Attribute filter for computer graphics applications

Abstract

A hardware filter for controlling the detectability, highlighting and invisibility of graphics primitives. Each of the three display attributes (detectability, highlighting, and invisibility) being controlled has associated with it both an inclusion counter and an exclusion counter, for a total of six counters. Associated with each class name is a flag, indicating whether the class name is a member of the currently active set, as well as a 6-bit string indicating the effect of the class on each of the six filters. On traversal of the structure hierarchy, upon encountering an order for the addition of a class name to the set, and if the flag corresponding to the class name has not already been set, the flag is set and the corresponding filter counters are incremented in parallel by gating an incrementing signal with the 6-bit string. Similarly, upon encountering an order for the removal of a class name to the set, and the flag corresponding to the class name has not been reset, the flag is reset and the corresponding filter counters are decremented by gating a decrementing signal with the 6-bit string. Subsequently encountered primitives are considered as detectable, highlighted or invisible if the corresponding inclusion filter counter is greater than zero, indicating that the primitive belongs to at least one included class, and the corresponding exclusion filter counter is zero, indicating that the primitive does not belong to any excluded class.