Hatching
A while ago I read this paper and finally got around to implementing it this week:
Real-Time Hatching
Emil Praun, Hugues Hoppe, Matthew Webb, Adam Finkelstein
Appears in SIGGRAPH 2001
The key concept in the paper is the "tonal art map", in which strokes are added to a texture array's mipmap levels to preserve coherence between levels and across tones - each stroke in an image is also present in all images above and to the right:
My possibly-novel contribution is to use the inverse (fast) Fourier transform (IFFT) to generate blue noise for the tonal art map generation. This takes a fraction of a second, compared to the many hours for void-and-cluster methods at large image sizes. The quality may be lower, but something to investigate another time - it's good enough for this hatching experiment. Here's a contrast of white and blue noise, the blue noise is perceptually much more smooth, lacking low-frequency components:
The other parts of the paper I haven't implemented yet, namely adjusting the hatching to match the principal curvature directions of the surface. This is more a mesh parameterization problem - I'm being simple and generating UVs for the bunny by spherical projection, instead of something complicated and good-looking.
My code is here:
git clone https://code.mathr.co.uk/hatching.git
Note that there are horrible hacks in the shaders for the specific scene
geometry at the moment, hopefully I'll find time to clean it up and make it
more general soon. You'll need to download the bunny.obj from
cs5721f07.