Models & Hierarchies

1. Models & Hierarchies CSE167: Computer Graphics Instructor: Steve Rotenberg UCSD, Fall 2006

2. Normals • The concept of normals is essential to lighting • Intuitively, we might think of a flat triangle as having a constant normal across the front face • However, in computer graphics, it is most common to specify normals and perform lighting at the vertices • This gives us a method of modeling smooth surfaces as a mesh of triangles with ‘shared’ normals at the vertices • We will talk about lighting in the next lecture, but for today, we will still think of our vertex as containing a normal

3. Models • We will extend our concept of a Model to include normals • We can do this by simply extending our vertex class: class Vertex { Vector3 Position; Vector3 Color; Vector3 Normal; public: void Draw() { glColor3f(Color.x, Color.y, Color.z); glNormal3f(Normal.x, Normal.y, Normal.z); glVertex3f(Position.x, Position.y, Position.z); // This has to be last } }

4. Model Data Structures • Everybody knows that a cube has 8 vertices • If we need to render a cube, however, each of those vertices requires 3 different normals. In other words, we might really need 3*8=24 vertices • If we render it as triangles, each 4-sided face actually requires 6 vertices, meaning that we might need to store and process 36 different vertices!

5. Indexed Models • So far, we have simply thought of a model as an array of triangles, each triangle storing 3 unique vertices • A more common method of storing a model is as an array of vertices, and an array of triangles • In the second method, each triangle stores an index (or pointer) to a vertex instead of storing the vertex data explicitly • This is called an indexed model or single indexed model • Indexing will almost always save memory, as models often have shared vertices that are used by several triangles • Large, smooth meshes will often share a single vertex between 4-6 triangles (or more) • Indexing can also save processing time as the vertex array can first be transformed and lit, and then the triangle array can be clipped and scan converted…

6. Single Indexed Model class Vertex { Vector3 Position; Vector3 Color; Vector3 Normal; }; class Triangle { Vertex *Vert[3]; // or int Vert[3]; }; class Model { int NumVerts,NumTris; Vertex *Vert; Triangle *Tri; };

7. Index vs. Pointer • Should we store the triangle verts as integers (indexing into the array of actual Vertex’s) ? int Vert[3]; • Or should we store them as pointers to the actual Vertex’s themselves ? Vertex *Vert[3]; • Memory: • In most systems an int is 4 bytes and a pointer is 4 bytes, so there isn’t a big difference in memory • However, for smaller models, you could benefit from using short ints, which are 2 bytes each. This would cut the triangle size in half, but limit you to 65536 vertices • Performance: • Storing Vertex*’s gives the triangle direct access to the data so should be faster • Other Issues: • It’s definitely more convenient to store the pointers instead of integers • One important reason to consider storing integers instead of pointers, however, is if you are using some type of dynamic array for the vertices (such as an STL vector). Pointers to members of these arrays are considered dangerous, since the array may have to reallocate itself if more vertices are added

8. Vertex Buffers • Hardware rendering API’s (like Direct3D and OpenGL) support some type of vertex buffer system as well (but everybody has a different name for it) • This is essentially an unindexed or single indexed model format • You start by defining your specific vertex type. Verts usually have a position and normal, and might have one or more colors, texture coordinates, or other properties • You then request a vertex buffer of whatever memory size you want. This memory is usually the actual video memory on the graphics board (not main memory) • The vertex buffer can then be filled up with vertex data as a single large array • One can then draw from the vertex buffer with a command like this: DrawSomething(int type,int first vert,int numverts); // type: triangles, strips, lines… • The advantage is that a large number of triangles can be drawn with a single CPU call and all of the work then takes place entirely on the graphics board

9. Index Buffers • An index buffer (or whatever name one calls it) is an array of (usually 2 byte or 4 byte) integers • It is stored in video memory like the vertex buffer • The integers index into a vertex buffer array • One can then draw triangles (or other primitives) by specifying a range of these indexes • Using vertex/index buffers is most likely going to be the fastest way to render on modern graphics hardware

10. Triangles, Strips, Fans • Graphics hardware usually supports slightly more elaborate primitives than single triangles • Most common extensions are strips and fans v8 v6 v4 v7 v2 v4 v3 v5 v5 v3 v0 v2 v6 v1 v1 v0 v7

11. Materials & Grouping • Usually models are made up from several different materials • The triangles are usually grouped and drawn by material

12. Hierarchical Transformations

13. Hierarchical Transformations • We have seen how a matrix can be used to place an individual object into a virtual 3D world • Sometimes, we have objects that are grouped together in some way • For example, we might have an articulated figure which contains several rigid components connected together in some fashion • Or we might have several objects sitting on a tray that is being carried around • Or we might have a bunch of moons and planets orbiting around in a solar system • Or we might have a hotel with 1000 rooms, each room containing a bed, chairs, table, etc. • In each of these cases, the placement of objects is described more easily when one considers their locations relative to each other • We will see how hierarchical transformations can be used to describe their placement

14. Scene Hierarchy • If a scene contains 1000 objects, we might think of a simple organization like this: Scene Object 1 Object 2 Object 3 … Object 1000

15. Scene Hierarchy • Or we could go for a more hierarchical grouping like: Scene Room 1 Room 2 Room 3 etc… Bed Dresser Chair 1 Chair 2 Table Book Monitor

16. Scene Hierarchy • It is very common in computer graphics to define a complex scene in some sort of hierarchical fashion • The individual objects are grouped into a tree like structure (upside down tree) • Each moving part is a single node in the tree • The node at the top is the root node • A node directly above another is that node’s parent • A node below another is a child and nodes with the same parent are called siblings • Nodes at the bottom of the tree with no children are called leaf nodes

17. Articulated Figures • An articulated figure is an example of a hierarchical object • The moving parts can be arranged into a tree data structure if we choose some particular piece as the ‘root’ • For an articulated figure (like a biped character), we usually choose the root to be somewhere near the center of the torso • Each joint in the figure has specific allowable degrees of freedom (DOFs) that define the range of possible poses for the figure

18. Example Articulated Figure

19. Hierarchical Transformations • We assume that each node in the tree graph represents some object that has a matrix describing its location and a model describing its geometry • When a node up in the tree moves its matrix, it takes its children with it (in other words, rotating a character’s shoulder joint will cause the elbow, wrist, and fingers to move as well)

20. Local Matrices • We will assume a tree structure where child nodes inherit transformations from the parent nodes • Each node in the tree stores a local matrix which is its transformation relative to its parent • To compute a node’s world space matrix, we need to concatenate its local matrix with its parent’s world matrix: W=Wparent·L

21. Recursive Traversal • To compute all of the world matrices in the scene, we can traverse the tree in a depth-first traversal • As each node is traversed, we compute its world space matrix • By the time a node is traversed, we are guaranteed that the parent’s world matrix is available

22. Forward Kinematics • In the recursive tree traversal, each joint first computes its local matrix L based on the values of its DOFs and some formula representative of the joint type: Local matrix L = Ljoint(φ1,φ2,…,φN) • Then, world matrix W is computed by concatenating L with the world matrix of the parent joint World matrix W = Wparent ·L

23. GL Matrix Stack • The GL matrix stack is set up to facilitate the rendering of hierarchical scenes • While traversing the tree, we can call glPushMatrix() when going down a level and glPopMatrix() when coming back up

24. Hierarchical Culling • Scene hierarchies can also assist in the culling process • Each object has a precomputed bounding sphere • This sphere is compared against the view volume to determine if the object is visible • We can also do hierarchical culling where each sphere contains all of its children as well • Culling a sphere automatically culls an entire subtree of the scene

25. Project 2 • In project 2, you are must create some sort of simple articulated figure, such as a hand • It must perform some simple animation (such as opening/closing the fingers) • It must be object oriented and make use of classes for key objects such as: Camera, Light, Model, Hand…

26. Cameras Camera { float FOV, Aspect, NearClip, FarClip; Vector3 Position, Target; public: Camera(); void DrawBegin(); void DrawEnd(); void SetAspect(float a); };

27. Camera void Camera::DrawBegin() { glClear(); glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(FOV,Aspect,NearClip,FarClip); glMatrixMode(GL_MODELVIEW); glLoadIdentity(); gluLookAt(Position.x,Position,y,Position.z, Target.x,Target.y,Target.z,0,1,0); glPushMatrix(); } void Camera::DrawBegin() { glPopMatrix(); glSwapBuffers(); }