Efficient rendering of semi-transparent geometry in real-time

Licentiate thesis, 2010

Rendering images of semi-transparent geometry representing e.g. windows, transparent cloth or architectural models made transparent for visualization purposes presents a difficult problem in interactive applications due to the limitation of the ubiquitous zbuffer of only storing one object per pixel, whereas correctly resolving the color of transparent geometry requires considering the color of all fragments within a pixel in the correct order. Very fine geometry, representing e.g. hair, fur or foliage that has triangles to thin to be correctly captured by anti-aliasing hardware must often be treated as semi-transparent and so suffer from the same limitations. Casting shadows from either of these types of geometry presents an analogous problem as standard real-time shadowing techniques do not take into account the opacity of materials. This thesis presents a number of novel solutions to the problem of rendering and casting shadows from semi-transparent and fine geometry. First, we present a method for simply sorting camera-facing billboards efficiently on the GPU. Second, the specific problem of rendering alpha-composited self-shadowing hair or fur, where the geometry is typically a large number of short line-segments is addressed. Third, we present a method that handles any type of transparent geometry under the assumption that alpha can be considered constant for all fragments and finally, we present a general algorithm that is able on current hardware to render any type of fine or transparent geometry which is equivalent to backwards Monte Carlo raytracing (with no changes in ray direction). For each new method introduced, comparisons to previous work are made considering performance as well as quality of the rendered images.