Unpremult
Divides RGB by alpha, converting premultiplied alpha back to straight alpha before color corrections.
Unpremult reverses premultiplication: every RGB value is divided by the pixel’s alpha, recovering the original un-darkened color in semi-transparent regions. Fully transparent pixels stay black, since there is no color left to recover.
Its main job is protecting edges during color work. Grading a premultiplied image shifts its semi-transparent edge pixels (they are part color, part black), which shows up as dark or bright fringes after compositing. Unpremult first, apply the correction, then Premult again and the edges stay clean.
At 8-bit precision the round trip is slightly lossy in the darkest semi-transparent pixels; in a 32-bit (float) document it is exact.
Parameters
Section titled “Parameters”Pins
Usage Tips
Section titled “Usage Tips”- Sandwich color corrections between Unpremult and Premult when working with premultiplied sources.
- Renders from 3D software are usually premultiplied; unpremult them before Levels, Curves, or Exposure adjustments.
- Prefer a 32-bit (float) document for repeated premult round trips; the conversion is exact there.