of the sub volume 14 is fixed in that plane referring now to fig5 after each instance of relocating or resizing of the sub volume prism 14 it is determined in step 64 whether or not the relocating and or resizing of the sub volume is complete in other words it is determined whether the right or left mouse button has been released in any of steps 32 40 54 or 62 in step 66 the coordinates x y z of the eight corners are then computed with these coordinates the volume of the sub volume is computed in step 68 also the display data is modified using the sub volume coordinates in order to locate the data which lies within the sub volume this data may be re scaled or displayed separately from the original data in step 70 the block diagram in fig6 summarizes an overall flow of the above described program the first step at 72 initializes the system hardware including the graphics display and the mouse the next step is to continuously loop and process user commands at step 74 including a continuous update of the cursor position and execution of commands received by the cursor the final step at 76 is to shutdown the display and mouse these steps are fully described in the next several paragraphs and are intended to be encompassed by the flow charts illustrated in fig3 through 6 the graphics display 10.