Introduction to OpenGL

Tom Kelliher, CS 320

Feb. 14, 2000

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Assignment

Read 3.1--3.4.

From Last Time

Primitive basics.

Outline

1. Finish up primitive basics: polygons, text, general curved surfaces.

2. Color and color models.

3. 2-D viewing.

Coming Up

Control, interaction.

OpenGL API

Polygon Basics

1. Convex and concave polygons.
   1. Definitions.

   2. Concave polygons can present problems.

   3. Often, hardware and software supports triangles.

   4. 2-D polygon convexity test: for all interior point, it is ``to the right'' when traversing the edges clockwise.

   5. Draw a polygon that won't pass Sze-Ling's ``turn right'' test.

2. OpenGL polygon types: GL_POLYGON, GL_TRIANGLES, GL_QUADS, GL_TRIANGLE_STRIP, GL_QUAD_STRIP, GL_TRIANGLE_FAN.

   What are the strip and fan types?

Text

1. Stroke text:
   1. Specified by vertices.

   2. Can send it through the graphics pipeline to manipulate it (point size, italics face).

   3. Computational-, storage-expensive

   4. Example: Postscript.

2. Raster text:
   1. Specified by a bit-map.

   2. A font is often stored as a strip. Individual characters are copied to the frame buffer via bitblt operations.

   3. Can't process through the graphics pipeline. Changing the point size by changing the pixel size: blockiness. Or by using several ``strips.''

   4. Computational-, storage-inexpensive.

General Curved Surfaces

Tessellation: Mesh of convex polygons. 2-D, 3-D.

Example: circle formed by GL_TRIANGLE_FAN.

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.