* Add support for sheenColor and sheenRoughness
This work is necessary to support the glTF extension KHR_materials_sheen.
This change effectively adds the specular lobe from the cloth material
model to the base material model. The cloth model remains useful for
its extra subsurface color feature but also because it's cheaper.
* Add support for KHR_materials_sheen to gltfio
* Update documentation
* Document default shading values
Emissive was previously defined in exposure compensation stops, which
was confusing to many. It is now a value in nits, with the alpha
channel controlling how much the camera exposure affects the emissive.
At 0, the emissive value is just added to the final pixel color, at
1 the emissive value is multiplied by the exposure just like with
regular lights.
The intensity of the emissive property can be computed from an
exposure value (EV) easily with the following formula:
emissive.rgb = emissive.rgb * pow(2.0, EV - 3.0);
This formula is available as Exposure::luminance(float) already
in Filament.
* Add support for bent normals
Bent normals can be enabled via the bentNormal property of a material.
When specular occlusion is enabled, bent normals improve the quality
of the computation.
* Save a couple of multiplications in bent specular AO
When dealing with thin objects we really have two thicknesses to
consider, the thickness in the direction of the normal, which
corresponds to the value used for solid objects, and the thickness of
the object's walls, which generally is a constant.
Reusing thickness in the later case is problematic for assets that have
a thickness map, but are rendered hollow.
In the future we can even imagine handling double-sided hollow objects
by using the thickness information -- e.g. a hollow cube.
Normally the IOR is deduced from the reflectance, but now we allow to
specify the IOR instead or in addition to the reflectance.
In the later case, it's possible to create physically impossible
materials, but this can be useful for artistic reasons.
This replaces the previous "curvature to roughness" method. Both are related
and rely on the screen space variance of geometric normals but this new
solution offers more control (the screen space variance and the clamping
threshold can be controlled).
Romain Guy
Mathias Agopian
Pixelflinger