OpenGL Programming Guide

Chapter 9
Texture Mapping

Chapter Objectives

After reading this chapter, you'll be able to do the following:

• Understand what texture mapping can add to your scene
  
  

• Specify a texture map and how its coordinates relate to those of the objects in your scene
  
  

• Control how a texture image is filtered as it's applied to a fragment
  
  

• Specify how the color values in the image combine with those of the fragment to which it's being applied
  
  

• Use automatic texture coordinate generation to produce effects like contour maps and environment maps

So far in this guide, every geometric primitive has been drawn as either a solid color or smoothly shaded between the colors at its vertices - that is, they've been drawn without texture mapping. If you want to draw a large brick wall without texture mapping, for example, each brick must be drawn as a separate polygon. Without texturing, a large flat wall - which is really a single rectangle - might require thousands of individual bricks, and even then the bricks would appear too smooth and regular to be realistic.

Texture mapping allows you to glue an image of a brick wall (obtained, perhaps, by scanning in a photograph of a real wall) to a polygon, and to draw the entire wall as a single polygon. Texture mapping ensures that all the right things happen as the polygon is transformed and rendered: When the wall is viewed in perspective, for example, the bricks decrease in size along with the wall as the wall gets farther from the viewpoint. Other uses for texture mapping include vegetation to be textured on large polygons representing the ground for a flight simulator; wallpaper patterns; and textures that make polygons appear to be made of natural substances such as marble, wood, or cloth. The possibilities are endless. Although it's most natural to think of applying textures to polygons, textures can be applied to all primitives - points, lines, polygons, bitmaps, and images. Plates 6, 8, 18-21, 24-27, and 29-31 all demonstrate the use of textures.

Because there are so many possibilities, texture mapping is a fairly large, complex subject, and you must make several programming choices when using it. For instance, you can map textures to surfaces made of a set of polygons or to curved surfaces, and you can repeat a texture in one or both directions to cover the surface. A texture can even be one-dimensional. In addition, you can automatically map a texture onto an object in such a way that the texture indicates contours or other properties of the item being viewed. Shiny objects can be textured so that they appear as if they were in the center of a room or other environment, reflecting the surroundings off their surfaces. Finally, a texture can be applied to a surface in different ways. It can be painted on directly (like a decal placed on a surface), it can be used to modulate the color the surface would have been painted otherwise, or it can be used to blend a texture color with the surface color. If this is your first exposure to texture mapping, you might find that the discussion in this chapter moves fairly quickly. As an additional reference, you might look at the chapter on texture mapping in Fundamentals of Three-Dimensional Computer Graphics by Alan Watt (Reading, Mass.: Addison-Wesley, 1990).

Textures are simply rectangular arrays of data - for example, color data, luminance data, or color and alpha data. The individual values in a texture array are often called texels. What makes texture mapping tricky is that a rectangular texture can be mapped to nonrectangular regions, and this must be done in a reasonable way.

Figure 9-1 illustrates the texture-mapping process. The left side of the figure represents the entire texture, and the black outline represents a quadrilateral shape whose corners are mapped to those spots on the texture. When the quadrilateral is displayed on the screen, it might be distorted by applying various transformations - rotations, translations, scaling, and projections. The right side of the figure shows how the texture-mapped quadrilateral might appear on your screen after these transformations. (Note that this quadrilateral is concave and might not be rendered correctly by OpenGL.)

Figure 9-1 : The Texture-Mapping Process

Notice how the texture is distorted to match the distortion of the quadrilateral. In this case, it's stretched in the x direction and compressed in the y direction; there's a bit of rotation and shearing going on as well. Depending on the texture size, the quadrilateral's distortion, and the size of the screen image, some of the texels might be mapped to more than one fragment, and some fragments might be covered by multiple texels. Since the texture is made up of discrete texels (in this case, 256 × 256 of them), filtering operations must be performed to map texels to fragments. For example, if many texels correspond to a fragment, they're averaged down to fit; if texel boundaries fall across fragment boundaries, a weighted average of the applicable texels is performed. Because of these calculations, texturing is computationally expensive, which is why many specialized graphics systems include hardware support for texture mapping.

This chapter covers the OpenGL's texture-mapping facility in the following major sections:

• "An Overview and an Example" gives a brief, broad look at the steps required to perform texture mapping. It also presents a relatively simple example of texture mapping.
  
  

• "Specifying the Texture" explains how to specify a two- or one-dimensional texture. It also discusses how to use a texture's borders, how to supply a series of related textures of different sizes, and how to control the filtering methods used to determine how an applied texture is mapped to screen coordinates.
  
  

• "Modulating and Blending" discusses the methods used for painting a texture onto a surface. You can choose to have the texture color values replace those that would be used if texturing wasn't in effect, or you can have the final color be a combination of the two.
  
  

• "Assigning Texture Coordinates" describes how to compute and assign appropriate texture coordinates to the vertices of an object. It also explains how to control the behavior of coordinates that lie outside the default range - that is, how to repeat or clamp textures across a surface.
  
  

• "Automatic Texture-Coordinate Generation" shows you how to have OpenGL automatically generate texture coordinates for you, in order to achieve such effects as contour and environment maps.
  
  

• "Advanced Features" explains how to manipulate the texture matrix stack and how to use the q texture coordinate.

Remember that you might be able to use display lists to store defined textures for maximum performance efficiency. See "Display-List Design Philosophy."

An Overview and an Example
Specifying the Texture
Modulating and Blending
Assigning Texture Coordinates
Automatic Texture-Coordinate Generation
Advanced Features

OpenGL Programming Guide