Introduction to OpenGL

Tom Kelliher, CS 320

Feb. 11, 2000

Administrivia

Announcements

Assignment

Read 2.5--2.6.

Project due 2/25.

From Last Time

Vertex specification, coordinate systems.

Outline

Coming Up

Viewing and Control.

OpenGL API

Support provided by a graphics API:

1. Primitive functions: points, line segments, polygons, text, curves, surfaces.

2. Attribute functions: color, fill, type face.

3. Viewing functions: attributes of the synthetic camera.

4. Transformation functions: rotation, translation, scaling. Matrix computations.

5. Input functions: keyboard, pointing device.

6. System communication: window system, OS, other workstations, other users.

OpenGL Library Organization:

Primitive Basics

Consider:

glBegin(primitiveType)
   glVertex*( ... );
   // ...
   glVertex*( ... );
glEnd();

1. GL_POINTS .

2. GL_LINES .

3. GL_LINE_STRIP .

4. GL_LINE_LOOP . When is this not a polygon?

Polygon Basics

1. Polygons have interiors which can be filled.

2. What is filling? Color, pattern (bitmap) fills.

3. Simple polygon: no edges cross.

4. When filling, how do we determine if a point is interior? Crossing test.

5. Fill example: five-pointed star. Scanlines. Is this the effect we want?

6. Winding test improvement:
   1. Pick any vertex and direction.

   2. Traverse the polygon, labeling each edge with direction of traversal.

   3. Scanline crosses ``up'' edge: add 1. Crosses ``down'' edge: subtract 1.

   4. Point's winding number starts at 0. In interior if final winding number not 0.

7. Complications: lines parallel to scanline, intersections with vertices.

8. 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 any interior point, it is ``to the right'' when traversing the edges clockwise.

9. 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.