Introduction to OpenGL

Tom Kelliher, CS 320

Feb. 16, 2000

Administrivia

Announcements

Monday is ``lab day.''

Assignment

Have a look at, and run, viewport.c and paint.c (available on the class home page).

From Last Time

Finished primitive basics.

Outline

1. Color and color models.

2. 2-D viewing.

3. OpenGL control functions.

Coming Up

Viewports, interaction.

Color

Additive color model:

Tristimulus value.

Notes:

1. Color is continuous.

2. Our visual system perceives light as a three-color system.

3. Basic tenet of three-color theory: If two colors produce the same tristimulus values, then they are visually indistinguishable.

4. Additive: RGB. Subtractive: CMY.

5. OpenGL has two color modes: RGB and indexed.

RGB Color Mode

1. Depth of the frame buffer. True color: 24 bits. Number of colors?

2. We specify a color as a point within the color cube:

   

   glColor3f(1.0, 0.0, 0.0);
   

3. Alpha value provides opacity factor:

   glClearColor(1.0, 1.0, 1.0, 1.0);
   

Indexed Color Mode

1. Many frame buffers have limited depth, say eight bits. However, the graphics controller may be able to display 16M colors.

2. Suppose: frame buffer has depth k bits. Each color is specified using m bits.

   At any point in time, we should be able to specify any colors from the total collection of colors.

   Use of a color-lookup table:

   

3. Shading, for 3-D graphics, requires a large # of colors. So, we'll stick with RGB mode.

Viewing

2-D viewing:

1. Objects inside the viewing/clipping rectangle are visible:

   What is clipping?

   

2. The default viewing volume is and centered at the origin.

The orthographic projection:

1. Projects the point onto .

2. In 2-D, we generally want to place objects at z=0.

3. OpenGL specification:

   void glOrtho(GLdouble left, GLdouble right,
                GLdouble bottom, GLdouble top,
                GLdouble near, GLdouble far);
   
   void glOrtho2d(GLdouble left, GLdouble right,
                  GLdouble bottom, GLdouble top);
   

   ``Multiply the current matrix with an orthographic matrix'' --- make the projection matrix the current matrix.

4. The OpenGL camera defaults to being positioned at the origin, pointed in the -z direction. The camera can ``see'' behind itself.

OpenGL matrices:

1. Modelview matrix: positions objects in front of the camera.

   Translation, rotation, scaling.

2. Projection matrix: determines shape of the viewing volume.

   Orthographic, perspective.

Control Functions

Several we've already seen:

1. glutInit(int *argcp, char *argv[])

2. glutCreateWindow(char *title)

3. glutInitDisplayMode(unsigned int mode) Example:

   glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH | GLUT_DOUBLE);
   

   Mode bit examples:
   1. GLUT_RGBA, GLUT_RGB, GLUT_INDEX.

   2. GLUT_SINGLE, GLUT_DOUBLE.

   3. GLUT_DEPTH.