Convert RGBA color to RGB

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

* A few comments have been raised that my original answer was not correct. It worked if alpha values were inverted from the normal. By definition, however, this won’t work in most cases. I’ve therefore updated the formula below to be correct for the normal case. This ends up being equal to @hkurabko’s answer below *

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).

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 Note – if the background colour is also transparent, then you’ll have to recurse the process for that first (again, choosing a matte) to get the source RGB for this operation.

  • Now, the conversion is defined as (in complete psuedo code here!):

    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)

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.

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