Sets the value of Transparency attribute. This attribute represents the transparency field that specifies how "clear" an object is, with 1.0 being completely transparent, and 0.0 completely opaque. Default value is 0.0 (opaque).

The definition of 1 being transparent and 0 being opaque is the opposite of the definition in alpha channels, where 0.0 is completely transparent and 1.0 is completely opaque. This definition is due to upward compatibility to previous versions of this standard in different to the definition in IfcIndexedColourMap.