This model tests the KHR_materials_unlit extension. There are two objects, one orange-ish (#FF7F00 in sRGB space) and one blue-ish (#007FFF in sRGB space). The visible surface of these two objects should appear as a uniform color, so much so that the 3D effect is intentionally lost.
The above screenshot shows a case where normal lighting calculations have been applied to an un-lit material. While the shadows on the ground are permissible, the various panels and edges visible on the orange and blue objects are not.
This model lists the unlit extension in both lists, like this:
"extensionsUsed": [
"KHR_materials_unlit"
],
"extensionsRequired": [
"KHR_materials_unlit"
]
The first list includes all extensions used by the current model, so there's no doubt that KHR_materials_unlit belongs there in this case. The second list is a subset of the first list, naming all the extensions that a given glTF viewer is required to support in order for this model to be loaded and displayed correctly. In this case, the sample model is a showcase for the visual appearance of the un-lit material, and any visible lighting effects are being declared as errors in the glTF viewer implementation.
For this reason, I decided to include KHR_materials_unlit in the extensionsRequired list in this case. A conforming glTF implementation that does not support the KHR_materials_unlit extension should find an unknown value in the extensionsRequired list, and report an error rather than loading or displaying the model without the unlit material.
In your own models, if you consider using a lit material to be a reasonable fallback behavior for implementations that don't support this extension, you should omit KHR_materials_unlit from the extensionsRequired list.
Copyright 2019 Analytical Graphics, Inc. CC-BY 4.0 https://creativecommons.org/licenses/by/4.0/ Model by Ed Mackey.