alpha_composite_premul — GFX2D composite op

• Data kinds: rgba_premul × rgba_premul → rgba_premul

• Call: import gfx2d; gfx2d.alpha_composite_premul(src, dst) (or opsgfx2d.get("alpha_composite_premul"))

Usage

Porter–Duff over on premultiplied colour: `src + dst*(1 - src.a)`.

This is the operator; :func:alpha_composite is this one with a conversion

on each side. Being affine in `dst`, it is exactly associative —

`over(over(A, B), C) == over(A, over(B, C))` to float64 rounding, which the

test suite checks as an equality rather than a tolerance.

References (sample data, literature)

• Sample-data catalog (download URLs / licences) — 2-D uses skimage.data (BSD/public domain) plus synthetic images; 3-D lists download URLs for real data sources (Stanford, PDS, …).

• Operator provenance and references — the sources of the research/methods this op family came from.

• The canonical algorithm (author, year) and its uses are named in the family usage guide above.

Runnable examples (verified samples that actually call this op)

• gfx2d_scene — py -3.11 examples/gfx2d_scene.py

Ops the type connects to (they accept rgba_premul as input)

unpremultiply

Same category (composite)

premultiply · unpremultiply · alpha_composite · blend_mode · layer_stack

*Provenance: gfx2d.py — GFX2D operator registry. This per-op note is generated by tools/opdocs.py md (do not hand-edit).*

© 2026 Kazufumi Furuse — Fullseye operator documentation. Licensed under Apache-2.0.