Programming Assignment A4:
Non-Photorealistic Rendering

For this assignment, write an OpenGL program that can display a 3-D object in various non-photorealistic styles, in real-time. Also turn in animation created with this program. You are to work individually on this assignment.

• There's no starter code for this assignment. You could work from your assignment 2 code, perhaps.
• It's OK if your program synthesizes geometry (e.g. subdivision surface, fractal, ...).
• Silhouette edges can be found in (at least) two ways: (1) 2-D post-process on image, looking for depth discontinuities, or (2) check all polygon edges to see if one adjacent face is front-facing while the other is backfacing, or if it's a boundary edge (only one adjacent face). The latter method is probably faster. If using that approach, make sure your silhouette edges are drawn into the z-buffer so they're properly occluded by surfaces in front. You'll probably want to use glPolygonOffset().
• Tips on 3-D silhouette-finding.
• Pointillist effects can be done in (at least) two ways: (1) 2-D post-process, where brushstrokes are (more or less) stationary with respect to screen space, or (2) drawing brushstrokes in 3-D as z-buffered splats or sprites. With either method, each brushstroke can be a square (ugly -- points off), circle, or any other shape, perhaps antialiased or drawn with a soft matte. It could even be a photograph or sketch of a single brushstroke. You'll have to tune the number and size of brushstrokes to avoid excessive gaps or overlaps.
• For better temporal coherence: if your pointillist method uses random numbers, you can make them repeatable by, for example, using vertex numbers or edge numbers and hashing them for lookup in an array of psuedo-random numbers.
• If generating animation by hand (rotate object, save image, repeat) the resulting motion can be jerky unless you're very smooth and consistent over time. To reduce this problem, you could make your interactive controls affect not the value of parameters (rotation angle, translation, ...) but their first derivatives. That way, the camera/rotation changes will have momentum, and things will continue to move linearly unless you change the settings. Or you could simply low pass filter (temporally blur) the mouse input.

• Craig Reynolds' collection of NPR links great set of links here
• http://www.dcs.shef.ac.uk/~dan/images.html
• cartoon: http://www.coolpowers.com/products/toonuppro/gallery.html
• http://www.davidgould.com/Gallery.htm, this links demonstrate the different rendering styles for NPR.
• Paul Haeberli's Impressionist java program based on his excellent SIGGRAPH 90 paper Paint By Numbers: Abstract Image Representations (see Reynolds' links above)
• Jeff Lander's articles on Cartoon Rendering
• Ramesh Raskar's silhouette edges, nice images
• Real-Time Hatching, Emil Praun, Hugues Hoppe, Matthew Webb, Adam Finkelstein, SIGGRAPH '01. Hatching (parallel lines for shading) in real time, using texture mapping.
• Illustrating smooth surfaces, Aaron Hertzmann, Denis Zorin, SIGGRAPH '00. We recommend this paper for its short section on fast (sublinear) computation of silhouette edges on a polygon model. That is, it's not necessary to visit all edges in order to find the silhouette edges! Note that use of the simple, linear-time algorithm is probably good enough for this assignment, however. Paper also describes a slow but good algorithm for cross-hatching.
• Pointillism carried to its extreme: polka dot style. A blobby model (PS) by Paul Heckbert and Andy Witkin, SIGGRAPH '94.
• Video Gogh image processing software from RE:Vision Effects
• A Non-Photorealistic Lighting Model For Automatic Technical Illustration, by Amy Gooch, Bruce Gooch, Peter Shirley and Elaine Cohen. SIGGRAPH 98. Modified Phong Model.
• Real-Time Non-photorealistic Rendering, by Lee Markosian, Michael A. Kowalski, Samuel J. Trychin, Lubomir Bourdev, Daniel Goldstein, and John F. Hughes. SIGGRAPH 97. Trades accuracy and detail for speed, uses a method for determining visible lines and surfaces which is a modification of Appel's hidden-line algorithm. Fast, approximate, stochastic silhouette algorithm.
• SIGGRAPH papers online: 2001 , 2000 , 1999 ,

• Paintings. See, e.g. Impressionism. You might want to check out the artists Seurat, Signac, Monet, and Van Gogh.
• Go to the Light exhibit currently on display at the Carnegie Museum of Art down Forbes Ave. (see announcement on 463 web page)

Grading Criteria

• 4 points: cartoon style (we'll check for speed & image quality)
• 4 points: pointillist style (ditto)
• 4 points: nice animation

• halftoning (use OpenGL stencil buffer?)
• hatching
• Kandinsky, Mondrian, or Matisse, ...
• photomosaics. This is just a variation on the pointillist idea above where small pictures are used as brushstrokes, and a picture is found whose color (and frequency content?) matches that desired. You could scour the web, collect a few thousand pictures, and zoom them down. A few sources for images: ditto.com , astronomy pic of day , Yahoo picture gallery , Getty Museum , Photo Bank , animal pictures . The .list files in the a4/pix directory contain lists of pictures found in a breadth-first traversal of those web pages to a depth of 2 or 3.
• interactive placement of brushstrokes using cursor
• characters (glyphs from a font) as brushstrokes
• Postscript for high resolution output (600 dpi on 10 inch page means 6000 pixel resolution!)
• other cool ideas

15-463, Computer Graphics 2
Paul Heckbert