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Subtract Color plugin (23rd Oct 2012)


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Effects > Color > Subtract Color

This is one approach to the "color to alpha" idea. The transparency of each pixel depends on how similar its color is to the selected color. The resulting image has the following property: if a layer of the selected color is placed underneath the resulting image, it will look (almost) exactly like the original image.


  • It is usually better to select an area to remove color from, rather than applying this effect to the whole image.
  • For best results, choose an extreme color (fully saturated, black or white) instead of a color close to an extreme value (e.g. choose RGB (255, 0, 100) rather than RGB (245, 10, 100)).


The parameters are:

Color - pixels of this color will have the maximum transparency. The closer a pixel's color is to this value, the more transparent it will be.

Function - "subtract color" (the main function), "show alpha as grayscale" (display fully transparent pixels as black, opaque pixels as white and semi-transparent pixels as gray) or "show original image" (for quick comparison).

Tolerance - affects how similar a pixel needs to be to the chosen color to be made transparent.

Tolerance sharpness - degree of "feathering" at the edge of the tolerance value (between transparent and non-transparent pixels).

Alpha intensity - maximum transparency to assign the pixels (100 means fully transparent).

Any feedback welcome!

Download: SubtractColor.zip

(For another plugin with a different approach, see Tanel's Color to Alpha: http://forums.getpai...?showtopic=7767)

Edited by davidf
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