Legacy ImageMagick Discussions Archive

And, of course, you can multiply instead of divide:
convert logo.png -alpha on -channel A -evaluate multiply 0.25 +channel result.png

This is the solution!
Works perfect, and instead of Divide you use Multiply! Clever, thank you!
That allow me to use 0-1 values for my level of transparency ...

Post your original image.

You need to turn on all the channels after selecting to process only the alpha channel.

Try

convert image -alpha on -channel a -evaluate set 30% +channel result

or

convert image -alpha on -background black -alpha background -channel a -evaluate set 30% +channel result ...

Thanks for the help guys!
I will try that out.

Best,

Hi all,

I need to change the opacity or alpha of my image based on a float variable from 0.0 to 1.0.
0 = transparent, 0.5 = half transparent and 1 = opaque.
This is a classic function that you find in many softs and also Imagic for PHP. See: http://www.php.net/manual/en/imagick.setimageopacity.php ...