On Mon, 26 Nov 2007 03:59:23 -0500
Lea Wiemann <[EMAIL PROTECTED]> wrote:

> Hi,
> I'd like to turn a black background into white, or transparency (in 
> which case it should look cut if displayed on a white background).  My 
> practical use case is photos of objects with black backgrounds that I'd 
> like to improve for Wikipedia.  The problem is that the boundary between 
> background and object is usually fuzzy.
> I've created a pair of example images (attached): Say I start off with 
> the image fuzzy-circle.png (a gray-patterned circle on a black 
> background); what I would like to get is something like 
> fuzzy-circle-goal.png (the gray-patterned circle on a white background) 
> -- but that seems pretty hard to achieve:  If I "select by color: black" 
> and then cut the selection, the corners look either aliased/"too sharp" 
> (for high thresholds in the select-by-color tool), or they have dark 
> pixels in them (for low thresholds).  Selecting the black background, 
> growing the selection (2px), feathering it (2px), and then cutting the 
> selection *kinda* works, but it cuts off the blurry part of the object 
> boundary.
> Any ideas on this one?

If you are using 2.4, Colours->Colour to alpha
or in 2.2 Filters->Colours->Colour to alpha

After that, make a new layer and fill with whatever colour you want

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