Opacity - Here you can assign opacity to the material where white is completely opaque and black is completely transparent. It should be noted that no principled model is able to represent all possible materials entirely accurately, and where those models fail - for example when the material isn’t viewed frontally - only approaches such as that of VRscans are able to capture the correct material representation. However with the introduction of the GGX model, all of these surfaces can be approximated well, thus reducing the need for using the other models. In the past, each model's characteristics resembled more closely a certain type of material, for example Phong could be used for plastics, Ward for cloth and metals, and Blinn for other common surfaces. The GGX model has a bright center and an even longer falloff (at default settings). The Ward model has an even broader center and falloff. The Blinn model has broader highlight center with a tight falloff. For example, the specular highlights with the Phong model have a very narrow and bright center with no falloff, but it doesn't work well with anisotropic reflections. Historically, the Phong, Blinn, Ward and GGX are successive reflectance models developed over the years in computer graphics where each model aimed to improve on the limitations of the previous ones.
