Mantaflow Color Mapping not ignoring alpha value #74220

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Andrew Price · 2020-02-26T04:39:03+01:00

Andrew Price commented

2020-02-26 04:39:03 +01:00

System Information
Operating system: Win 10
Graphics card: dual RTX Titans

Blender Version
Broken: 2.82

Short description of error
The gradient color mapping of smoke simulation will fill empty space with alpha if any color besides black is given

Exact steps for others to reproduce the error

Create any smoke simulation
In domain smoke settings, enable Color Mapping

Set the left hand field to any color besides black and it will ignore the alpha value and fill it

**System Information** Operating system: Win 10 Graphics card: dual RTX Titans **Blender Version** Broken: 2.82 **Short description of error** The gradient color mapping of smoke simulation will fill empty space with alpha if any color besides black is given **Exact steps for others to reproduce the error** - Create any smoke simulation - In domain smoke settings, enable Color Mapping # Set the left hand field to any color besides black and it will ignore the alpha value and fill it ![2020-02-26_13-34-26.png](https://archive.blender.org/developer/F8370628/2020-02-26_13-34-26.png)

Andrew Price commented

2020-02-26 04:39:03 +01:00

Added subscriber: @AndrewPrice

Richard Antalik commented

2020-02-26 14:31:56 +01:00

Changed status from 'Needs Triage' to: 'Confirmed'

Jacques Lucke commented

2020-02-26 15:35:11 +01:00

Added subscriber: @JacquesLucke

Jacques Lucke commented

2020-02-26 15:35:11 +01:00

Simply adding straight_to_premul_v4(out); to the end of BKE_colorband_evaluate seems to make it behave as I'd expect. No idea of this is the correct fix. workbench_volume_frag.glsl seems to expect that rgb = 0 when alpha = 0.

Simply adding `straight_to_premul_v4(out);` to the end of `BKE_colorband_evaluate` seems to make it behave as I'd expect. No idea of this is the correct fix. `workbench_volume_frag.glsl` seems to expect that `rgb = 0` when `alpha = 0`.

Jacques Lucke commented

2020-03-27 14:10:05 +01:00

Added subscriber: @Jeroen-Bakker

Jacques Lucke commented

2020-03-27 14:10:05 +01:00

@Jeroen-Bakker, do you know if the fix mentioned above makes sense?

Clément Foucault commented

2020-05-20 18:28:51 +02:00

Added subscriber: @fclem

Clément Foucault commented

2020-05-20 18:28:51 +02:00

I coded the volume shader for workbench. If I remember correctly it expects premultiplied color so that you can have purely emissive material (without alpha) which is exactly what @AndrewPrice is showing here.

In my opinion, the real issue is that the color widgets don't support/show premultiplied colors. So we could fix it after the colorband eval if we don't plan on improving the widget but to me it feels like removing a feature.

I coded the volume shader for workbench. If I remember correctly it expects premultiplied color so that you can have purely emissive material (without alpha) which is exactly what @AndrewPrice is showing here. In my opinion, the real issue is that the color widgets don't support/show premultiplied colors. So we could fix it after the colorband eval if we don't plan on improving the widget but to me it feels like removing a feature.

Brecht Van Lommel commented

2020-05-21 00:17:00 +02:00

Added subscriber: @brecht

Brecht Van Lommel commented

2020-05-21 00:17:00 +02:00

For volume shading there is no alpha strictly speaking, instead there is density. It's a matter of finding the most useful interpretation.

Current: alpha is the density for absorption (the "smoke") only, not emission (the "fire")
straight_to_premul_v4: alpha is the density for both absorption and emission

Since this color mapping is used for visualizing one volume grid at a time, I think it's most convenient if the alpha controls the density of both.

If the purpose was to show smoke and fire together you'd want separate control over the density of each, but to me that seems unnecessary here.

For volume shading there is no alpha strictly speaking, instead there is density. It's a matter of finding the most useful interpretation. * Current: alpha is the density for absorption (the "smoke") only, not emission (the "fire") * `straight_to_premul_v4`: alpha is the density for both absorption and emission Since this color mapping is used for visualizing one volume grid at a time, I think it's most convenient if the alpha controls the density of both. If the purpose was to show smoke and fire together you'd want separate control over the density of each, but to me that seems unnecessary here.

blender-admin commented

2020-06-29 14:19:44 +02:00

This issue was referenced by 9feb0d5f2e

This issue was referenced by 9feb0d5f2eb87cc5f19b00c8928514e44edc4dde

Clément Foucault commented

2020-06-29 14:20:22 +02:00

Changed status from 'Confirmed' to: 'Resolved'

Clément Foucault closed this issue

2020-06-29 14:20:22 +02:00

Clément Foucault self-assigned this 2020-06-29 14:20:22 +02:00

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Mantaflow Color Mapping not ignoring alpha value #74220

Set the left hand field to any color besides black and it will ignore the alpha value and fill it