Intro to OpenGL

Actually, we will be using Mesa instead of OpenGL. The difference is not important from our point of view.

My first program

Download the code in threeDots.cc to your linux account. Compile and execute with g++ -o threeDots -L/usr/X11R6/lib -L/usr/lib/X11 -lglut -lGLU -lGL -lm -lX11 -lXi -lXmu threeDots.cc ./threeDots You can download and use the Makefile to ease building your OpenGL applications. In this case, you would just have to [arnold@cslinux ~/320]$ make threeDots g++ -L/usr/X11R6/lib -L/usr/lib/X11 -lglut -lGLU -lGL -lm -lX11 -lXi -lXmu threeDots.cc -o threeDots You can also try out f60.cc and bounce.cc (this shows you how to do animations, the key is glutIdleFunc(idle)). We won't discuss these in this tutorial.

References

Local man page
OpenGL
OpenGL Reference Manual (more on this can be found here)
OpenGL Programmers Guide(more on this can be found here)
OpenGL superbible online resource (access only at UofT or via UofT Proxy Server)
OpenGL tutorial
Introduction to OpenGL
Some neat OpenGL projects
An overview of OpenGL

Understanding OpenGL

OpenGL: device-independent, platform-independent

OpenGL libraries

OpenGL	GLU	GLUT	GLUI
main library; graphics routines header file: gl.h #include <GL/gl.h> command prefix: gl; constant prefix: GL example: glLineWidth(), GL_POLYGON	utility library; commands to set up matrices for viewing orientations and projections, perform polygon tessellation, and render surfaces header file: glu.h #include <GL/glu.h> command prefix: glu; constant prefix: GLU example: gluLookAt()	utility toolkit; window and event management header file: glut.h #include <GL/glut.h> command prefix: glut; constant prefix: GLUT example: glutSolidSphere(), GLUT_DOUBLE	user interface toolkit; buttons, etc. header file: glui.h #include <GL/glui.h>

OpenGL is a State Machine

OpenGL is an immediate-mode API. Each command has an immediate effect based on the current rendering state. Rendering states are flags that specify which features are on and off and how they should be applied. Some examples are such questions as, "Is lighting enabled? Is texturing turned on? Is fog enabled, and what is its density?" Each state variable is either on or off or contains some numeric value. States can be set and read by the OpenGL functions.

See: glEnable, glDisable, glIsEnabled.

Overview of an OpenGL program

Main
- open window and configure frame buffer (GLUT)
- initialize GL states
- initialize display (double buffer, colour mode, etc.)
Loop (GLUT)
- check for events
- if window event
  - modify viewport (if needed)
  - redraw
- else if mouse or keyboard events
  - do something (e.g. move camera, change state, etc.)
Redraw
- clear the screen (and buffers -> Z buffer)
- change states (if desired)
- render
- swap buffers

OpenGL order of operations

in your data structures (OpenGL does not have any)
- construct shapes from primitives (e.g. mathematical representation)
use OpenGL
- arrange shapes in 3D and select your vantage point
- calculate colour or texture properties for each
- convert shapes into pixels on screen

Syntax of OpenGL functions

always with prefix gl (gl, glu, glut, etc.)
example: glVertex3f()
- Vertex - command
- 3 - number of arguments
- f - type of argument
  - i - integer
  - f - float
  - d - double

OpenGL primitives

glBegin(GL_LINES)
    glVertex3f(__, __, __);
    glVertex3f(__, __, __);
    ...
glEnd();

the first two vertices form a line segment, the next two vertices form another line segment, and so on
GL_LINES can be replaced by GL_POINTS, GL_LINE_LOOP, GL_TRIANGLES, GL_POLYGON...(try: man glBegin)

OpenGL states

on/off
- glEnable(GL_POINT_SMOOTH)
- glDisable(GL_POINT_SMOOTH)
- state variables: GL_POINT_SMOOTH, GL_DEPTH_TEST, etc.
mode states
- example: glShadeModel(GL_FLAT), glShadeModel(GL_SMOOTH)
  - GL_FLAT means uniform shading
  - GL_SMOOTH means OpenGL will interpret colours from one vertex to another
- once a mode state is set, it continues to be in effect until reset

Viewing transformations

glMatrixMode(...)
- argument can be GL_MODELVIEW, GL_PROJECTION or GL_TEXTURE
glLoadIdentity()
- sets the current matrix to the identity matrix
glTranslate3f(x, y, z)
glRotate3f(angle, x, y, z)
glScale3f(x, y, z)

3D projection

glFrustum(...) - perspective projection
glOrtho(...) - orthographic projection

Lighting

2 types of lights sources
- directional
- positional
glLightfv(__, __, __) - type and colour of source
- arguments can be (GL_LIGHT0, GL_POSITION, position)
glEnable(GL_LIGHTING)
glEnable(GL_LIGHT0)
can have at least 8 light sources (LIGHT0 to LIGHT7)