Convert RGBA color to RGB?

I've upvoted Johannes' answer because he's right about that.

I've upvoted Johannes' answer because he's right about that. A more specific answer, however, incorporates the alpha value into the actual colour result based on an opaque background colour (or 'matte' as it's referred to). For example - Assuming that 0 is opaque and 255 is transparent, then 255,255,255,127 RGBA with a black matte will result in 127,127,127 RGB value.

There is an algorithm for this (from this wikipedia link): Normalise the RGBA values so that they're all between 0 and 1 - just divide each value by 255 to do this. We'll call the result Source. Normalise also the matte colour (black, white whatever).

We'll call the result BGColor Now, the conversion is defined as (in complete psuedo code here! ): Source => Target = (BGColor + Source) = Target. R = ((1 - Source.

A) * Source. R) + (Source. A * BGColor.

R) Target. G = ((1 - Source. A) * Source.

G) + (Source. A * BGColor. G) Target.

B = ((1 - Source. A) * Source. B) + (Source.

A * BGColor. B) Sorry for the dodgy formatting, couldn't figure out how else to do it :) To get the final 0-255 values for Target you simply multiply all the normalised values back up by 255, making sure you cap at 255 if any of the combined values exceed 1.0 (this is over-exposure and there are more complex algorithms dealing with this that involve whole-image processing etc. ). EDIT: In your question you said you want a white background - in that case just fix BGColor to 255,255,255.

1 In the light of the question edit this is a very accurate and good answer. I deleted mine and hope you bubble up, since you explained the matters way better :) – Joey Jan 12 '10 at 14:08 thanks johannes - very kind :) – Andras Zoltan Jan 12 '10 at 14:10.

Hm... regarding to en.wikipedia.org/wiki/Alpha_compositing#... solution provided by Andras Zoltan should be slightly changed to: Source => Target = (BGColor + Source) = Target. R = ((1 - Source. A) * BGColor.

R) + (Source. A * Source. R) Target.

G = ((1 - Source. A) * BGColor. G) + (Source.

A * Source. G) Target. B = ((1 - Source.

A) * BGColor. B) + (Source. A * Source.

B) This changed version works fine for me, because in prev. Version rgba(0,0,0,0) with matte rgb(ff,ff,ff) will be changed to rgb(0,0,0).

I believe your are right. Wikipedia and Andras Zoltan's answer is a little bit different. – nacho4d Jul 17 '10 at 6:52 I agree, use this version, not @Andras Zoltan's.

For a test scenario, convert rgba(0,0,0,.7) on background rgb(255,255,255) using both formulas; hkurabko's forumala clearly produces the correct answer. – Toast Jul 29 at 19:21.

This depends on the color space you use. If the RGBA is in pre-multiplied color-space and is semi-transparent, you need to divide out alpha to get the correct RGB color. If the color is in non pre-multiplied color-space, then you can just discard the alpha channel.

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