[x3d-public] Comment of X3D V4, glTF, and Lighting

[Leonard Daly]

Posted on behalf of Khronos' 3D Formats Working Group - owners of the 
glTF specification.

This message addresses the proposed use of glTF in X3D V4.0, 
specifically the means for lighting glTF V2.0 models.

glTF V2.0 primarily uses materials that are rendered using 
physically-based rendering 
(https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#materials 
<https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#materials>) 
with the metallic-roughness model. Because of the potential for highly 
reflective materials, it is beneficial to simulate real-world lighting 
for the rendering to approximate what the eye would see with a 
real-world object. This simulation is typically done with image-based 
lighting (IBL). While IBL does not cover all situations (e.g., internal 
lighting in a model of a lamp), it does provide an excellent 
approximation of ambient and environment light source when there is a 
single model and the model/light interaction is not time dependent.

To describe and illustrate lighting issues I wrote a blog post 
"Importance of Environment Lights" 
(https://realism.com/blog/importance-environment-lights) with an 
accompanying interactive demonstration "PBR Model & Lighting Inspector" 
(https://realism.com/Examples/pbr-lighting/). The tool allows the user 
to pick a model, image lighting, and environment image (Background node 
in X3D) and shows the real-time lighting effects. The tool uses Google's 
<model-viewer> custom HTML tag. Note that the lighting available in the 
demo is not indicative real-world lighting, but is simpler to illustrate 
the point. Another 3D Formats WG member (Eric Chadwick from Wayfair) 
rendered a different model (Fight Helmet) using closer to studio 
lighting and 3 Point lights (separate renders). The comparison image is 
at http://realism.com/Examples/pbr-lighting/FlightHelmet-3Points-IBL.png

In the review copy of the X3D V4 specification, section 17 
(https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-WD3/Part01/components/lighting.html) 
describes the lighting component. There is no discussion of image based 
lighting. Section 9.4.2 Inline 
(https://www.web3d.org/specifications/X3Dv4Draft/ISO-IEC19775-1v4-WD3/Part01/components/networking.html#Inline) 
discusses the importing of glTF models into the X3D scene. There is no 
discussion of lighting there.

Not including IBL with glTF model rendering creates appearances that are 
inaccurate and do not show off the capabilities of glTF materials and 
appearance. The 3D Formats Working Group highly recommends including IBL 
capability when rendering glTF models.

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