1. OpenGL and Projections
David E. Johnson

2. Goals Quick overview of OpenGL
Some derivation of projections and transformations

3. OpenGL
OpenGL is nothing more than a set of functions you call from your program
Most functions map to GPU hardware
Hides the details of the display adapter, operating system, etc.
Has grown to become a flexible general purpose compute platform

4. As a programmer, you need to
Specify the location/parameters of camera.
Specify the geometry (and appearance).
Specify the lights.
OpenGL will compute the resulting 2D image!

5. Basic Pipeline

6. Basic Pipeline

7. OpenGL Hierarchy Several levels of abstraction are provided GL GLU
Lowest level: vertex, matrix manipulation
glVertex3f(point.x, point.y, point.z)
GLU
Helper functions for shapes, transformations
gluPerspective( fovy, aspect, near, far )
GLUT
Highest level: Window and interface management
glutSwapBuffers()
All fairly raw – most people have their own point class which gets transformed to GL arrays as needed.

8. OpenGL Implementations
OpenGL is an API
#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>
Windows, Linux, UNIX, etc. all provide a platform specific implementation.
Windows: opengl32.lib glu32.lib glut32.lib
Linux: -l GL -l GLU –l GLUT

9. OpenGL Conventions OpenGL is largely state based
Calls persist
This instead of having functions with 100s of options
Transformations live on stacks
Layers of transformations possible
Work is done on buffers (color, depth,…)
Many functions have multiple forms:
glVertex2f, glVertex3i, glVertex4dv, etc.
Number indicates number of arguments
Letters indicate type
f: float, d: double, v: pointer, etc.
Programs tend to be event based

10. A simple GLUT program // A simple OpenGL and glut program
#include <GL/gl.h>
#include <GL/glut.h>
void display() {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
glFlush(); }
int main(int argc, char **argv)
glutInit(&argc, argv);
glutInitWindowSize(512,512);
glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH );
glutCreateWindow("The glut hello world program");
glutDisplayFunc(display);
glClearColor(0.0, 0.0, 0.0, 1.0);
glutMainLoop(); // Infinite event loop
return 0;

11. OpenGL: Camera Two things to specify:
Physical location of camera in the scene
Where is the camera?
Which direction is it pointing?
What is the orientation of the camera?
Projection properties of the camera
Depth of field?
Field of view in the x and y directions?

12. The OpenGL Camera Initially the object and camera frames are the same
Default model-view matrix is an identity
The camera is located at origin and points in the negative z direction
OpenGL also specifies a default view volume that is a cube with sides of length 2 centered at the origin
Default projection matrix is an identity

13. Moving Camera Back default frames frames after translation by –d

14. Moving the Camera
We can move the camera to any desired position by a sequence of rotations and translations
Example: side view
Rotate the camera
Move it away from origin
Model-view matrix C = RT

15. OpenGL code
Remember that last transformation specified is first to be applied
glMatrixMode(GL_MODELVIEW)
glLoadIdentity();
glRotatef(90.0, 0.0, 1.0, 0.0);
glTranslatef(0.0, 0.0, -d);

16. The LookAt Function
The GLU library contains the function gluLookAt to form the required modelview matrix through a simple interface
Note the need for setting an up direction
Still need to initialize
Can concatenate with modeling transformations
Example: isometric view of cube aligned with axes
glMatrixMode(GL_MODELVIEW):
glLoadIdentity();
gluLookAt(1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0., );

17. gluLookAt
glLookAt(eyex, eyey, eyez, atx, aty, atz, upx, upy, upz)

18. Projections
See notes