Drawtext+Alphamerge results in incorrect alpha value

[jnvsor]

I was trying to create a video of a timer for use in other videos with a transparent background. There doesn't seem to be a transparent video source so I used alphamerge to achieve the same effect.

Unfortunately, drawtext with alphamerge results in a max alpha of 235 not 255.

Running the following 3 commands generates the 3 attatched files. Note that in alpha.png the text has 235 alpha, not 255, and that I can't reproduce without using drawtext directly.

# Drawtext to alphamerge
ffmpeg -f lavfi -r 30 -t 10 -i "color=c=#FFFFFF" -f lavfi -r 30 -t 10 -i "color=c=#000000" -filter_complex "[1]drawtext=text="FFmpeg":fontcolor=white:fontsize=150:fontfile=/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf[text];[0][text]alphamerge[out]" -map "[out]" -vframes 1 -c:v png -pix_fmt rgba alpha.png

# Saving the drawtext mask on it's own
ffmpeg -f lavfi -r 30 -t 10 -i "color=c=#FFFFFF" -f lavfi -r 30 -t 10 -i "color=c=#000000" -filter_complex "[1]drawtext=text="FFmpeg":fontcolor=white:fontsize=150:fontfile=/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf[out]" -map "[out]" -vframes 1 -c:v png -pix_fmt rgba noalpha.png

# And applying that mask (No bug here)
ffmpeg -f lavfi -r 30 -t 10 -i "color=c=#FFFFFF" -i noalpha.png -filter_complex "[0][1]alphamerge[out]" -map "[out]" -vframes 1 -c:v png -pix_fmt rgba third.png

[jnvsor]

Alpha of text is 235 not 255

[jnvsor]

The mask in itself shows color value is 255

[jnvsor]

Applying the mask separately works

[Carl Eugen Hoyos]

Please provide your failing command line together with the complete, uncut console output to make this a valid ticket.

[jnvsor]

$ ffmpeg -f lavfi -r 30 -t 10 -i "color=c=#FFFFFF" -f lavfi -r 30 -t 10 -i "color=c=#000000" -filter_complex "[1]drawtext=text="FFmpeg":fontcolor=white:fontsize=150:fontfile=/usr/share/fonts/truetype/dejavu/DejaVuSansMono.ttf[text];[0][text]alphamerge[out]" -map "[out]" -vframes 1 -c:v png -pix_fmt rgba alpha.png
ffmpeg version N-59884-gfd334b9 Copyright (c) 2000-2014 the FFmpeg developers
  built on Jan 18 2014 02:17:50 with gcc 4.8 (Debian 4.8.2-12)
  configuration: --arch=amd64 --enable-pthreads --enable-libopencv --enable-librtmp --enable-libopenjpeg --enable-libopus --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-vaapi --enable-runtime-cpudetect --enable-libvorbis --enable-zlib --enable-swscale --enable-libcdio --enable-bzlib --enable-libdc1394 --enable-frei0r --enable-gnutls --enable-libgsm --enable-libmp3lame --enable-libpulse --enable-vdpau --enable-libvpx --enable-gpl --enable-x11grab --enable-libx264 --enable-libfreetype
  libavutil      52. 62.100 / 52. 62.100
  libavcodec     55. 48.101 / 55. 48.101
  libavformat    55. 23.103 / 55. 23.103
  libavdevice    55.  5.102 / 55.  5.102
  libavfilter     4.  1.100 /  4.  1.100
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Input #0, lavfi, from 'color=c=#FFFFFF':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
Input #1, lavfi, from 'color=c=#000000':
  Duration: N/A, start: 0.000000, bitrate: N/A
    Stream #1:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 320x240 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
File 'alpha.png' already exists. Overwrite ? [y/N] y
Output #0, image2, to 'alpha.png':
  Metadata:
    encoder         : Lavf55.23.103
    Stream #0:0: Video: png, rgba, 320x240 [SAR 1:1 DAR 4:3], q=2-31, 200 kb/s, 90k tbn, 30 tbc (default)
Stream mapping:
  Stream #0:0 (rawvideo) -> alphamerge:main
  Stream #1:0 (rawvideo) -> drawtext
  alphamerge -> Stream #0:0 (png)
Press [q] to stop, [?] for help
frame=    1 fps=0.0 q=0.0 Lsize=N/A time=00:00:00.03 bitrate=N/A    
video:3kB audio:0kB subtitle:0 global headers:0kB muxing overhead -100.641774%

[Elon Musk]

Forcing rgba as format after drawtext filter fixes issue.