The first set of scenarios are for demonstrating material capabilities, so these are primarily fidelity tests. Following these are simpler cases that test glTF spec conformance, but are not particularly interesting for fidelity. At the bottom are the Furnace Test and the Directional Light Test. The Furnace Test checks energy conservation and the correct result is the whole 7x7 grid of metal-rough spheres being invisible (indistinguishable from the background). The top (metal) and bottom (dielectric) rows are passing, while the mixtures lose some energy. The Directional Light Test checks the worst-case scenario for environmental lighting approximations: the case where a single bright pixel represents a directional light.
We show a simple logarithmic metric on an average of the pixel differences, excluding the transparent background. This is not a perceptual metric, so take it with a grain of salt. It is mostly there to help us identify regressions.