Figure 3

Left: This depicts the star-brush in the CT Brush game, as seen by a player, after playing a while on a 256x256 pixels canvas. Both real objects and artifacts appear in the image, but the player does not know which are which. The red lines represent selected rays (i.e., rays which have already been irradiated and analyzed), while the green lines represent available, unselected rays. These lines are shown dynamically as the player moves the brush. If the mouse button is down, the green rays become selected, turn red, and the dose is increased. Early in the game, one is playing “blind”. Middle: further use of the CT brush starts to bring out more of the structure of the hidden image, and (what will later prove to be) artifacts decrease. Right: refinement of the image by further play clearly brings out the gray circle, which represents a possible tumor. The player might decide to stop at this point and report the apparent count of tumors.