Simulate color transparency

Question

I have RGB color value and alpha value. How can I get new RGB value assuming that I have white backgound and alpha is applied?

score 14 · Accepted Answer · answered Jul 12 '12 at 08:42

14

The formula to be applied to each of the color channels is the following:

cr = cf * af + cb * ab * (1 - af)

where cr is the resulting color of the pixel, cf is the foreground color, cb the background color, af foreground alpha and ab background alpha.

Note that often color values are stored already premultiplied by alpha in which case the formula simplifies to

cr = cf + cb * (1 - af)

answered Jul 12 '12 at 08:42

Adam Zalcman

Assume I have red color (ff0000) with alpha 0.5, and the background is white. Trying to get result color: ff0000*0.5 + ffffff*0.5 = ff7fff, but this is not what I expect... http://www.colorhexa.com/ff7fff What am I doing wrong? – Timofei Davydik Jul 12 '12 at 09:02
2

Apply the formula to each color individually (red, green, blue) – Aaron Digulla Jul 12 '12 at 09:16
What about the alpha channel of the new color? – Sebastian Barth Oct 26 '16 at 11:11

1 Answers1