By default, the ImageMapperGL2D sets the opacity of the LookupTable to the value of opacity property:
LookupTableProp->GetLookupTable()->ChangeOpacityForAll( opacity ); LookupTableProp->GetLookupTable()->ChangeOpacity(0, 0.0);
Sometimes is desired to use custom opacity values for each color, using the opacity configuration of the LookupTable property.