We describe a method for combining and visualizing a set of overlapping volume images with high resolution but limited spatial extent. Our system combines the calculation of a registration metric with ray casting for direct volume rendering into a combined operation performed on the graphics processing unit (GPU). The combined calculation reduces memory traffic, increases rendering frame rate, and makes possible interactive-speed, usersupervised, semi-automatic combination of many component

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... me images. For volumes that do not overlap any other imaged volume, the system uses contextual information provided in the form of an overall 2D background image to calculate a registration metric.