Legacy ImageMagick Discussions Archive

I've got some images which have an alpha value of 240 where it should be fully opaque. Even the transparent areas should become more opaque.

For this i wanted to add 15 to the alpha channel of the whole image using convert.

I tried it with
convert --channel alpha -evaluate Add 15 +channel input.png output.png

But the alpha stays the same. If I use Multiply instead of Add it's possible to change the value with this command, but I want just Add 15 to every value.

What does "convert -version" say? Probably that you are using Q16, where values go from 0 to 65535, not 0 to 255. So a 8-bit 240 becomes 240*256+240 = 61680, so you need to add 65535-61680 = 4335.

You can also add by percent. So compute 100*15/quantumrange where quantumrange is 255 for Q8 IM and 655535 for Q16 IM

or get percent from

convert xc: -format "%[fx:15*100/quantumrange]" info:

This is the output of the version command:

Code: Select all

Version: ImageMagick 6.7.7-10 2014-08-21 Q16 http://www.imagemagick.org
Copyright: Copyright (C) 1999-2012 ImageMagick Studio LLC
Features: OpenMP

So I'm using Q16. When I add 4335 it works.

Legacy ImageMagick Discussions Archive

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