Hatching

hatched bunny

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:

tonal art map

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:

white noise

blue noise

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.