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

[Robert Krüger]

On Tue, May 21, 2013 at 7:34 PM, Paul B Mahol <onemda at gmail.com> wrote:
> On 5/21/13, Robert Krueger <krueger at lesspain.de> wrote:
>> On Tue, May 21, 2013 at 6:56 PM, Paul B Mahol <onemda at gmail.com> wrote:
>>> 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.
>>
>> I saw that but then I thought the geq filter would automatically use a
>> format with alpha if an alpha expression was there, regardless of what
>> was used in the previous member of the filter chain. Then I started
>> looking for a way to force the source to have alpha. I tried
>> - using -pix_fmt rgba as an input option but got an error message
>> telling me that pixel format is an unsupported option
>> - using color=0x80808080 as the source because I hoped setting an
>> alpha value would somehow make it use a pixel format with alpha
>> support
>>
>> So how do I force alpha to be there in the source other than
>> generating a dummy video with alpha and using that as an input (which,
>> of course I can do but I was looking for the more elegant solution
>> first because I thought I must be missing some option to force a pixel
>> format)?
>
> Add format filter before geq filter, like this: -vf format=rgba,geq.....

adding format=rgba or gbrap both worked and did what I was looking
for. Thanks a lot!

Sloppy research on my part. I was searching the docs for an option to
the scale filter to specify the pixel format and did not even know
that the format filter existed.