On 05/05/2013 03:56 AM, Richard Gitschlag wrote:


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Date: Sat, 4 May 2013 22:04:41 +0200
From: ofn...@laposte.net
To: gimp-user-list@gnome.org
Subject: Re: [Gimp-user] How to make transparency gradiate?

On 05/04/2013 03:54 PM, Richard Gitschlag wrote:



    You can also achieve the same result using paint tools.

    1 - Eyedrop the background color.
2 - Switch to the Paintbrush and the "color erase" blending mode. Color erase is also a color-to-alpha transition.
    3 - Start painting the background.


Hmm... this begs the question: what is the difference between "Colors/Color-to-alpha" and the bucket-fill tool in color-erase mode?

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For starters, Color to Alpha is a plugin, color erase is part of the program core.

Color erase can be used "on the fly" with any drawing tool, and it can benefit from all the tool's options (such as brush hardness and mouse/tablet dynamics). Combine it with the "Behind" blending mode (its exact opposite) it's almost like having a different Eraser tool.

To cite some of my personal experience, when I create a traditional color pencil drawing, I typically want to clean up the background paper. Not the paper grain (it mostly washes out anyway and is not an issue), but things like stray pencil flecks and so on. I also wanted to be able to digitally tint the background (say, by gradient), so I needed to erase the background. The problem is you can't use the eraser to do this - you have a flat layer with RGB values gradiating from color RGB to white background so you can't just erase out the alpha channel (leaving the RGB values otherwise unchanged); you need a Color to Alpha transition.

So, for a while what I did was I copied the layer, performed a Color to Alpha transition (relative to white) on the lower copy, then used the Eraser on the upper copy. But once I wrapped my head around what the "color erase" blending mode actually IS, I realized that was a much more efficient way of doing the same thing. I didn't have to duplicate the layer; I could just paint over it in "Color Erase" mode; any mistakes I can just paint over again in "Behind" mode. The only downside is not having a way to easily toggle between these two modes.

-- Stratadrake
strata_ran...@hotmail.com
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Numbers may not lie, but neither do they tell the whole truth.



Part of the question was whether we do get the very same results using either technique. So I did some testing and yes, they seem strictly identical.

For your mode changes, you can write a trivial script that uses gimp-context-set-paint-mode and assign it to a keyboard shortcut.

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