# Why do transparent edges fringe after resizing or blending, and does my library premultiply for me?

- id: art_01M5MEMPGA5P9V6Y09G6CPE6HV
- 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, T17 · tags: none
- updated: 2026-10-23T13:19:11.365Z
- url: https://veyrul.com/articles/art_01M5MEMPGA5P9V6Y09G6CPE6HV

Filtering straight-alpha RGBA bleeds the colour of invisible pixels into visible edges; Pillow (9de06c8a) avoids this by converting RGBA to premultiplied RGBa for resize (all filters except NEAREST) and reduce, then converting back. Un-premultiplying fails where alpha is 0 (OpenEXR), and PNG says compositing should use linear intensity, not gamma-encoded values.

Filtering straight-alpha RGBA bleeds the colour of invisible pixels into visible edges; Pillow (9de06c8a) avoids this by converting RGBA to premultiplied RGBa for resize (all filters except NEAREST) and reduce, then converting back. Un-premultiplying fails where alpha is 0 (OpenEXR), and PNG says compositing should use linear intensity, not gamma-encoded values.

Curator qualifications: Specifications and code read 2026-10-22; no images processed (the producer experiment was not re-run). Producer experiment separating library behaviours not re-run.

Limitations: Specifications and code read 2026-10-22; no images processed (the producer experiment was not re-run).

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

```json
{
  "citations": [
    {
      "url": "https://www.w3.org/TR/png-3/",
      "note": "Pillow 9de06c8a src/PIL/Image.py L2415-L2418 (resize) and L2468-L2471 (reduce); OpenEXR TechnicalIntroduction.rst L1262-L1274; W3C PNG-3 Alpha channel processing"
    },
    {
      "url": "https://github.com/AcademySoftwareFoundation/openexr/blob/dbdc101a3b754ee21f0e70ed28ffec1cba32c78e/website/TechnicalIntroduction.rst",
      "note": "Pillow 9de06c8a src/PIL/Image.py L2415-L2418 (resize) and L2468-L2471 (reduce); OpenEXR TechnicalIntroduction.rst L1262-L1274; W3C PNG-3 Alpha channel processing"
    },
    {
      "url": "https://github.com/python-pillow/Pillow/blob/9de06c8a53ab71e724edc84cf59bbf843c973984/src/PIL/Image.py",
      "note": "Pillow 9de06c8a src/PIL/Image.py L2415-L2418 (resize) and L2468-L2471 (reduce); OpenEXR TechnicalIntroduction.rst L1262-L1274; W3C PNG-3 Alpha channel processing"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Filtering straight-alpha RGBA bleeds the colour of invisible pixels into visible edges; Pillow (9de06c8a) avoids this by converting RGBA to premultiplied RGBa for resize (all filters except NEAREST) and reduce, then converting back. Un-premultiplying fails where alpha is 0 (OpenEXR), and PNG says compositing should use linear intensity, not gamma-encoded values. · locator: Pillow 9de06c8a src/PIL/Image.py L2415-L2418 (resize) and L2468-L2471 (reduce); OpenEXR TechnicalIntroduction.rst L1262-L1274; W3C PNG-3 Alpha channel processing · source: https://www.w3.org/TR/png-3/

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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