Drawing in Space: Geometric Primitives and Buffers
This chapter is from the book
This chapter is from the book
This chapter is from the book
What you'll learn in this chapter:
|
How To |
Functions You'll Use |
|
Draw points, lines, and shapes |
glBegin/glEnd/glVertex |
|
Set shape outlines to wireframe |
glPolygonMode or solid objects |
|
Set point sizes for drawing |
glPointSize |
|
Set line drawing width |
glLineWidth |
|
Perform hidden surface removal |
glCullFace/glClear |
|
Set patterns for broken lines |
glLineStipple |
|
Set polygon fill patterns |
glPolygonStipple |
|
Use the OpenGL Scissor box |
glScissor |
|
Use the stencil buffer |
glStencilFunc/glStencilMask/glStencilOp |
If you've ever had a chemistry class (and probably even if you haven't), you know that all matter consists of atoms and that all atoms consist of only three things: protons, neutrons, and electrons. All the materials and substances you have ever come into contact with—from the petals of a rose to the sand on the beach—are just different arrangements of these three fundamental building blocks. Although this explanation is a little oversimplified for almost anyone beyond the third or fourth grade, it demonstrates a powerful principle: With just a few simple building blocks, you can create highly complex and beautiful structures.
The connection is fairly obvious. Objects and scenes that you create with OpenGL also consist of smaller, simpler shapes, arranged and combined in various and unique ways. This chapter explores these building blocks of 3D objects, called primitives. All primitives in OpenGL are one- or two-dimensional objects, ranging from single points to lines and complex polygons. In this chapter, you learn everything you need to know to draw objects in three dimensions from these simpler shapes.
Drawing Points in 3D
When you first learned to draw any kind of graphics on any computer system, you probably started with pixels. A pixel is the smallest element on your computer monitor, and on color systems, that pixel can be any one of many available colors. This is computer graphics at its simplest: Draw a point somewhere on the screen, and make it a specific color. Then build on this simple concept, using your favorite computer language to produce lines, polygons, circles, and other shapes and graphics. Perhaps even a GUI...
With OpenGL, however, drawing on the computer screen is fundamentally different. You're not concerned with physical screen coordinates and pixels, but rather positional coordinates in your viewing volume. You let OpenGL worry about how to get your points, lines, and everything else projected from your established 3D space to the 2D image made by your computer screen.
This chapter and the next cover the most fundamental concepts of OpenGL or any 3D graphics toolkit. In the upcoming chapter, we provide substantial detail about how this transformation from 3D space to the 2D landscape of your computer monitor takes place, as well as how to transform (rotate, translate, and scale) your objects. For now, we take this ability for granted to focus on plotting and drawing in a 3D coordinate system. This approach might seem backward, but if you first know how to draw something and then worry about all the ways to manipulate your drawings, the material in Chapter 4, "Geometric Transformations: The Pipeline," is more interesting and easier to learn. When you have a solid understanding of graphics primitives and coordinate transformations, you will be able to quickly master any 3D graphics language or API.