[x3d-public] glTF Materials

[Leonard Daly]

The topic of glTF material libraries came up during the X3D WG meeting. 
I volunteered to report back on what is available. This is the report. 
Because all of this is publicly available and generally useful, I am 
also posting it to 'x3d-public'

The glTF V2 specification 
(https://github.com/KhronosGroup/glTF/tree/master/specification/2.0) 
uses Physically Based Rendering (PBR).

Note: PBR Articles are plentiful. Listed here are a couple I used when 
writing this up. These are useful to help in term definition and 
understanding
  * 
https://www.marmoset.co/posts/basic-theory-of-physically-based-rendering/
  * 
https://www.marmoset.co/posts/physically-based-rendering-and-you-can-too/

Khronos works by defining a specification and allowing extensions - 
either Khronos or vendor developed. This discussion is just about the 
approved spec and does not include draft or extension documents. At this 
time there is no  common material library; however, there is a draft 
document for glTF V1 that is posted.


The glTF V2 specification that discusses materials is at 
https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#materials. 
It includes an illustrative (image) example plus example (JSON) code. 
Appendix B is currently unwritten.

PBR is mostly used to handle metals. Most non-metallic surfaces are 
special cases or otherwise covered in the metallic-roughness model. This 
model specifies what happens to light when it strikes the surface of the 
object - it is the bidirectional reflectance distribution function 
(BRDF). It is the responsibility of the lighting system to use these 
values with the various light sources (including reflection from other 
objects) to determine an objects perceived appearance.

For each supplied pixel, the objects BRDF is calculated from six number, 
(RGBA, metallic-factor, roughness-factor; described in Appendix B - 
currently unwritten). Each value comes from a texmap (image; 2 total - 
metallic and roughness are combined) so the BRDF is a function of 
surface coordinates. If the inputs for the six numbers are constant over 
the object (i.e., the texmaps are a single color), then the object's 
BRDF will produce the same value for each point on the object's surface.

The glTF system does not specify a "rust" texture (as in variation over 
the surface). That is content.

As far as I can tell, there is no effort at any organization affiliated 
with glTF to provide a library of various textures (e.g., dirt, brick, 
stone, rust, wood, leaf-litter, etc.). This is content and out-of-scope 
of the glTF work. It is more along the lines of A-Frame asset or Unity 
libraries -- somewhat akin to the Universal Media library for X3D content.

-- 
*Leonard Daly*
3D Systems & Cloud Consultant
LA ACM SIGGRAPH Chair
President, Daly Realism - /Creating the Future/
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