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‘Unpremultiple’ is the process of dividing the alpha out of the image. Then the colour corrections can be done. After this stage, the alpha can then again be multiplied back in. This can often be done within the colour correction node itself, or manually. from Digital Compositing

Many 3D programs have a specific compositor node for this.

I tried to find an unpremult node in Blender compositor but I didn't find such a node.

How would I perform unpremult

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  • $\begingroup$ What is unpremult? $\endgroup$
    – Blunder
    Jul 16 at 16:45
  • $\begingroup$ Blender already provides separate access to colors and alpha. Unless I misunderstand your question there should be no need for this operation in the Compositor. $\endgroup$
    – Robert Gützkow
    Jul 22 at 14:19
  • $\begingroup$ Can be useful for color correction though @RobertGützkow $\endgroup$
    – brockmann
    Jul 22 at 14:30
  • $\begingroup$ @brockmann Ok, your answer is technically correct, if you want straight alpha. Maybe I'm not aware of a particular use case but when would this actually be necessary? $\endgroup$
    – Robert Gützkow
    Jul 22 at 14:42
  • $\begingroup$ Premult is used when creating transparency with a background. Un is used to undo the effect. $\endgroup$ Jul 22 at 16:02
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Add an Alpha Convert node and set it to 'Straight'. The operation divides the rgb channels by its alpha which is also known as 'unpremult' in other compositors:

enter image description here

From the manual:

For compositing and rendering, Premultiplied Alpha is the standard in Blender. Render layers will be premultiplied alpha, and images loaded into rendering or compositing will be converted to this.

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