2.1.9 [ISO32000-1] Section 8.7.4.5.5, Type 4 Shadings (Free-Form Gouraud-Shaded Triangle Meshes)
V0009:
The specification states:
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Type 4 shadings (free-form Gouraud-shaded triangle meshes) are commonly used to represent complex coloured and shaded three-dimensional shapes.
Microsoft Edge:
Free-form Gouraud-shaded triangle mesh shadings are not supported (their presence is ignored).