1. Computer Graphics: Programming, Problem Solving, and Visual Communication
Steve Cunningham
California State University Stanislaus and Grinnell College
PowerPoint Instructor’s Resource

2. Implementing Modeling in OpenGL
Going from concepts to images

3. Modeling Topics Specifying geometry GLU and GLUT tools Transformations
Transformation Stack
Drawing text
Inverting the eyepoint transformation
Display lists

4. Specifying Geometry
General model is glBegin(grouping_mode); vertex list glEnd();
The grouping mode identifies how the vertices are to be used
The vertices in the vertex list can be done manually or by computation

5. Specifying Geometry (2)
There are many different grouping modes that OpenGL recognizes
Simple modes:
GL_POINTS
GL_LINES
GL_TRIANGLES
GL_QUADS
GL_POLYGON

6. Specifying Geometry (3)
Geometry compression modes:
GL_LINE_STRIP
GL_TRIANGLE_STRIP
GL_TRIANGLE_FAN
GL_QUAD_STRIP

7. Specifying Geometry (4)
Some details of OpenGL geometry:
All OpenGL polygons are assumed to be convex
Polygons use vertices as if they were from a triangle fan
Quad strips use vertices as if they were for triangle strips, so the order is different from GL_QUADS

8. Specifying Geometry (5)
The vertex list can use vertices of many different types
The vertex data type is specified in the the particular glVertex* function name
glVertex[2|3][i|f|d]{v}
There are additional options, but they are not often used

9. Specifying Geometry (6)
As you specify the geometry with the vertices, you can specify other data for each vertex (e.g. appearance data)
glColor*(…)
glNormal*(…)
glTexCoord*(…)
Vertices can be given individually or as vertex arrays, reducing function calls

10. Some Sample OpenGL Objects
Triangle fan used to create a cone
Triangle strips used to create a surface (one strip shown)

11. Appearance Information
Simple OpenGL drawing uses edges that are one pixel wide and are not antialiased
You can specify point sizes and line width
You can antialias points, lines, and polygon edges
glEnable(type) as appropriate
glHint(type, value)

12. Clipping
Besides the automatic clipping on the view volume, you can specify other clipping by defining clip planes
Clip planes are defined by the coordinates in the plane equation
glClipPlane(number,plane)
Clip planes may be enabled or disabled as your program runs

13. GLU Quadric Objects
The GLU utilities include a number of useful objects
gluSphere
gluCylinder
gluDisk
All require that you first create a general GLUquadric pointer:
GLUquadric* gluNewQuadric(void)

14. GLU Quadric Objects (2)
GLU quadric objects are created by invoking a function that uses the GLUquadric* value
The function also defines granularity and size
You can select whether the object is solid or wireframe
You can specify if you want to have normals or texture coordinates generated automatically

15. GLUT Objects
GLUT provides a number of additional graphical objects you can use easily
Cone
Sphere
Cube
Torus
Dodecahedron
Icosahedron
Octahedron
Tetrahedron
Teapot
Each of these can be either wireframe or solid

16. Examples of GLU and GLUT Objects

17. Transformations in OpenGL
Rotation
glRotatef(angle, x, y, z)
Translation
glTranslatef(tx, ty, tz)
Scaling
glScalef(sx, sy, sz)
Individually these are simple, but they can be composed to create complex transformations

18. Transformation Stacks
You can create a stack for any of the standard OpenGL transformations
The modelview transformation stack is critically important to manage the scene graph
glMatrixMode(GL_MODELVIEW);
glPushMatrix(); …
glPopMatrix();

19. Text in OpenGL
Text is handled by a GLUT function that writes a character at a time in the font (usually bitmapped) you choose
glutBitmapCharacter(font, char)
Text you create this way is simply placed on the screen and cannot be selected

20. Inverting the Eyepoints
We saw that it can be useful to move the eyepoint in your model
Of course, you could call glutLookAt(…) with new viewing data each time you redraw the scene
The viewing data uses raw coordinates, though, and this could be difficult to calculate with many eye motions

21. Inverting the Eyepoint (2)
The eyepoint motion will probably be set by using simple transformations
If these simple transformations are T1*T2*T3* … *TN then their inverse is
TN-1*…*T3-1*T2-1*T1-1
and each of the inverses is simple

22. Inverting the Eyepoint (3)
In order to make the eyepoint placement easy, you can simply apply the needed inverse transformations at the top of the scene graph (at the beginning of the display function) and then use the default view

23. Creating Display Lists
Display lists are OpenGL’s way to compile geometry
This lets the system work on geometry that has been optimized, making it faster to draw complex objects
glNewList(i); …
glEndList();
You can then use this geometry with
glCallList(i);