 Alpha compositing

Alpha compositing combines image contributions using their colour values and alpha channels. The source-over compositing rule places a source layer over a destination while retaining the destination's contribution where the source is partly transparent. Other compositing rules select different contributions; the alpha values alone do not identify the chosen rule.^[ref]

Write $C_s,C_b$ for source and background colour triples, with alpha values ${\alpha }_s,{\alpha }_b$. In premultiplied alpha form, $c_s={\alpha }_s{C}_s$ and $c_b={\alpha }_b{C}_b$. Then source-over compositing givesWhen ${\alpha }_o>0$, the corresponding straight alpha colour is $C_o=c_o/{\alpha }_o$. The output can remain partly transparent, ready to be combined with another background.

For an original Linear-light RGB example, place red $C_s=(1,0,0)$ with ${\alpha }_s=1/2$ over blue $C_b=(0,0,1)$ with ${\alpha }_b=1/4$. The result isReversing the layers instead gives $c_o=(3/8,0,1/4)$ and $C_o=(3/5,0,2/5)$, with the same output alpha. Layer order changes the colour even when it leaves the output alpha unchanged.

If the background is opaque, ${\alpha }_b=1$ and ${\alpha }_o=1$, so the rule reduces to $C_o={\alpha }_s{C}_s+(1-{\alpha }_s)C_b$. For a physical light-mixture interpretation, carry out that arithmetic in Linear-light RGB; encoded sRGB values first need their appropriate conversion.^[ref] Separately specify whether stored colour samples already contain their alpha multiplier, so that the source contribution is not multiplied twice.