Specifies the threshold (transparency range) for a specified category.
The threshold is a value from 0 through 1 that specifies a cutoff point for each color component. For example, suppose the threshold is set to 0.7, and suppose you are rendering a color whose red, green, and blue components are 230, 50, and 220. The red component, 230, is greater than 0.7×255, so the red component will be changed to 255 (full intensity). The green component, 50, is less than 0.7×255, so the green component will be changed to 0. The blue component, 220, is greater than 0.7×255, so the blue component will be changed to 255.