Cycles base_color parameter BUG?

[aurora]

I got a simple USD with a cube and a cycles bsdf shader assigned.
Shade "base_color" parameter set to a red color.

Shader load fine in Gaffer but on inspection I see no base_color parameter. On render cube  is the default 0.8 grey.

If a texture is assigned to the base color, cube renders textured... still can see the parameter.

ShaderQuery and ShaderTweaks cant see the parameter either.

Is this a bug?

Here is the usd file

simple_shader.usda

A

[aurora]

Here is a USD file with texture connected to the base_color parameter:

simple_shader_with_texture.usda

[aurora]

Ah... I see where the problem may lay... its Blender not Gaffer.

Blender stores RGB and Alpha for color parameters:

float4 inputs:base_color = (0.53920454, 0.121217735, 0.5191846, 0.5191846)

Gaffer expects just RGB:

color3f inputs:base_color = (0.53920454, 0.121217735, 0.5191846)

¯\_(ツ)_/¯

A

[John Haddon]

On Sat, Jan 21, 2023 at 7:09 AM aurora <auro...@gmail.com> wrote:

Ah... I see where the problem may lay... its Blender not Gaffer.

Yep, this looks like a Blender export bug to me. In the Cycles source code it is clear that `base_color` is a `color` (RGB) and not a `float4` (RGBA). It also seems bogus that the alpha that Blender is writing out is just a copy of the B.

Regardless, I've opened a PR to fix the loading on the Gaffer side, since we should be able to load `float4` parameters, and there might be cases where they are legitimate : https://github.com/ImageEngine/cortex/pull/1327.

In the same Cycles source code, you can see that `BSDF` is capitalised, which is why Gaffer also gets upset about the lower case `bsdf` in the `simple_shader.usda` (but not by the upper case `BSDF` in the `simple_shader_with_texture.usda`. With the above fix and with the `BSDF` fixed, I am able to render your cube.

Is there some way we can get these bugs reported to the Blender team? There's a danger with USD that everyone ends up writing their own unique flavours, and ideally we'd have as few workarounds as possible for this sort of thing...

Cheers...

John

[aurora]

Thank you John. Wasn't expecting you to take action. I also think Blender export should be fixed instead. I actually started pocking the Blender guys for it. I really dont see the use of Alpha in color values like Base color or SSS color... even within Cycles.
Cheers!

[aurora]

Seems like Blender is working a new version of BSDF:

https://www.youtube.com/watch?v=DQeP363Xmn4