Make glTF PBR thickness input control thin-walledness

[pablode]

Attempts to fix #1936. Needs some testing.

[pablode]

Just a heads-up: looks good in MaterialXView, but I'm waiting for USD with MaterialX 1.39 so that this can be tested with a path tracer.

[jstone-lucasfilm]

@pablode I believe it's now possible to test MaterialX 1.39 functionality in Omniverse Kit 106, if you'd like to give this a try.

[pablode]

@jstone-lucasfilm I just tested this with MaterialX 1.39.1 and the latest MDL SDK. Unfortunately I'm getting an MDL compilation error:

[12:57:32.022] (ERROR) [MDL] <string>(80,121): C219 'mx_surface' : cannot convert argument 'mxp_thin_walled' from 'bool' to 'uniform bool': depends on parameter type
[12:57:32.022] (ERROR) [MDL] <string>(80,121): C219 'mx_surface' : cannot convert argument 'mxp_thin_walled' from 'bool' to 'uniform bool': depends on parameter type

At first glance, the problem seems to be that thickness is declared with uniform="false" in the gltf_pbr, while the surface node has thin_walled declared as uniform="true".

[jstone-lucasfilm]

Thanks for testing this, @pablode, and I'm CC'ing @niklasharrysson for his thoughts on which direction we ought to go with the uniformity of thickness and thin-walled properties in MaterialX.

[niklasharrysson]

Since the thin_walled input is uniform it doesn't work to connect a network with a varying / non-uniform parameter upstream. In order for this to work the thickness input must also be set as uniform.

thin_walled is set to uniform since a surface must be either a closed volume (thin_walled = false) or a thin doublesided sheet (thin_walled = true). It can't be both, depending on a varying variable.

It is MDL in particular that enforces this rule, as it has the concept of uniform variables built into the language. And thin_walled is a uniform in the MDL material model.