Projective Grid Mapping for Planetary Terrain
Joseph D. Mahsman · 2010
Visualizing planetary terrain presents unique challenges not found in traditional terrain visualization: displaying features that rise from behind the horizon on a curved surface; rendering near and far surfaces with large differences in scale; and using appropriate map projections, especially near the poles. Projective Grid Mapping (PGM) is a hybrid terrain rendering technique that combines ray casting and rasterization to create a view-dependent mesh in GPU memory. We formulate a version of PGM for planetary terrain that simplifies ray-sphere intersection to two dimensions and ray casts the area behind the horizon. We minimize the number of rays that do not contribute to the final image by creating a camera that samples only the area of the planet that could affect the final image. The GPU-resident mesh obviates host-to-device communication of vertices and allows for dynamic operations, such as compositing overlapping datasets. We adapt two map projections to the GPU for equatorial and polar data to reduce distortion. Our method achieves realtime framerates for a full-planet visualization of Mars.