Our goal was to transform a 3-D rendering with an already existing lighting model into a “cartoony” version.  We focused on the creation of thick lines to give the final image a two-dimensional and perhaps even hand-draw quality.  Based on ideas in Takafumi Saito and Tokiichiro Takahashi’s paper, Comprehensible Rendering of 3-D Shapes, from 1990, we applied edge detection filters on a ray-traced image implementing the Phong model. 

             The paper focused on post-processing techniques applied after the geometric and physical processes.  Geometric processes include shape modeling and camera setup, while physical processes include lighting and textures.  We can modify the outputted pixels of these during the abstract process, which allows for an “artistic” touch.  The paper suggested using the Sobel filter for thicker lines and their own variation of a second order filter for thinner lines. 

             Although the notion of G-Buffers, which provide geometric and physical information for each pixel, was another central concept, we did not implement these as most of the edge detection processes do not require them.

CS384G Final Project

Sarah Abraham and Sarah Imboden

December 12, 2008

             Instead we took an existing ray-tracer and modified it to apply our choice of edge-detecting filters.  We implemented the Sobel filter (along with the paper’s recommendation for normalizing it), but their second order filter resulted in strange, if nifty, artefacts (see left), so we decided to use a Laplacian filter for thin lines.  Based on these filters’ outputs, we added black lines to the ray-traced image achieving a cartoon style. 

             Finally we implemented two versions of anti-aliasing.  Based on the pixel intensities after filtering, we hoped by picking a suitable threshold to interpolate over, it would smooth out the transition between line and image.  Because it did not, we also implemented a version that “blurred” the line by modifying the pixels adjacent to it.  The result was satisfactory so long as the images have a high enough resolution.  Indeed the nature of the technique (modifying pixels after rendering) requires enough of them that lines do not appear “smeared” as they do at lower resolutions (see below)

Text Box: Original Second Order FilterText Box: Hi Resolution BlurredText Box: Low Resolution BlurredText Box: High Resolution No Anti-Aliasing