Unpremultiply

Unpremultiply refers to reversing premultiplied alpha in digital images and color pipelines. In a premultiplied representation, the red, green, and blue channels are stored already multiplied by the alpha channel: R' = R × A, G' = G × A, B' = B × A, where A is the alpha value. Unpremultiply computes the original color values by dividing the stored channels by alpha: R = R'/A, G = G'/A, B = B'/A.

If alpha is zero, the original color is mathematically undefined since division by zero would occur. In practice, implementations typically handle this by setting the RGB components to zero or by adopting a pipeline-specific rule when A is zero. When A is greater than zero, the division yields the unpremultiplied color, which is then clamped to the valid color range if needed (for example, [0,1] in floating point or [0,255] in 8-bit integer formats).

Applications of unpremultiply include image compositing, color correction, texture sampling, and converting between premultiplied and straight (unpremultiplied) alpha representations. Many graphics APIs and image formats support premultiplied alpha to speed up blending; in such cases, unpremultiply is used when straight alpha values are required for processing or output.

Common considerations include handling division by zero safely, managing numerical precision and rounding artifacts, and performing operations in a linear color space to avoid gamma-related errors. Some pipelines perform unpremultiply in hardware or shader code, while others rely on image processing libraries to implement the operation.