1. CS 248 OpenGL Help Session CS248 Presented by Sean Walker
(adapted from Ian Buck’s slides – see ’03 web page)
Stanford University
Nov. 5, 2004

2. Overview What is OpenGL? Basic primitives and rendering in OpenGL
Transformations and viewing
GLUT and the interaction / display loop
Buffers, Lighting, and Texturing
Other features
Development tips
Note: all page references refer to the OpenGL Programming Guide, 4th Edition ver. 1.4 (aka “The Red Book”) unless noted otherwise.

3. OpenGL is a software interface to graphics hardware.
What is OpenGL?
OpenGL is a software interface to graphics hardware.
Application
3-D world
Simulation
User Interface
Graphics Hardware
(rasterizer, texturing,
lighting, transformations,
etc.)
OpenGL
Geometry, vertices, normals, colors, texture, etc.

4. OpenGL Basics
Read the Red Book! It’s a great resource and is very readable.
OpenGL is a state machine: polygons are affected by the current color, transformation, drawing mode, etc.
Everything in the OpenGL specification is supported in every implementation

5. Specifying Object Vertices (Ch.2 p.42, Ch.4 p.171)
Every object is specified by vertices
glVertex3f (2.0, 4.1, 6.0);
glVertex2i (4, 5);
glVertex3fv (vector);
Current color affects any vertices
glColor3f (0.0, 0.5, 1.0);
glColor4ub (0, 128, 255, 0);
glColor3dv (color);

6. Specifying Object Vertices (Ch.2 p.42)
Vertices are specified only between glBegin(mode) and glEnd(), usually in a counter-clockwise order for polygons.
glBegin (GL_TRIANGLES); glVertex2i (0, 0); glVertex2i (2, 0); glVertex2i (1, 1);
glEnd();

7. Primitive Types in glBegin (Ch.2, p.44)
Points GL_POINTS
Lines GL_LINES, GL_LINE_STRIP, GL_LINE_LOOP
Triangles GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN
Quads GL_QUADS, GL_QUAD_STRIP
Polygons GL_POLYGON
Nate Robins' Tutorial: shapes
(show page 45)

8. Transformations and Viewing (Ch.3)
OpenGL has 3 different matrix modes:
GL_MODELVIEW
GL_PROJECTION
GL_TEXTURE
Choose the matrix with: glMatrixMode(…);

9. OpenGL: Modelview matrix
Transforms objects within the scene.
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
glTranslatef(10.5, 0, 0);
glRotatef(45, 0, 0, 1);
DrawCube();
Remember that the operations are right multiplied, so the transformation just before DrawCube() takes effect first.
Tutorial: transformation

10. OpenGL: Projection Matrix
Sets up a perspective projection. (page 127)
glFrustrum (...);
gluPerspective (fovy, aspect, near, far);
glOrtho (...);
gluLookAt (...);
(often applied to modelview matrix)

11. OpenGL: Projection Matrix
Example: glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(64, (float)windowWidth / (float)windowHeight, 4, 4096);
gluLookAt(0.0, 0.0, 2.0, // camera position
0.0, 0.0, 0.0, // target position
0.0, 0.0, 2.0); // up vector
Tutorial: projection

12. GLUT – OpenGL Utility Toolkit (Appendix D)
GLUT is a library that handles system events and windowing across multiple platforms
Includes some nice utilities
We strongly suggest you use it
Find it at:

13. GLUT – Starting Up int main (int argc, char *argv[])
{ glutInit(&argc, argv);
glutInitDisplayMode (GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);
glutInitWindowSize (windowWidth, windowHeight);
glutInitWindowPosition (0, 0);
glutCreateWindow (“248 Video Game!");
SetStates(); // Initialize rendering states*
RegisterCallbacks(); // Set event callbacks*
glutMainLoop(); // Start GLUT
return 0; }
* Your code here

14. Setting Up Rendering States
OpenGL is a state machine: polygons are affected by the current color, transformation, drawing mode, etc.
Enable and disable features such as lighting, texturing, and alpha blending.
glEnable (GL_LIGHTING);
glDisable (GL_FOG);
Forgetting to enable something is a common source of bugs!

15. GLUT Event Callbacks
Register functions that are called when certain events occur.
Examples:
glutDisplayFunc( Display );
glutKeyboardFunc( Keyboard );
glutReshapeFunc( Reshape );
glutMouseFunc( Mouse );
glutPassiveMotionFunc( PassiveFunc );
glutMotionFunc( MouseDraggedFunc );
glutIdleFunc( Idle );

16. OpenGL Buffers Multiple types of buffers Clearing buffers:
Color buffers (front/back, left/right)
Depth buffer (hidden surface removal)
Stencil buffer (allows masking or stenciling)
Accumulation buffer (antialiasing, depth of field)
Clearing buffers:
// Clear to this color when screen is cleared. glClearColor (0.0, 0.0, 0.0, 0.0); // Clear color and depth buffers. glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

17. OpenGL Double Buffering
Draw on back buffer while front buffer is being displayed.
When finished drawing, swap the two, and begin work on the new back buffer.
glutSwapBuffers();
Primary purpose: eliminate flicker

18. OpenGL: Normals and Lighting
OpenGL can do lighting computations for you
Normal vectors should be of unit length (normalized) in most cases.
Normal vector kept as state – each vertex is assigned the most recently set normal vector
...
glNormal3fv (n0); glVertex3fv (v0); glVertex3fv (v1); glVertex3fv (v2);

19. OpenGL: Lighting (Ch.5 p.178)
glEnable (GL_LIGHTING);
OpenGL supports a minimum of 8 lights.
glEnable (GL_LIGHT0); glEnable (GL_LIGHT7);
Lights have a position, type, and color, among other things (more details in text).
Types of lights are point light, directional light, and spotlight. (page 191)
Tutorial: lightposition

20. OpenGL: Shading
OpenGL supports 2 basic shading models: flat and smooth.
glShadeModel(GL_FLAT); glShadeModel(GL_SMOOTH);

21. OpenGL: Material Properties (Ch.5)
Material properties are associated with each polygon (corresponding light properties)
glMaterial*(GLenum face, GLenum pname, TYPE param);
Some properties (pname), page 206:
GL_AMBIENT: Ambient color of material
GL_DIFFUSE: Diffuse color of material
GL_SPECULAR: Specular component (for highlights)
GL_SHININESS: Specular exponent (intensity of highlight)
Tutorial: lightmaterial

22. OpenGL: Texturing

23. OpenGL: Texturing Loading your data Setting texture state
This can come from an image: ppm, tiff
Or create at run time
Final result is always an array
Setting texture state
Creating texture names with “binding”, scaling the image/data, building Mipmaps, setting filters, etc.

24. OpenGL: Texturing Mapping the texture to the polygon
specify (s,t) texture coordinates for (x,y,z) polygon vertices
texture coordinates (s,t)are from 0,1: glTexCoord2f(s,t);
(x3,y3,z3)
(x1,y1,z1) t
0,1
1,1
1,1 +
0,0
1,0
0,0 s
(x0,y0,z0)
(x2,y2,z2)
Tutorial: Texture
pg 403

25. OpenGL: Advanced Texturing
Advanced texturing techniques
Mipmapping
Multitextures
Automatic texture generation
Let OpenGL determine texture coordinates for you
Environment Mapping
Texture matrix stack
Fragment Shaders
Custom lighting effects

26. OpenGL: Alpha Blending Ch 6, pg 225
When enabled, OpenGL uses the alpha channel to blend a new fragment’s color value with a color in the framebuffer
Useful for overlaying textures or other effects ? + =
New color
Color in framebuffer
(r’,g’,b’,a’)
(r1,g1,b1,a1)
(r0,g0,b0,a0)
“source”
“destination”

27. OpenGL: Fog Simulate atmospheric effects glFog (): Sets fog parameters
glEnable (GL_FOG);
Tutorial: fog

28. OpenGL: Other Features
Display Lists (ch 7): Speed up your game!
Quadrics (ch 11): Pre-made objects
Also look at GLUT’s objects
Evaluators (ch 12): Bezier curves and surfaces
Selection (ch 13): Clicking on game objects with a mouse

29. Development Headers On Windows: On Linux:
Download the GLUT libraries (linked off the proj3 webpage).
You want to link your project with: opengl32.lib, glut32.lib, and glu32.lib. This is under Project->Settings->Link in MS Visual Studio.
On Linux:
GLUT is already installed on the graphics lab PCs.
In your Makefile, compile with flags: -L/usr/lib -lGL -lGLU –lglut
Headers
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
Call glutReportErrors() once each display loop for debugging.
This will report any errors that may have occurred during rendering, such as an illegal operation in a glBegin/glEnd pair.

30. Questions?