Premult
Multiplies RGB by alpha, converting straight (unassociated) alpha to premultiplied alpha for correct compositing math.
Premult associates an image’s color with its coverage: every RGB value is multiplied by the pixel’s alpha, so fully transparent pixels go black and semi-transparent edges darken proportionally. Compositing operations that add or filter light (Merge’s Plus, glows, blurs across edges) are only correct on premultiplied images.
Most ArcBrush nodes handle alpha for you, so you rarely need Premult in simple graphs. It matters at the boundaries: preparing straight-alpha artwork for a Merge Plus pass, matching renders that come out of 3D software premultiplied, or building a compositing chain that mirrors a Nuke-style premultiplied workflow.
Parameters
Section titled “Parameters”Pins
Usage Tips
Section titled “Usage Tips”- Premultiply before Merge’s Plus operator so semi-transparent edges add correctly instead of glowing too bright.
- If an image already looks premultiplied (dark fringes on transparent edges), do not premultiply it again; that darkens edges further.
- Pair with Unpremult around color corrections: unpremult, grade, premult again keeps edge pixels from shifting.