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Alpha Transparency

Started by andrew_j_w Jan 7, 2003 at 6:25 AM 4 replies 400+ views

Original Post

Jan 7, 2003 at 6:25 AM

Hi, I''m trying to manually merge two images with alpha transparency, but I can''t come up with a formula that looks good. I''ve got as far as R = Rs*(1-An) + Rn*An G = Gs*(1-An) + Gn*An B = Bs*(1-An) + Bn*An where Rs,Gs,Bs are the colurs of the bottom image and Rn,Gb,Bn are the colours of the top image. An is the alpha value (0..1) for the top image. This works fine, but I can''t get a formula to give me a final alpha for the combined image. What should the alpha value be for two pixels with 0.5 as their alpha value? What about 0.75 and 0.3? Any help would be apprciated, Andrew

edotorpedo

Jan 7, 2003 at 10:16 AM

Your formula seems to be the correct formula. About the alpha value of two pixels, the most logical would be:

(AlphaPixel1 + AlphaPixel2) / 2

so alpha = (0.75 + 0.3) / 2 = 0.525

Have you tried this already?

Edo

andrew_j_w

Jan 7, 2003 at 10:18 AM

The problem with that is if the bottom pixel has an alpha value of 1.0 and the top pixel has a value of 0.5 then the final alpha value would be 0.75, not 1.0.

I hope this makes my problem clearer,
Andrew