unpremultiplied

Unpremultiplied refers to a representation of color with an alpha channel where the red, green, and blue components are stored as independent color values and have not been multiplied by the alpha value. In this form, the alpha channel expresses opacity, while the color channels represent the full color intensities of the fragment before blending.

This contrasts with premultiplied alpha, in which the color channels are stored already multiplied by alpha (R' = R times A, G' = G times A, B' = B times A). Premultiplied is common in GPU pipelines because blending can be performed without division, which can improve performance and reduce artifacts at edges.

In image formats, PNG and most non-animated formats typically use unpremultiplied alpha; some graphics engines and textures are stored as premultiplied alpha internally, and many rendering pipelines convert to premultiplied form for compositing.

Blending with unpremultiplied data uses the over operation: out alpha equals As plus Ad times (1 minus As). If the resulting alpha is nonzero, the output color channels are computed as (Rs times As plus Rd times Ad times (1 minus As)) divided by the output alpha, for each channel. With premultiplied data, the over operation can be performed as out color equals Cs plus Cd times (1 minus As) and out alpha as above, where Cs and Cd are premultiplied color values.

Practical notes: ensure consistent representation when compositing; converting to a common form avoids artifacts. Working in linear color space helps achieve correct blending of translucent surfaces and reduces color issues at edges.