OpenGL review session

1. Introduction – why OpenGL

• What does OpenGL do

  • Draw stuff "onscreen", in 2D and 3D

• What doesn't it do

  • Windows, menus, define what "onscreen" is, share the display, input devices

• GLUT as windowing interface

  • Makes up for most of the things in the "What doesn't it do" section

  • OpenGL + GLUT + {insert your windowing system here} make a pretty complete combination.

  • OGL/GLUT app should be portable across the various Unix platforms and Win32

2. GLUT overview

• Mostly one-time initialization

See sample code in /usr/class/cs248/assignments/assigment3/example/simple

Example:

  // GLUT Window Initialization:
  glutInit (&argc, argv);
  glutInitWindowSize (g_Width, g_Height);
  glutInitDisplayMode ( GLUT_RGB | GLUT_DOUBLE | GLUT_DEPTH);
  glutCreateWindow ("CS248 GLUT example");

• Callback model (like xsupport)

Example:

  // Register callbacks:
  glutDisplayFunc (display);
  glutXXXXXXXFunc (callbackName);
  ...
  glutMainLoop();

• More info:

GLUT documentation at http://reality.sgi.com/mjk/glut3/glut3.html

3. OpenGL overview

• Function names like glXXX

• Data types like 2i, 3f, fv

• glBegin, glEnd to specify polygons

• glVertex, glNormal to specify vertices inside polygons

• state management

  • many functions apply to current state

  • have lots of other functions to select/modify current state

4. Rendering in OpenGL – basic 2D

   • How to get a basic 2D drawing onscreen

   • Clearing screen – glClearColor, glClear

   • Viewport – glViewport

   • Orthographic projection – glOrtho

   • Specifying colors – glColor*

   • Specifying vertices – glVertex*

   • Making polygons out of vertices (glBegin/glEnd)

5. Rendering in OpenGL – 3D

Overview of pipeline

Viewing

Transformation, 2 matrices
Backface culling
Clipping
Depth test

Lighting

Diffuse, specular, ambient
Multiple lights
Depends on normal vectors -- glNormal

Shading

Based on light position, materials, normals
Calculated for each vertex, interpolated to fill polygon (Gouraud shading)

Transformation model

2 matrices: projection, modelview

glLoadIdentity

Projection

glOrtho, gluPerspective

Common modelview transformations

for viewing

for model: glRotate, glTranslate, glScale

General transformations: glMultMatrix

glPushMatrix, glPopMatrix

Rendering primitives

Vertices, polygons, glBegin/glEnd as before

Z buffer (hidden surface removal)

clear depth buffer when clearing frame buffer

glEnable (GL_DEPTH_TEST), glDepthFunc

Texture mapping

Enabling texture modes

Enable texturing – glEnable (GL_TEXTURE_2D)

Bind texture to context -- glBindTexture

Build mipmaps – gluBuild2DMipmaps

Set filtering options – glTexParameteri

Specifying texture coordinates

By hand – glTexCoord*

Automatically -- glTexGen

6. Performance considerations

Triangle strips – good

Vertex arrays – often good

Display lists – sometimes good

Texture mapping

Mipmaps

Hardware vs. software

7. Gotchas

Lighting

Clipping – esp. near/far planes

Avoid the dreaded “black screen”

 make small reversible changes to your code until you get a feel for what does what

8. More hints

Use man pages on SGI's

man -k concept | grep ^gl

last year's review session notes

http://graphics.stanford.edu/courses/cs248-99/OpenGLSession/248opengl.html