Programming Assignment A4:
Non-Photorealistic Rendering

Revision 2

Demo: 10:30am Tue. 1 May 2001
Animation Due: 11:59pm Wed. 2 May

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.

Requirements

Read in an OBJ or SMF model file.
Display in least the following three styles:
- realistic: standard OpenGL rendering with lighting.
- cartoon: Saturday morning cartoon style, with silhouette edges overdrawn, and highly quantized shading. Edges are traditionally drawn in black, but you can use any (visible) color. Quantization can be explained by an example: An realistic apple normally has a smooth gradation of shades of red, from dark red to red, and red to white. Your stylized results should map those colors to three classes: dark red, red, and white. Thus, most regions of the image will consist of areas of constant color.
- pointillist: display the object as if it's drawn with dabbed brushstrokes.
The user should be able to switch styles interactively.
Do the above styles in real-time (redisplay at 30Hz or faster) while the user spins the object around or moves the camera.
Your images should show reasonable temporal coherence. For example, as the object is rotated, it's not acceptable for your pointillist brushstrokes to be jumping around randomly. It's ok for things to pop or jump occasionally, but not on every frame. (See tip below)
Demonstrate the above on the demo date (we'll ask you to read in a model file consisting of between 3000 and 6000 triangles for testing). Generate animation by the animation date, turned in in the conventional way.

Tips

The following are tips, not requirements.

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().
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 drawn with a soft matte. It could even be a photograph or sketch of a single brushstroke. You'll have to experiment with point placement to tune brush size and number 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 into an integer that is then used to look up an array of psuedo-random numbers.
If generating animation by hand (rotate object, save image, repeat) the resulting motion can be very 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.

Non-Photorealistic Rendering Examples
We suggest you look at the pictures at the following web sites for visual inspiration or for optimization tips. Reading the papers below in full is not necessary to do this assignment, but you might find them interesting.

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

Traditional Paintings

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

Out of 12 points:

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

Extra credit for real-time

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 desired color (and frequency content?) matches that desired. You could scour the web to collect a few thousand pictures. A few sources for images: ditto.com , astronomy pic of day , Yahoo picture gallery , Getty Museum , Photo Bank ,
interactive placement of brushstrokes using cursor
other cool ideas

Change log:
16 Apr. 2001: revision 1
18 Apr. 2001: minor clarifications & photo source list added

15-463, Computer Graphics 2
Paul Heckbert

Programming Assignment A4: Non-Photorealistic Rendering

Requirements

Tips

Grading Criteria

Programming Assignment A4:
Non-Photorealistic Rendering