# What does Blender's compositor define for premultiplication, Over, Disjoint Over, Conjoint Over and alpha conversion, and where do its code comments and manual disagree with other sources?

- id: art_01M5MS09ZG4EYCE2HE7KY0J093
- 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 · tags: none
- updated: 2026-10-23T16:20:17.514Z
- url: https://veyrul.com/articles/art_01M5MS09ZG4EYCE2HE7KY0J093

Blender's compositor (f46990f) implements Alpha Over as premultiplied Over (fg + bg*(1-fa)), optionally premultiplying a straight foreground first; Disjoint Over un-premultiplies the background when fa+ba >= 1; Conjoint Over returns the foreground when fa > ba, else fg + bg*(1-fa/ba), with a source comment correcting the blog formula it cites; Convert Alpha un-premultiplies by dividing by alpha except at alpha exactly 0, where the pixel passes through unchanged.

Blender's compositor (f46990f) implements Alpha Over as premultiplied Over (fg + bg*(1-fa)), optionally premultiplying a straight foreground first; Disjoint Over un-premultiplies the background when fa+ba >= 1; Conjoint Over returns the foreground when fa > ba, else fg + bg*(1-fa/ba), with a source comment correcting the blog formula it cites; Convert Alpha un-premultiplies by dividing by alpha except at alpha exactly 0, where the pixel passes through unchanged.

Curator qualifications: Spec/code read 2026-10-22; nothing composited. Nuke equivalence and Blender manual statements per producer.

Limitations: Spec/code read 2026-10-22; nothing composited.

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

```json
{
  "citations": [
    {
      "url": "https://learn.foundry.com/nuke/content/reference_guide/merge_nodes/merge.html",
      "note": "compositor/shaders/library/gpu_shader_compositor_alpha_over.glsl at f46990f L13-75; gpu_shader_compositor_convert_alpha.glsl L17"
    },
    {
      "url": "https://github.com/blender/blender/blob/f46990f5f55e36cc771b0ce476512b89ecfcea23/source/blender/compositor/shaders/library/gpu_shader_compositor_alpha_over.glsl",
      "note": "compositor/shaders/library/gpu_shader_compositor_alpha_over.glsl at f46990f L13-75; gpu_shader_compositor_convert_alpha.glsl L17"
    },
    {
      "url": "https://github.com/blender/blender/blob/f46990f5f55e36cc771b0ce476512b89ecfcea23/source/blender/compositor/shaders/library/gpu_shader_compositor_convert_alpha.glsl",
      "note": "compositor/shaders/library/gpu_shader_compositor_alpha_over.glsl at f46990f L13-75; gpu_shader_compositor_convert_alpha.glsl L17"
    },
    {
      "url": "https://docs.blender.org/manual/en/latest/compositing/types/color/alpha_convert.html",
      "note": "compositor/shaders/library/gpu_shader_compositor_alpha_over.glsl at f46990f L13-75; gpu_shader_compositor_convert_alpha.glsl L17"
    },
    {
      "url": "https://docs.blender.org/manual/en/latest/compositing/types/color/alpha_over.html",
      "note": "compositor/shaders/library/gpu_shader_compositor_alpha_over.glsl at f46990f L13-75; gpu_shader_compositor_convert_alpha.glsl L17"
    }
  ]
}
```

Evidence claims:
- [sourced_fact] Blender's compositor (f46990f) implements Alpha Over as premultiplied Over (fg + bg*(1-fa)), optionally premultiplying a straight foreground first; Disjoint Over un-premultiplies the background when fa+ba >= 1; Conjoint Over returns the foreground when fa > ba, else fg + bg*(1-fa/ba), with a source comment correcting the blog formula it cites; Convert Alpha un-premultiplies by dividing by alpha except at alpha exactly 0, where the pixel passes through unchanged. · locator: compositor/shaders/library/gpu_shader_compositor_alpha_over.glsl at f46990f L13-75; gpu_shader_compositor_convert_alpha.glsl L17 · source: https://learn.foundry.com/nuke/content/reference_guide/merge_nodes/merge.html

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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