The present invention discloses a method and system, which discards invalid pixels by using cache memories storing depth values of all pixels having inputted which satisfy a Z Test Mode. The present invention is applied in a three-dimensional graphic system, and uses a plurality of pre-test units to process input pixels in a parallel manner to avoid system performance degeneration due to sequential execution used in prior art. The plurality of pre-test units read the depth values stored in a pre-test Z cache in the system and compare them with the depth values of input pixels being processed according to the Z Test Mode. If the answer is yes, the input pixels are discarded. Otherwise, the content of the pre-test Z cache is updated.