Fill alpha channel with a constant?

[rling]

Hi, is there a quick way to fill an image's alpha channel with a constant? -fx and -recolor are overkill and are quite slow. i would like to do what -dissolve does but 'convert' doesnt support it. thanks

[rling]

I've just noticed there is a -compose operator, copy-opacity, which might work. I would have to create a canvas using xc:, with a solid color and the required opacity and the size of the image whose alpha channel I want to set, and composite it onto the image with copy-opacity. The downside is I need to know the size of the image in advance which is really a pain... oh well back to the documentation

[anthony]

Easy way is to just set all that channels values...

Code: Select all

    -channel A -evaluate set 50% +channel

will replace all alpha/matte channel values with a 50% transparency.