For the purpose of tracking the ball visibly from the monitor screen, a red cross is displayed at the estimated position. Actually, the color red is displayed by overwriting the video image, using only one bit of the Z-mode pixel plane.
At each even vertical retrace, the red cross is drawn. As soon as a pixel has been displayed, it is replaced with a new one, since the image in the frame memory is being rebuilt in real time. The new pixel always contains all zeros (black) in the planes containing the red image, in other words, the cross is erased as soon as it is displayed.