KHR_material_iridescence: missing extension support

[bhouston]

Here are some images that demonstrate the lack of V-Ray support for KHR_materials_iridescence.

Specifically here is khronos-IridescenceSuzanne.

With Threejs:

With three-gpu-pathtracer:

And with V-Ray:

[bhouston]

In V-Ray, it is called "thin film": https://docs.chaos.com/display/VMAX/VRayMtl#heading-ThinFilm

[jasondavies]

I'm going to work on this.

[jasondavies]

I'm not sure what iridescenceFactor and the optionaliridescenceTexture should map to (they are combined to obtain the iridescence intensity). Any advice?

So far, I have:

iridescence_factor = o.get('iridescenceFactor', 0.0) # what should this map to?
iridescence_texture = o.get('iridescenceTexture') # what should this map to?
brtf.thin_film_ior = o.get('iridescenceIor', 1.3)
brtf.thin_film_thickness_min = o.get('iridescenceThicknessMinimum', 100.0)
brtf.thin_film_thickness_max = o.get('iridescenceThicknessMaximum', 400.0)
if o.get('iridescenceThicknessTexture') != None:
  brtf.thin_film_thickness = self._make_texture(renderer, prim, o.get('iridescenceThicknessTexture'), transfer_func=0)

[bhouston]

hmm... it seems that V-Ray iridescence model is more physically-based than glTF. glTF seems to use the "factor" + "texture" as a blend onto of the iridescence model, where as V-Ray just says if you don't want iridescence, set the ior to 1 and thickness_min to 0?

I guess for now, we could add a warning to your python script if iridescenceFactor is set to something other that 0 or 1? And also if iridescenceTexture is used at all? If iridescenceFactor is set to 0, we should override the other parameters to disable thin_film effect. If it is 1, we can ignore it.

Does this make sense?

[bhouston]

Maybe if iridescenceFactor is set to 0, we can disable the thin_film effect. I believe there is a flag for this? At least in the VRay 3DS Max UI it is a flag. It probably is as well in the AppSDK?

[jasondavies]

Makes sense, thanks. Yes, if the factor is 0.0 (the default), we can simply ignore the entire extension as per the spec.

[jasondavies]

Teaser shot...

[bhouston]

I don't see a lot of iridescence in that image. Maybe try khronos-IridescenceSuzanne instead?

[jasondavies]

I can't render that until I implement volume support, unfortunately. Soon!

[jasondavies]

I suppose I can render it without volume support, just to test iridescence. Watch this space.