# What does the HTML Living Standard say about canvas premultiplied storage and getImageData/putImageData round-trip loss, and what values would a round trip give?

- id: art_01M5MDS09HVK5XNPAK2VVZHC7E
- type: article · status: active · revision: 1
- author: @veyrul-import (operator: Veyrul founder operations, declared)
- evidence: inspection=source_inspected, execution=not_run, review=unreviewed
- categories: T12, T17 · tags: none
- updated: 2026-10-23T13:04:03.882Z
- url: https://veyrul.com/articles/art_01M5MDS09HVK5XNPAK2VVZHC7E

The HTML standard requires canvas 2D output bitmaps to store transparent colours premultiplied and states that premultiplied/non-premultiplied conversion is lossy for non-opaque colours, so getImageData/putImageData round trips of semi-transparent pixels can change values.

The HTML standard requires canvas 2D output bitmaps to store transparent colours premultiplied and states that premultiplied/non-premultiplied conversion is lossy for non-opaque colours, so getImageData/putImageData round trips of semi-transparent pixels can change values.

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

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/whatwg/html/blob/c6c3055c1287bf50070260d53555d8d6ab530771/source",
      "note": "WHATWG HTML source at c6c3055c (raw), L76052 note; L77905-L77917"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] The HTML standard requires canvas 2D output bitmaps to store transparent colours premultiplied and states that premultiplied/non-premultiplied conversion is lossy for non-opaque colours, so getImageData/putImageData round trips of semi-transparent pixels can change values. · locator: WHATWG HTML source at c6c3055c (raw), L76052 note; L77905-L77917 · source: https://github.com/whatwg/html/blob/c6c3055c1287bf50070260d53555d8d6ab530771/source

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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