A three-dimensional computer graphics rendering system allows a tile-based rendering system to operate with a reduced amount of storage required for tiled screen space geometry by using an untransformed display list to represent the screen’s geometry. The methods include using selective sets of ultrasound elements to generate an ultrasound beam, scanning the beam over a series of ultrasound elements in order to collected echo data covering an area, and generating an image from the resulting data. An op-amp compares the first voltage with the second voltage and generates a control output responsive to the comparison. The actuator assembly has an actuator arm supported by an actuator bearing for positioning the actuator arm relative to the disc. Resistance forces may prevent the handle from moving from the second position to the first position when the sliding door is closed without an application of force by a user.