so what the title says. I've never used ffmpeg before but I really need to extract an alpha mask for a project. The problem is a grey color which should be white so when I merge it with the main video there is transparency where there shouldn't be.
I used the command
ffmpeg -i angry.mov -filter:v alphaextract mask.mov
and
ffmpeg -i angry.mov -vf alphaextract,format=yuv420p 123.mov
is there any other way to do it? I did the original mov file from png images.
Thanks
We may use lutyuv filter, but it may be too complicated if your never used FFmpeg before.
Example for converting all gray values in the alpha channel to white:
ffmpeg -i angry.mov -vf alphaextract,lutyuv="y=(val-(maxval-1))*maxval" -pix_fmt gray 123.mov
Example:
Input image: peppers_alpha.png:
As you can see there are semi-transparent pixels.
(You may download the above image for executing the code samples below).
Extracting alpha channel using alphaextract filter:
ffmpeg -y -i peppers_alpha.png -vf alphaextract -pix_fmt gray alpha.png
Result:
As you can see the semi-transparent pixels turned to gray levels (and the opaque pixels are zeros).
Turning the semi-transparent pixels to black pixels using alphaextract and lutyuv filters:
ffmpeg -y -i peppers_alpha.png -vf alphaextract,lutyuv="y=(val-(maxval-1))*maxval" -pix_fmt gray new_alpha.png
Result:
As you can see, the semi-transparent pixels turned to black.
Note: maxval = 255 for 8 bits images.
We may adjust adjust the threshold by subtracting 50 instead of 1 (for example):
ffmpeg -y -i peppers_alpha.png -vf alphaextract,lutyuv="y=(val-(255-50))*255" -pix_fmt gray new_alpha.png
Result:
Why y=(val-254)*255 sets pixels in range [0, 254] to 0?
Explanation:
All the mathematical operations are applied with clipping to [0, 255].
Values below 0 are clipped to 0 and values above 255 are clipped to 255.
val-254 is 0 (after clipping) for all values except 255 (for values in range [0, 254]).
So for val in [0, 254] the result is:
0*255 = 0
And for val = 255 the result is:
(255-254)*255 = 255
