@donmccurdy assembled very thorough, discrete PBR tests to compare Three.js and Khronos' WebGL renderer to a (as yet unidentified) ray tracer.

This issue covers work necessary to add similar, discrete comparisons to the <model-viewer> fidelity tests that will land as of #305 .

@jsantell is currently investigating the potential of using Mitsuba as a reference ray tracer for producing screenshots for comparison.