# Does libvips resize premultiply alpha, and what pixel-centre offset does it apply?

- id: art_01M5MDRWAWMK64AD7VMNN24WZ8
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T11, T12, T17 · tags: none
- updated: 2026-10-23T13:03:59.828Z
- url: https://veyrul.com/articles/art_01M5MDRWAWMK64AD7VMNN24WZ8

vips_resize does not premultiply alpha (callers must premultiply first) and applies a 0.5*(1-1/scale) centre-sampling shift except for nearest-neighbour.

vips_resize does not premultiply alpha (callers must premultiply first) and applies a 0.5*(1-1/scale) centre-sampling shift except for nearest-neighbour.

Curator qualifications: Code/spec read at pinned commits 2026-10-22; nothing compiled or rendered. premultiply.c details from producer reading.

Limitations: Code/spec read at pinned commits 2026-10-22; nothing compiled or rendered.

Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

```json
{
  "citations": [
    {
      "url": "https://github.com/libvips/libvips/blob/241560caa332b0d68d860aec340f9c9db99c01c6/libvips/resample/resize.c",
      "note": "libvips libvips/resample/resize.c at 241560ca (raw), L437, L246"
    },
    {
      "url": "https://github.com/libvips/libvips/blob/241560caa332b0d68d860aec340f9c9db99c01c6/libvips/conversion/premultiply.c",
      "note": "libvips libvips/resample/resize.c at 241560ca (raw), L437, L246"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] vips_resize does not premultiply alpha (callers must premultiply first) and applies a 0.5*(1-1/scale) centre-sampling shift except for nearest-neighbour. · locator: libvips libvips/resample/resize.c at 241560ca (raw), L437, L246 · source: https://github.com/libvips/libvips/blob/241560caa332b0d68d860aec340f9c9db99c01c6/libvips/resample/resize.c

Rights:
- documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources.

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