[FFmpeg-user] Alpha not working as expected with geq filter

[Paul B Mahol]

On 5/21/13, Robert Krueger <krueger at lesspain.de> wrote:
> Hi,
>
> I am trying to generate a test file using nullsrc and the geq filter
> to obtain an image that has color and alpha channel values defined by
> equations like so:
>
> ffmpeg -f lavfi -i nullsrc=s=256x256:d=0.01 -vf
> scale,geq='r=255*(X/W):g=255*(Y/H):b=255*((X/W)*(Y/H)):alpha_expr=255*(1-Y/H)'
> -pix_fmt rgba -y ~/tmp/generated-sd-alpha.png
>
> The generated png file does not contain any transparent pixels as far
> as I can see and I wonder what I have done wrong. I found trac ticket
> #2520 which has some similarities but it is marked as fixed. Full
> command line and output:
>
> ffmpeg -f lavfi -i nullsrc=s=256x256:d=0.01 -vf
> scale,geq='r=255*(X/W):g=255*(Y/H):b=255*((X/W)*(Y/H)):alpha_expr=255*(1-Y/H)'
> -pix_fmt rgba -y ~/tmp/generated-sd-alpha.png
> ffmpeg version N-53322-gb1bbd71 Copyright (c) 2000-2013 the FFmpeg
> developers
>   built on May 21 2013 16:18:03 with llvm-gcc 4.2.1 (LLVM build 2336.11.00)
>   configuration: --enable-gpl
>   libavutil      52. 33.100 / 52. 33.100
>   libavcodec     55. 10.101 / 55. 10.101
>   libavformat    55.  7.100 / 55.  7.100
>   libavdevice    55.  1.100 / 55.  1.100
>   libavfilter     3. 68.101 /  3. 68.101
>   libswscale      2.  3.100 /  2.  3.100
>   libswresample   0. 17.102 /  0. 17.102
>   libpostproc    52.  3.100 / 52.  3.100
> Input #0, lavfi, from 'nullsrc=s=256x256:d=0.01':
>   Duration: N/A, start: 0.000000, bitrate: N/A
>     Stream #0:0: Video: rawvideo (I420 / 0x30323449), yuv420p, 256x256

Input does not have any alpha, thus alpa can't be changed.

> [SAR 1:1 DAR 1:1], 25 tbr, 25 tbn, 25 tbc
> [swscaler @ 0x7f9438803e00] No accelerated colorspace conversion found
> from yuv420p to gbrp.
> [swscaler @ 0x7f943881dc00] No accelerated colorspace conversion found
> from yuv420p to gbrp.
> [swscaler @ 0x7f9438825400] No accelerated colorspace conversion found
> from yuv420p to gbrp.
> Output #0, image2, to '/Users/krueger/tmp/generated-sd-alpha.png':
>   Metadata:
>     encoder         : Lavf55.7.100
>     Stream #0:0: Video: png, rgba, 256x256 [SAR 1:1 DAR 1:1], q=2-31,
> 200 kb/s, 90k tbn, 25 tbc
> Stream mapping:
>   Stream #0:0 -> #0:0 (rawvideo -> png)
> 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:145kB audio:0kB subtitle:0 global headers:0kB muxing overhead
> -100.014851%
>
> Thanks in advance,
>
> Robert
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