High speed z-buffer with dynamic random access memory

Abstract

The present invention provides apparatus and methods which are most advantageously used in conjunction with a computer display system incorporating the use of a Z-buffer to provide three dimensional hidden surface elimination. A buffer memory is provided which is sufficiently large such that each display element (pixel) on the display is represented by a 16-bit Z value. The Z value corresponds to the Z axis depth of the object at the particular point corresponding to the pixel. The buffer comprises a plurality of dynamic random access memories (D-RAMs) having two operation modes: Normal and Read-Modify-Write (RMW). A counter/pointer register is provided which successively addresses values in the buffer representing successive pixels along scan lines of the display. A graphics processor is provided with coordinates defining a three dimensional image to be displayed and, for each point of the object, computes a current Z.sub.c value beginning at an initial coordinate address in memory. The initial address is loaded into the graphics processor and a D-RAM read-modify write (RMW) cycle is initiated concurrent with the calculation of Z.sub.c. This address corresponds to the address for the point at which the graphics processor is computing Z.sub.c. The value of Z.sub.p, the prior Z value for that address location, is read from the buffer memory and stored in a data-out register. The dynamic RAM in the buffer then waits in the RMW cycle until the processor fetches the Z.sub.p value from the data-out register and compares it to Z.sub.c. The value of Z.sub.p in the buffer is updated if Z.sub.c is less than Z.sub.p. If Z.sub.c is greater than or equal to the Z.sub.p the RMW cycle is aborted, and the current value of Z.sub.p in the buffer is maintained.