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

Advanced Computer Graphics 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. 18/06/2019 Advanced Computer Graphics

Advanced Computer Graphics 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). 18/06/2019 Advanced Computer Graphics

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. 18/06/2019 Advanced Computer Graphics

Advanced Computer Graphics 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). 18/06/2019 Advanced Computer Graphics

Advanced Computer Graphics 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. 18/06/2019 Advanced Computer Graphics

Advanced Computer Graphics 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). 18/06/2019 Advanced Computer Graphics

Advanced Computer Graphics 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. 18/06/2019 Advanced Computer Graphics

Advanced Computer Graphics 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. 18/06/2019 Advanced Computer Graphics

Advanced Computer Graphics 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 18/06/2019 Advanced Computer Graphics