Pricipled BSDF doesn't allow negative or above 1 colors for its specular/metallic part #54218

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iszotic · 2018-03-03T18:38:22+01:00

iszotic commented

2018-03-03 18:38:22 +01:00

System Information
Kubuntu 16.04.3 kernel 4.10, AMDGPUpro 17.50 (legacy opecl)
CPU AMD R7 1700
GPU AMD Rx 480 8gb

Blender Version
Broken: 2.79.2
Worked: using regular glossy shader.

Short description of error

Pricipled BSDF doesn't allow negative colors for its specular part, so when we add the negative principled shader with the positive one it should render a black object but instead, it's only rendering the positive shader.
I tested the diffuse, subsubscattering and transmission parts of the principled shader and only the specular one it's not allowing the negative colors or above 1, in other words only allowing clampled colors between 0 and 1.

Exact steps for others to reproduce the error

create a material and in this material, create 2 colors, one negative, the other positive
connect each color to a principled shader
set the "Metallic" parameter of the shaders to 1 and the "Specular" to 0
add these shaders
apply the material to a sphere
the rendered sphere should be black but it's colored.

negative.blend

**System Information** Kubuntu 16.04.3 kernel 4.10, AMDGPUpro 17.50 (legacy opecl) CPU AMD R7 1700 GPU AMD Rx 480 8gb **Blender Version** Broken: 2.79.2 Worked: using regular glossy shader. **Short description of error** Pricipled BSDF doesn't allow negative colors for its specular part, so when we add the negative principled shader with the positive one it should render a black object but instead, it's only rendering the positive shader. I tested the diffuse, subsubscattering and transmission parts of the principled shader and only the specular one it's not allowing the negative colors or above 1, in other words only allowing clampled colors between 0 and 1. **Exact steps for others to reproduce the error** 1) create a material and in this material, create 2 colors, one negative, the other positive 2) connect each color to a principled shader 3) set the "Metallic" parameter of the shaders to 1 and the "Specular" to 0 4) add these shaders 5) apply the material to a sphere 6) the rendered sphere should be black but it's colored. [negative.blend](https://archive.blender.org/developer/F2411699/negative.blend)

iszotic commented

2018-03-03 18:38:22 +01:00

Added subscriber: @iszotic

iszotic changed title from ~~Pricipled BSDF doesn't allow negative colors for its specular/metallic part~~ to Pricipled BSDF doesn't allow negative or above 1 colors for its specular/metallic part

2018-03-03 18:50:06 +01:00

Brecht Van Lommel commented

2018-03-03 21:34:06 +01:00

Added subscriber: @brecht

Brecht Van Lommel commented

2018-03-03 21:34:06 +01:00

I can confirm that, though using negative base colors is not supported at all for the Principled BSDF. It combines the various components in a physically based way and negative values don't make much sense in that context. The proper solution may not be to support negative values but to clamp them away, I need to look into it further.

Sybren A. Stüvel commented

2020-02-04 12:26:59 +01:00

Changed status from 'Confirmed' to: 'Archived'

Sybren A. Stüvel closed this issue

2020-02-04 12:26:59 +01:00

Sybren A. Stüvel self-assigned this 2020-02-04 12:26:59 +01:00

Evan Wilson commented

2020-02-05 04:12:08 +01:00

Added subscriber: @EAW

Evan Wilson commented

2020-02-05 04:12:08 +01:00

Note: this is only the case for Cycles.

Using EEVEE (which wasn't in Blender at the time of this report), it is possible to create a negative color using the a value input node and combine RGB node, that will then create a specular black sphere following the above steps.

Note: this is only the case for Cycles. Using EEVEE (which wasn't in Blender at the time of this report), it is possible to create a negative color using the a value input node and combine RGB node, that will then create a specular black sphere following the above steps.

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Pricipled BSDF doesn't allow negative or above 1 colors for its specular/metallic part #54218