#5: Models & ScenesCSE167: Computer GraphicsInstructor: Ronen BarzelUCSD, Winter 20061Outline For TodayScene Graphs Shapes Tessellation2Modeling by writing a program First two projects: Scene hard-coded in the model The scene exists only in the drawScene() method Advantages: Simple, Direct Problems Code gets complex Special-purpose, hard to change Special-purpose, hard to make many variants Can’t easily examine or manipulate models• Can only “draw”3Sample Scene4Schematic Diagram (Top View)5Top view with Coordinates6Hierarchical Transforms Last week, introduced hierarchical transforms Scene hierarchy:7Data structure for hierarchical scene Want: Collection of individual models/objects Organized in groups Related via hierarchical transformations Use a tree structure Each node: Has associated local coordinates Can define a shape to draw in local coordinates Can have children that inherit its local coordinates Typically, different classes of nodes: “Transform nodes” that affect the local coordinates “Shape nodes” that define shapes8Scene Tree9Node base class A Node base class might support: getLocalTransform() -- matrix puts node’s frame in parent’s coordinates getGeometry() -- description of geometry in this node (later today) getChild(i) -- access child nodes• addChild(), deleteChild() -- modify the scene Subclasses for different kinds of transforms, shapes, etc. Note: many designs possible Concepts are the same, details differ Optimize for: speed (games), memory (large-scale visualization),editing flexibility (modeling systems), rendering flexibility(production systems), … In our case: optimize for pedagogy & projects10Node base classclass Node {// dataMatrix localTransform;Geometry *geometry;Node *children[N];int numChildren; // methods:getLocalTransform() { return localTransform; }getGeometry() { return geom; }getChild(i) { return children[i]; }addChild(Node *c) { children[numChildren++] = c; }}11Draw by traversing the treedraw(Node node) { PushCTM(); Transform(node.getLocalTransform()); drawGeometry(node.getGeometry()); for (i=0; i<node.numChildren; ++i) { draw(node.child[i]); } PopCTM();} Effect is same hierarchical transformation as last week12Modify the scene Change tree structure Add nodes Delete nodes Rearrange nodes Change tree contents Change transform matrix Change shape geometry data Define subclasses for different kinds of nodes Subclass has parameters specific to its function Changing parameter causes base info to update13Example: Translation Nodeclass Translation(Transformation) { private: float x,y,z; void update() { localTransfom.MakeTranslation(x,y,z); } public: void setTranslation(float tx, float ty, float tz) { x = tx; y = ty; z = tz; update(); } void setX(float tx) { x = tx; update(); } void setY(float ty) { y = ty; update(); } void setZ(float tz) { z = tz; update(); }}14Example: Rotation Nodeclass Rotation(Transformation) { private: Vector3 axis; float angle; void update() { localTransfom.MakeRotateAxisAngle(axis,angle); } public: void setAxis(Vector3 v) { axis = v; axis.Normalize(); update(); } void setAngle(float a) { angle = a; localTransfom.MakeRotateAxisAngle(axis,angle); }}15More detailed scene graph16Building this sceneWORLD = new Node();table1Trans = new Translation(…); WORLD.addChild(table1Trans);table1Rot = newRotation(…); table1Trans.addChild(table1Rot);table1 = makeTable(); table1Rot.addChild(table1);top1Trans = new Translation(…); table1Rot.addChild(top1Trans);lampTrans = new Translation(…); top1Trans.addChild(lampTrans);lamp = makeLamp(); lampTrans.addChild(lamp);book1Trans = new Translation(…); top1Trans.addChild(book1Trans);book1Rot = newRotation(…); book1Trans.addChild(book1Rot);book1 = makebook(); book1Rot.addChild(book1);book2Trans = new Translation(…); top1Trans.addChild(book2Trans);book2Rot = newRotation(…); book2Trans.addChild(book2Rot);book2 = makebook(); book2Rot.addChild(book1);table2Trans = new Translation(…); WORLD.addChild(table2Trans);table2Rot = newRotation(…); table2Trans.addChild(table2Rot);table2 = makeTable(); table2Rot.addChild(table2);top2Trans = new Translation(…); table2Rot.addChild(top2Trans);… Still building the scene hardwired in the program But now can more easily manipulate it…17Change scene Change a transform in the tree: table1Rot.setAngle(23); Table rotates, everything on the table moves with it Allows easy animation Build scene once at start of program Update parameters to draw each frame e.g. Solar system:drawScene() { sunSpin.setAngle(g_Rotation); earthSpin.setAngle(3*g_Rotation); earthOrbit.setAngle(2*g_Rotation); moonOrbit.setAngle(8*g_Rotation); draw(WORLD);} Allows interactive model manipulation tools e.g. button to add a book• Create subtree with transforms and book shape• Insert as child of table18Not just transform nodes Shape nodes Contain geometry:• cube, sphere (later today)• curved surfaces (next week)• Etc… Can have nodes that control structure Switch/Select: parameters choose whether or which children to enable Group nodes that encapsulate subtrees Etc… Can have nodes that define other properties: Color Material Lights Camera Etc… Again, different details for different designs19Java3D Scene Graph20OpenInventor Scene Graph21Maya “Hypergraph”22Scene vs. Model No real difference between a scene and a model A scene is typically a collection of “models” (or “objects”) Each model may be built from “parts” Use the scene graph structure Scene typically includes cameras, lights, etc. in the graph;Model typically doesn’t (but can)23Parameteric models Parameters for: Relationship between parts Shape of individual parts Hierarchical relationship between parts Modeling robots separate rigid parts Parameters for joint angles Hierarchy:• Rooted at pelvis: Move pelvis, whole body moves• Neck & Head: subtree; move neck and head, or just move head• Arms: Shoulder, Elbow, Wrist joints• Legs: Hips, Knee, Ankle joints This model idiom is known as: an Articulated figure Often talk about degrees of freedom (DOFs)• Total number of float parameters in the model24Robot25Screen Graph, not Tree
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