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An alpha channel stores an additional value for each pixel that controls its contribution when images are combined by alpha compositing. In a common normalized convention, $\alpha =0$ makes the source pixel fully transparent and $\alpha =1$ makes it fully opaque. An alpha channel can describe partial coverage as well as transparency; its meaning includes the compositing rule, rather than merely a fourth color component. PNG represents its explicit alpha channel with 8-bit or 16-bit values, with zero fully transparent and the maximum value fully opaque.^[ref]

For source color $C_s$ and an opaque background color $C_b$, the source-over rule in Linear-light RGB isFor an original example, a red source $C_s=(1,0,0)$ at $\alpha =1/4$ over a blue background $C_b=(0,0,1)$ produces $(1/4,0,3/4)$. These are linear color components; applying the same arithmetic directly to nonlinear display codes generally gives a different physical mixture.^[ref]

Storage conventions matter too. With straight alpha the stored color remains $C_s$; with premultiplied alpha it is $\alpha C_s$. The red source in the example is therefore stored as either $(1,0,0,1/4)$ or $(1/4,0,0,1/4)$. Using the first formula on an already premultiplied color would multiply its source contribution by alpha twice. PNG explicitly uses non-premultiplied color samples.^[ref]

With premultiplied alpha, a stored colour triple already includes its alpha channel multiplier: $c=\alpha C$, where $C$ is the corresponding straight alpha colour. The alpha value itself is retained. This is also called associated alpha. For $\alpha >0$, recover $C=c/\alpha$; at zero alpha the multiplication loses the original colour. PNG uses unassociated samples rather than this convention.^[ref]

For an original Linear-light RGB example, red $C=(1,0,0)$ with $\alpha =1/4$ becomes the stored quadruple $(1/4,0,0,1/4)$. The source-over compositing contribution of this sample is already $(1/4,0,0)$. Multiplying its first three entries by alpha again would make the red contribution $1/16$ instead of $1/4$.

Premultiplied alpha also makes a useful linear interpolation example transparent. Let sample A be opaque red, $(C_A,{\alpha }_A)=((1,0,0),1)$, and sample B be fully transparent blue, $(C_B,{\alpha }_B)=((0,0,1),0)$. Their premultiplied quadruples are $(1,0,0,1)$ and $(0,0,0,0)$. Averaging them givesPlaced over black, this contributes $(1/2,0,0)$. Averaging the straight colour triples and alphas separately would instead give $C=(1/2,0,1/2)$, $\alpha =1/2$, contributing $(1/4,0,1/4)$ over black. That blue contribution came from colour attached to a fully transparent sample. The calculation explains how an interpolation convention can create a coloured fringe.

The multiplication convention and the color space are separate choices. The example uses Linear-light RGB throughout. A file decoder, texture sampler and alpha compositing stage must agree on both the colour encoding and whether the samples use straight alpha or premultiplied alpha.