 Alpha channel

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]