core 142 r2495 6a301b6 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 lookahead_threads=2 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=24 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, matroska, to 'pipe:':   0KB vq=    0KB sq=    0B f=0/0
  Metadata:
    encoder         : Lavf56.15.102
    Stream #0:0: Video: h264 (libx264) (H264 / 0x34363248), yuv420p, 854x480 [SAR 1:1 DAR 427:240], q=-1--1, 24 fps, 1k tbn, 24 tbc (default)
    Metadata:
      encoder         : Lavc56.13.100 libx264
    Stream #0:1: Audio: vorbis (libvorbis) (oV[0][0] / 0x566F), 48000 Hz, 5.1(side), fltp
    Metadata:
      encoder         : Lavc56.13.100 libvorbis
Stream mapping:
  Stream #0:0 (mpeg4) -> overlay:main (graph 0)
  Stream #1:0 (png) -> fade (graph 0)
  overlay (graph 0) -> Stream #0:0 (libx264)
  Stream #0:1 -> #0:1 (ac3 (native) -> vorbis (libvorbis))
Press [q] to stop, [?] for help

The only workaround to have the image fade in from transparent (which is what everyone would usually like to do) is to add the format=pix_fmts=yuva420p filter before the fade in.

As this filter also works when the original image contains an alpha channel, wouldn't it make sense to change the fade filter in such a way that it automatically fades from transparent when its alpha option is enabled? The defaulting to black does not make sense when the user wants alpha channel fading (i.e. "transparency fading").

[Michael Niedermayer]

Fixed in 03637762d94cdd47b06c6311c8116edf9de73256