OpenGL 4 Shading Language Cookbook - Third Edition

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Rather than making Material.Metal a Boolean value, one could choose to make it a continuous value between 0 and 1.  Indeed, this is exactly what some implementations do. The value would then be used to interpolate between the two models (metallic and dielectric). However, that makes the parameter somewhat less intuitive for artists and you may find that the extra configurability may not be all that useful.