OpenGL Programming Guide

Chapter 7Blending, Antialiasing, and Fog

Antialiasing

You might have noticed in some of your OpenGL pictures that lines, especially nearly horizontal or nearly vertical ones, appear jagged. These jaggies appear because the ideal line is approximated by a series of pixels that must lie on the pixel grid. The jaggedness is called aliasing, and this section describes antialiasing techniques to reduce it. Figure 7-2 shows two intersecting lines, both aliased and antialiased. The pictures have been magnified to show the effect

Figure 7-2 : Aliased and Antialiased Lines

Figure 7-2 shows how a diagonal line one pixel wide covers more of some pixel squares than others. In fact, when performing antialiasing, OpenGL calculates a coverage value for each fragment based on the fraction of the pixel square on the screen that it would cover. The figure shows these coverage values for the line. In RGBA mode, OpenGL multiplies the fragment's alpha value by its coverage. You can then use the resulting alpha value to blend the fragment with the corresponding pixel already in the framebuffer. In color-index mode, OpenGL sets the least significant 4 bits of the color index based on the fragment's coverage (0000 for no coverage and 1111 for complete coverage). It's up to you to load your color map and apply it appropriately to take advantage of this coverage information.

Figure 7-3 : Determining Coverage Values

The details of calculating coverage values are complex, difficult to specify in general, and in fact may vary slightly depending on your particular implementation of OpenGL. You can use the glHint() command to exercise some control over the trade-off between image quality and speed, but not all implementations will take the hint. void glHint(GLenum target, GLenum hint);

Controls certain aspects of OpenGL behavior. The target parameter indicates which behavior is to be controlled; its possible values are shown in Table 7-2 . The hint parameter can be GL_FASTEST to indicate that the most efficient option should be chosen, GL_NICEST to indicate the highest-quality option, or GL_DONT_CARE to indicate no preference. The interpretation of hints is implementation-dependent; an implementation can ignore them entirely.

For more information about the relevant topics, see "Antialiasing" for the details on sampling and "Fog" for details on fog. The GL_PERSPECTIVE_CORRECTION_HINT parameter refers to how color values and texture coordinates are interpolated across a primitive: either linearly in screen space (a relatively simple calculation) or in a perspective-correct manner (which requires more computation). Often, systems perform linear color interpolation because the results, while not technically correct, are visually acceptable; textures, however, in most cases require perspective-correct interpolation to be visually acceptable. Thus, an implementation can choose to use this parameter to control the method used for interpolation. Perspective projection is discussed in Chapter 3 , color is discussed in Chapter 5 , and texture mapping is discussed in Chapter 9 .

Table 7-2 : Values for Use with glHint() and Their Meaning

Parameter	Meaning
GL_POINT_SMOOTH_HINT, GL_LINE_SMOOTH_HINT, GL_POLYGON_SMOOTH_HINT	Specify the desired sampling quality of points, lines, or polygons during antialiasing operations
GL_FOG_HINT	Specifies whether fog calculations are done per pixel (GL_NICEST) or per vertex (GL_FASTEST)
GL_PERSPECTIVE_CORRECTION_HINT	Specifies the desired quality of color and texture-coordinate interpolation

Antialiasing Points or Lines
Antialiasing Polygons

OpenGL Programming Guide