Advanced GPU Raycasting

Abstract

Modern GPUs offer a degree of programmability that opens up a wide field of applications far beyond processing millions of triangles at ever increasing speed. Raycasting is one of these applications that can make heavy use of the built-in features of todays graphics cards. With the possibilities offered by this technology, there is a lot of room for new techniques that do not simply convert existing algorithms to the GPU, but use the very strengths of this architecture to create more realistic images at interactive frame rates. This paper presents an approach to hardware based raycasting in the fragment shader of a shader model 3 compatible graphics card that not only allows for both orthogonal and perspective projection, but enables the user to move the viewpoint into the dataset for fly-through applications like virtual endoscopy. This hardware-based approach can also be used to correctly intersect the rendered dataset with normal OpenGL geometry, allowing arbitrary 3D-meshes, pointers or grids to be rendered in the same scene. The last section deals with the biggest problem of GPUbased raycasting- the limited amount of available video RAM- and how it can be circumvented by applying a cached blocking scheme that loads only blocks of interest into the memory.