Pixel position of the moving surgical instruments (Large Needle Driver and Grasping Retractor) compared with initial position. Color bar represents the position density. (a) Pixel coordinates density of the tracking point while the single surgical instrument moving. (b) Distances between the target position and center of the view while tracking the single surgical instrument. (c) Pixel coordinates density of the tracking point while the double surgical instrument moving simultaneously. (d) Distances between the target position (center of the two instruments) and center of the view.