Advanced Computer Graphics: Teddy

1. Advanced Computer Graphics:Teddy James Gain Department of Computer ScienceUniversity of Cape Town jgain@cs.uct.ac.za Advanced Computer GraphicsCollaborative Visual Computing Laboratory

2. Modelling Interfaces • Most modelling systems are difficult and tedious to use: • Expose the underlying mathematics. Problematic for artists. • 2D Input and Output to an inherently 3D task. • Teddy (A Sketching Interface for 3D Free-Form Design): • Interactively and automatically constructs 3D polygonal surfaces from 2D silhouettes. • Uses: gesture recognition, polygon-mesh representation, pencil shaded rendering, virtual trackball camera manipulation and 3D from 2D extrapolation. • Good for exploratory design. • Limitations: no holes (objects are topologically equivalent to a sphere), inflation approach means that broad and thin models are more difficult to achieve. Advanced Computer Graphics

3. Creation Operations • Creation: user sketches a closed 2D curve. Converted to a polygon mesh by inflation. Degree of inflation depends on width – wide areas become fat, narrow areas become thin. • Rotation: right mouse button changes camera viewpoint using a virtual trackball (cf textbook). Advanced Computer Graphics

4. Modification Operations • Surface Detail: An unclosed curve drawn on the canvas within the silhouette of the object is projected onto the object’s surface as a set of 3D line segments. • Extrusion: Two stroke operation. A closed curve within the silhouette acts as the extrusion base. The model is rotated. A curve starting and ending at the base ring is then used to define the extrusion shape. Advanced Computer Graphics

5. Deletion Operations • Cutting: A stroke which runs across the object, starting and terminating outside the silhouette defines the removal of object material. Spot the usability problem. • Erasing, smoothing: A scribble can be used to erase surface detail (within the silhouette) or smooth the surface (within a previously sketched base ring). Advanced Computer Graphics

6. Creation Process • Delauney triangulation of stroke interior • Categorize triangles • (terminal – 2 exterior edges, sleeve – 1 exterior edge, junction – 0 exterior edges) • Prune terminal triangles • Create central spine • Elevate spine in proportion to average distance from connected exterior vertices. Advanced Computer Graphics

7. Surface Painting • Create swathe from camera viewpoint through each stroke line segment. • Find closest surface line segments by intersecting the object and swathe. • Algorithm fails if surface line segments cannot be connected (due to fold in object). Advanced Computer Graphics

8. Extrusion • Base ring projected onto object’s surface. • 2D extruding stroke projected onto plane perpendicular to object’s surface. • Base ring swept and resized along extruding stroke. Advanced Computer Graphics

9. Cutting • Create swathe from camera viewpoint through each stroke line segment. • Find and join by triangulation the front and back intersections of the object and swathe. • Remove original object triangles on the left of the stroke, splice in new cutting triangles. Advanced Computer Graphics

10. Teddy Summary • Simple, intuitive sketching interface. • Some work also done on deforming (bending and distorting) objects. • Advantages: • Overcomes some of the limitations of designing 3D objects in a 2D environment. • Hides mathematical details. • Disadvantages: • Can fail for pathological input strokes. • Cannot create completely general objects. • Reference: T. Igarashi, S. Matsuoka and H. Tanaka, “Teddy: A Sketching Interface for 3D Freeform Design”, Proceedings of SIGGRAPH ’99, August 1999, pp. 409-416. (hand out) • Web-site: http://www-ui.is.s.u-tokyo.ac.jp/~takeo/teddy/teddy.htm Advanced Computer Graphics