Thursday, 24 January 2013

Wireframe Shader

Discussion at work led to talking about implementing a wireframe shader in Nuke, so I decided to see how difficult it would be in Imagine. As long as the mesh consists of polygons of a single type - i.e. triangles or quads: not difficult at all it turns out.

For triangles, it's easy enough to emulate a wireframe surface by simply working out how close a hit position on a triangle is to an edge by transforming the triangle's points into world space and then using the standard point-to-line method of a perpendicular vector to each edge. This gives you the distance in world space of the hit position to each edge, and based on which distance is the closest, you can then apply a step function to the colour of the surface, based on the distance and a line thickness amount.

For quads, I first tried using the same algorithm as for triangles, but ignoring the edge of the triangle that  was shared within the quad. This worked to some degree, but each quad had two opposing wedges in the corners where the point-to-line formula meant that parts of some edges weren't shaded correctly.

So instead, I decided to use the Barycentric coordinates of the hit position within the triangle. This allowed me to correctly isolate all four edges of the quad based on a fixed threshold, but I then had to work out the line width and keep it uniform to the length of any edges. In the end I multiplied the Barycentric coordinates of both the the hit position and the inverse hit position (for the opposing edge of the quad) by the length of each of the non-shared edges of the triangle, giving a distance. The smallest of these distances I then used to step the colour, as I did for triangles. While this might not be perfectly accurate and work in all situations, it seems to work very well in practice and also allowed me to (almost) match the line thickness to the triangle method. It also looks very nice:

There's a very slight (~1%) overhead in shading, as triangles have to be fetched and transformed to world space, but both of the renderings above at full HD finished in under six minutes with 676 samples per pixel.

When I sort out the texturing infrastructure to make it more flexible, it should be very easy to apply this texture as an alpha texture for a fully-3-dimensional mesh that is able to be seen through and cast shadows.

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