Level of transparency

[koxon]

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.se ... pacity.php

I can't seem to find how to do it with the command line tool "convert".
Here is what I tried:
$> convert logo.png -alpha on -channel A -evaluate divide 3 result.png
This works fine but as I do a "divide" I can't reach 0 and create full transparency. It also hard for me to translate my input variable 0>1 to the proper division factor.


$> convert logo.png -alpha on -channel A -evaluate set 30% result.png
This sounded great! ... However the quality obtain is not. The color is bleeding, I get a black BG and the quality is not good at all. See below:


Do you guys have a solution for me?
I need to be able to convert my float input variable 0>1 to a parameter that "convert" will understand to perform transparency.

Thanks a lot!

Koxon

[fmw42]

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

[snibgo]

And, of course, you can multiply instead of divide:

Code: Select all

convert logo.png -alpha on -channel A -evaluate multiply 0.25 +channel result.png

[koxon]

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

Best,

[koxon]

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

This makes the background transparent but the quality is not there. I got pixels bleeding on the edges ...
Thanks though.

[koxon]

And, of course, you can multiply instead of divide:

Code: Select all

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.

Thanks a lot!