Nodes: Exclusion (Mix Color) clamps negative values even when Clamp Result is disabled #114242

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quackarooni · 2023-10-29T08:38:19+01:00

quackarooni commented

2023-10-29 08:38:19 +01:00

System Information
Operating system: Windows 10 Home
Graphics card: Intel(R) UHD Graphics 605

Blender Version
Broken: since 3.5 when Exclusion was first implemented, (still persists in 4.0 and 4.1 daily builds)

Short description of error
When the blend type of the Mix Color node is set to Exclusion, its output clamps all negative values to 0.0 regardless of whether Clamp Result is toggled or not. This is true for Shader, Geometry, and Compositor versions of the node.

Most sources I could find only mention that Exclusion's formula is 'A + B - 2AB', and don't explicitly mention clamping for negative numbers. This behavior is also inconsistent with other blend types from the same group (i.e. Subtract, & Divide), which allow for negative values. Difference is an exception as it has an absolute value baked into its formula.

Exact steps for others to reproduce the error

Combine two colors using Mix Color set to Exclusion. (Make sure any of the values lie outside of [0-1] so that the output value goes outside of range)
Either a.) compare with a node implementation of the formula, or b.) compare for values that are exactly 0.0. In both cases, a discrepancy arises in regions where the value would've been negative if it wasn't clamped.

An example .blend file is also attached with examples of both.

**System Information** Operating system: Windows 10 Home Graphics card: Intel(R) UHD Graphics 605 **Blender Version** Broken: since 3.5 when Exclusion was first implemented, (still persists in 4.0 and 4.1 daily builds) **Short description of error** When the blend type of the `Mix Color` node is set to `Exclusion`, its output clamps all negative values to 0.0 regardless of whether `Clamp Result` is toggled or not. This is true for Shader, Geometry, and Compositor versions of the node. Most sources I could find only mention that Exclusion's formula is `'A + B - 2AB'`, and don't explicitly mention clamping for negative numbers. This behavior is also inconsistent with other blend types from the same group *(i.e. Subtract, & Divide)*, which allow for negative values. *Difference* is an exception as it has an absolute value baked into its formula. **Exact steps for others to reproduce the error** 1. Combine two colors using `Mix Color` set to `Exclusion`. *(Make sure any of the values lie outside of [0-1] so that the output value goes outside of range)* 2. Either a.) compare with a node implementation of the formula, or b.) compare for values that are exactly 0.0. In both cases, a discrepancy arises in regions where the value would've been negative if it wasn't clamped. An example .blend file is also attached with examples of both.

Exclusion Bug.blend

123 KiB

quackarooni added the

labels 2023-10-29 08:38:19 +01:00

quackarooni commented

2023-10-29 08:44:32 +01:00

Example images attached below: (White areas represented negative values that were clamped at 0.0)

Shader Nodes

Geometry Nodes

Compositor Nodes

Example images attached below: *(White areas represented negative values that were clamped at 0.0)* | Shader Nodes | |--------| |![Exclusion Clamping - Shader Nodes.png](/attachments/ac2d68e4-b67a-4536-ab7b-5dd5a344c4c5)| | Geometry Nodes | |--------| |![Exclusion Clamping - Geometry Nodes.png](/attachments/26162e3e-72fa-4620-8809-31a277deafc7)| | Compositor Nodes | |--------| |![Exclusion Clamping - Compositor Nodes.png](/attachments/761190b4-f52e-48c4-a8ea-d249f9b3831f)|

Exclusion Clamping - Compositor Nodes.png

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Exclusion Clamping - Shader Nodes.png

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Exclusion Clamping - Geometry Nodes.png

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Germano Cavalcante commented

2023-10-30 19:12:41 +01:00

I can confirm what was described. But I'm not sure what's really expected.

Exclusion mode was added in bea5fe6505 by @CharlieJolly

@CharlieJolly, could you provide some insight here?

I also noticed that the manual does not include this mode:
https://docs.blender.org/manual/en/4.1/render/shader_nodes/converter/mix.html?utm_source=blender-4.1.0-alpha#bpy-types-shadernodemix

Cc. @brecht, @OmarEmaraDev

I can confirm what was described. But I'm not sure what's really expected. Exclusion mode was added in bea5fe6505 by @CharlieJolly @CharlieJolly, could you provide some insight here? I also noticed that the manual does not include this mode: https://docs.blender.org/manual/en/4.1/render/shader_nodes/converter/mix.html?utm_source=blender-4.1.0-alpha#bpy-types-shadernodemix Cc. @brecht, @OmarEmaraDev

Germano Cavalcante added

Status

Needs Info from Developers

and removed

Status

Needs Triage

labels 2023-10-30 19:12:54 +01:00

Omar Emara commented

2023-10-30 19:31:24 +01:00

This is the intended behavior, but I guess it needs to be documented. This was discussed in details in the PR: https://archive.blender.org/developer/D16543.

Germano Cavalcante commented

2023-10-30 19:45:56 +01:00

Thanks for the info!

For documentation, we have another repository. I think maybe it can be tracked down however at blender/blender-manual#104446 (Cc @Blendify)

Closing then.

Thanks for the info! For documentation, we have another repository. I think maybe it can be tracked down however at https://projects.blender.org/blender/blender-manual/issues/104446 (Cc @Blendify) Closing then.

Germano Cavalcante closed this issue

2023-10-30 19:45:59 +01:00

Blender Bot added

Status

Archived

and removed

Status

Needs Info from Developers

labels 2023-10-30 19:46:11 +01:00

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