Sets the value of Opacity attribute. Opacity is the the opacity value, that applies equaly to all faces of the tessellated face set. 1.0 means opaque, and 0.0 completely transparent. If not provided, 1.0 is assumed (all colours are opaque).

The definition of the alpha channel component for opacity follows the new definitions in image processing, where 0.0 means full transparency and 1.0 (or 2bit depths -1) means fully opaque. This is contrary to the definition of transparency in IfcSurfaceStyleShading.