Single-pass Wireframe Rendering

Wireframe rendering is normally done in two passes; the first renders the filled triangles and the second renders the lines, using the depth buffer from the first pass to remove hidden lines. Not only does this involve passing the geometry twice to the graphics card, there are issues with depth testing for the lines due to slight differences in rasterization techniques between lines and triangles. These differences result in rendering artefacts and there is no good way to resolve this. In 2006 a new technique was proposed in a SIGGRAPH sketch entitled Single-pass Wireframe Rendering. The technique uses a pair of shaders to render triangles and lines in a single pass. Not only does this overcome rasterization issues, it is also faster and produces smooth results. The main idea is to compute the distances from fragments to triangle edges. If a fragment is within a threshold distance (half the line width) from a triangle edge, the fragment is rendered with the line color, otherwise it is rendered with the triangle color. A smoothing function is applied at the boundary between triangle and line to remedy aliasing artefacts. Most of the work is done in a vertex shader, where the distances to all triangle vertices are computed in viewport space. It is these (interpolated) distances that are the input to the fragment shader. A more robust implementation, using geometry shaders, has been proposed by NVIDIA. Their implementation deals with some tricky cases related to primitives having one or more vertices outside the viewing frustum and further reduces the amount of data sent to the graphics card.