# How much do pixels differ across GPUs and browsers for the same WebGL or WebGPU scene, and what tolerance do conformance tests allow?

- id: art_01M5NN6PSG91SBWTDRHWC37ZZF
- 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, T19, T20 · tags: none
- updated: 2026-10-24T00:33:07.370Z
- url: https://veyrul.com/articles/art_01M5NN6PSG91SBWTDRHWC37ZZF

A 2012 study rendered one simple antialiased WebGL scene on 270 crowd-sourced machines: the images looked identical but formed 50 distinct pixel-level renders, mostly consistent within a graphics-card model (the text also mentions 51 groups). Current WebGPU conformance tests assume small differences: integer formats compare exactly by default, float and normalised formats can use ULP or fractional thresholds, a canvas colour-space reftest allows up to 2 levels per channel, and WGSL leaves the rounding mode open when quantising. So pixel-exact equality across devices should not be expected; no recent cross-device measurement was found.

A 2012 study rendered one simple antialiased WebGL scene on 270 crowd-sourced machines: the images looked identical but formed 50 distinct pixel-level renders, mostly consistent within a graphics-card model (the text also mentions 51 groups). Current WebGPU conformance tests assume small differences: integer formats compare exactly by default, float and normalised formats can use ULP or fractional thresholds, a canvas colour-space reftest allows up to 2 levels per channel, and WGSL leaves the rounding mode open when quantising. So pixel-exact equality across devices should not be expected; no recent cross-device measurement was found.

Curator qualifications: Paper and test sources read 2026-10-22; no rendering performed. 2012 data (WebGL 1, Firefox 11-era browsers, self-reported hardware) may not reflect current GPUs.

Limitations: Paper and test sources read 2026-10-22; no rendering performed.

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

```json
{
  "citations": [
    {
      "url": "https://hovav.net/ucsd/dist/canvas.pdf",
      "note": "Mowery & Shacham, Pixel Perfect (canvas.pdf), pdftotext lines 415, 481, 491, 790-812; gpuweb/cts src/webgpu/util/texture/texture_ok.ts @8de0e0a7 lines 20-29, 63; canvas_colorspace_rgba8unorm.https.html line 17; docs/fp_primer.md line 62"
    },
    {
      "url": "https://github.com/gpuweb/cts/blob/8de0e0a7da5c62e1e4e194b929c431002610155a/src/webgpu/util/texture/texture_ok.ts",
      "note": "Mowery & Shacham, Pixel Perfect (canvas.pdf), pdftotext lines 415, 481, 491, 790-812; gpuweb/cts src/webgpu/util/texture/texture_ok.ts @8de0e0a7 lines 20-29, 63; canvas_colorspace_rgba8unorm.https.html line 17; docs/fp_primer.md line 62"
    },
    {
      "url": "https://github.com/gpuweb/cts/blob/8de0e0a7da5c62e1e4e194b929c431002610155a/docs/fp_primer.md",
      "note": "Mowery & Shacham, Pixel Perfect (canvas.pdf), pdftotext lines 415, 481, 491, 790-812; gpuweb/cts src/webgpu/util/texture/texture_ok.ts @8de0e0a7 lines 20-29, 63; canvas_colorspace_rgba8unorm.https.html line 17; docs/fp_primer.md line 62"
    },
    {
      "url": "https://github.com/gpuweb/cts/blob/8de0e0a7da5c62e1e4e194b929c431002610155a/src/webgpu/web_platform/reftests/canvas_colorspace_rgba8unorm.https.html",
      "note": "Mowery & Shacham, Pixel Perfect (canvas.pdf), pdftotext lines 415, 481, 491, 790-812; gpuweb/cts src/webgpu/util/texture/texture_ok.ts @8de0e0a7 lines 20-29, 63; canvas_colorspace_rgba8unorm.https.html line 17; docs/fp_primer.md line 62"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] A 2012 study rendered one simple antialiased WebGL scene on 270 crowd-sourced machines: the images looked identical but formed 50 distinct pixel-level renders, mostly consistent within a graphics-card model (the text also mentions 51 groups). Current WebGPU conformance tests assume small differences: integer formats compare exactly by default, float and normalised formats can use ULP or fractional thresholds, a canvas colour-space reftest allows up to 2 levels per channel, and WGSL leaves the rounding mode open when quantising. So pixel-exact equality across devices should not be expected; no recent cross-device measurement was found. · locator: Mowery & Shacham, Pixel Perfect (canvas.pdf), pdftotext lines 415, 481, 491, 790-812; gpuweb/cts src/webgpu/util/texture/texture_ok.ts @8de0e0a7 lines 20-29, 63; canvas_colorspace_rgba8unorm.https.html line 17; docs/fp_primer.md line 62 · source: https://hovav.net/ucsd/dist/canvas.pdf

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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