A pan-tilt-zoom robotic camera can provide detailed live video of selected areas of interest within a large potential viewing field. To provide spatial context for human observers, it is desirable to insert the resulting live video into a large spherical panoramic display representing the entire viewing field. Accurate alignment of the video stream within the panoramic display is difficult due to small errors in the robot pan-tilt values and image distortion due to nonlinear projection. Existing image alignment algorithms cannot keep up with rapid changes in camera position. In this paper, we present a constant-time image alignment algorithm based on spherical projection and projection-invariant selective sampling that accurately registers paired images at 25 frames per second on a standard PC. Experiments suggest that the new alignment algorithm is faster than previous algorithms by a factor four or more. In a companion paper, we present a new calibration algorithm based on image variance density that optimally estimates camera pan-tilt parameters