premultiply — GFX2D composite op

• Data kinds: rgba → rgba_premul

• Call: import gfx2d; gfx2d.premultiply(rgba) (or opsgfx2d.get("premultiply"))

Usage

Straight-alpha `rgba to **premultiplied** rgba: (C*A, A)`.

Lossy in one place, on purpose: the colour of a fully transparent pixel is

multiplied to zero and cannot be recovered. That colour never affects a

composite, so nothing downstream needs it — but a round trip through this

pair is the identity only where `alpha > 0`.

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 · alpha_composite_premul

Same category (composite)

unpremultiply · alpha_composite · alpha_composite_premul · 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.