unpremultiply — GFX2D composite op

• Data kinds: rgba_premul → rgba

• Call: import gfx2d; gfx2d.unpremultiply(rgba_premul) (or opsgfx2d.get("unpremultiply"))

Usage

Premultiplied `rgba back to straight alpha: (C/A, A)`.

Pixels with `alpha == 0` come back as transparent black (see

:func:premultiply on why that is the only defensible answer).

Raises ValueError: if the input is not premultiplied, detected as

`colour > alpha` at some pixel.

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 as input)

premultiply · alpha_composite · sprite_blit · sprite_transform · sprite_sheet_slice · nine_slice

Same category (composite)

premultiply · 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.