OpenGL Functions

GL function names follow a standardized scheme

All begin with "gl", followed by the basic name of the command

When there's only one possible form of arguments:

OpenGL Functions

When arguments can take multiple forms:

e.g. the Color function has the forms:

• glColor3f - 3 floating point arguments
• glColor3i - 3 integer arguments
• glColor3ub - 3 unsigned byte arguments
• glColor3fv - 1 argument, a vector of 3 floating point numbers
• etc.

OpenGL Constants

Constants are defined for certain function arguments

Constant names are all in capital letters, in the form:

e.g. possible arguments for glBegin() are:

• GL_POINTS
• GL_LINES
• GL_TRIANGLES
• GL_TRIANGLE_STRIP
• etc.

GLUT

OpenGL Utility Toolkit

GLUT provides a simple interface for opening & managing windows, and getting input from the keyboard & mouse.

It also defines a basic program structure - an event loop, with callback functions.

GLUT documentation

GLUT Framework

GLUT Events

Events in GLUT include:

• Key pressed
• Key released
• Mouse button pressed
• Mouse button released
• Mouse moved
• Mouse entered/left window
• Window resized
• Window needs redrawing
• Menu selection
• Special devices - tablet, spaceball, dial/buttonbox
• Timer elapsed
• Idle (no events pending)

GLUT Functions - Setup

glutInit(sys.argv)

Initialize GLUT
sys.argv is the command-line arguments

glutInitDisplayMode(mode)

Tell GLUT what GL-related options the window should have
mode is a bitwise-or'ed (the | operator) subset of:

• GLUT_SINGLE - single-buffered
• GLUT_DOUBLE - double-buffered
  
  
• GLUT_RGB - use RGB color
• GLUT_INDEX - use color indexing
  
  
• GLUT_DEPTH - allow depth-buffering

(other options exist, but we won't be using them in this class)

glutInitWindowSize(x,y)

Set the initial size of the window (in pixels)

glutInitWindowPosition(x,y)

Set the window's initial location on the screen (in pixels, from the upper left corner)

glutCreateWindow(title)

Tell GLUT to create the window. The window will not actually appear immediately.
title is the name of the window.

GLUT Functions - Callbacks

glutDisplayFunc(function)

Gives the name of a function to call whenever the graphics must be drawn.

glutKeyboardFunc(function)

Gives the name of a function to call whenever a key is pressed.
function takes 3 arguments - key, x, y. key is the key that was pressed; x & y are the position of the mouse when the key was pressed.

glutKeyboardUpFunc(function)

Gives the name of a function to call whenever a key is released.
function takes 3 arguments - key, x, y. key is the key that was released; x & y are the position of the mouse when the key was pressed.

glutSpecialFunc(function)

Gives the name of a function to call whenever a 'special' key is pressed.
function takes 3 arguments - key, x, y. key is the key that was pressed, e.g. GLUT_KEY_UP, GLUT_KEY_HOME; x & y are the position of the mouse when the key was pressed.

glutMouseFunc(function)

Gives the name of a function to call whenever a mouse button is pressed or released.
function takes 4 arguments - button, state, x, y. button is the button that was pressed - GLUT_LEFT_BUTTON, GLUT_MIDDLE_BUTTON, GLUT_RIGHT_BUFFON; state is either GLUT_DOWN or GLUT_UP; x & y are the position of the mouse when the button's state changed.

glutTimerFunc(msecs, function, value)

Gives the name of a function to call when msecs (or more) milliseconds have elapsed. value is an argument to pass to function.

glutIdleFunc(function)

Gives the name of a function to call when there are no events left to be handled.

GLUT Functions - Other

glutMainLoop()

Starts GLUT's event loop. This function never returns, so code after it will not be executed.

glutPostRedisplay()

Generates a "window needs redrawing" event, forcing GLUT to call your draw function
This can be called from an event callback function, to force redrawing when something changes. If called from the idle function, the window will be constantly redrawn.

GLUT Example

# Minimal program to open & clear a window.
import sys
from OpenGL.GLUT import *
from OpenGL.GL import *
from OpenGL.GLU import *

def draw():
    glClear(GL_COLOR_BUFFER_BIT)
    glFlush()

def keyboard(key, x, y):
    if key == 'q':
        sys.exit(0)
    elif key == ' ':
        print 'Mouse is at', x, y

glutInit([])
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB)
glutInitWindowSize(200, 200)
glutInitWindowPosition(0,0)
glutCreateWindow(sys.argv[0])
glutDisplayFunc(draw)
glutKeyboardFunc(keyboard)
glutMainLoop()

Photorealism

Perception

Color

Colors in the physical world can be any wavelength, or combination of wavelengths, of light

The Eye

Rods & cones absorb light, send signal to brain

Color Perception

Any visible wavelength is perceived the same as some combination of 3 basic colors
(roughly blue, green, and red)

RGB Color

RGB = Red , Green , Blue
Each component (R, G, or B), ranges from a minimum (no intensity) to a maximum (full intensity), typically 0.0 to 1.0.

Color on the Computer

Computer numbers have a finite resolution - how many distinct values can be represented

24 bit color = 8 bits red + 8 bits green + 8 bits blue
(a.k.a. 8 bits per component)
8 bits = 256 possible values

32 bit color usually means 8 bits red + 8 bits green + 8 bits blue + 8 bits alpha
16 bit color can be 5 bits red + 6 bits green + 5 bits blue

HDRI: High Dynamic Range Imaging - uses 16 or 32 bits per component

RGB Photography

Prokudin-Gorskii, ca. 1910

RGB Photography

CMY Color

CMY = Cyan , Magenta , Yellow
    C = 1.0 - R
    M = 1.0 - G
    Y = 1.0 - B

CMYK = Cyan , Magenta , Yellow , Black

HSV Color

HSV = Hue , Saturation , Value

Color Gamuts

Luminance

The "brightness" of a color.

Formula, used in NTSC television standard, based on human perception:

    0.30 * R + 0.59 * G + 0.11 * B

Real-world Luminance

Rods & cones adapt to average level of illumination

Rods most sensitive at low levels (scotopic vision)

Cones more sensitive at higher levels (photopic vision)

This document is by Dave Pape, and is released under a Creative Commons License.