If you look closely and critically at lines drawn on the screen, particularly lines that are nearly horizontal or nearly vertical, they may appear to be jagged. The screen is a grid of pixels and the line is approximated by lighting spans of pixels on that grid. The jaggedness is called aliasing; examples of aliased lines are shown in Figure 4.5(a). Anti-aliasing techniques reduce the jaggedness, as shown in Figure 4.5(b), by partially coloring neighbouring pixels to simulate partial pixel coverage.

Figure 4.5 Aliased and anti-aliased lines.
These lines are drawn at a resolution of 50 pixels/inch in order to exaggerate the jagged edges of the aliased lines and highlight the widening and blending in the anti-aliased lines. These lines are examples of the general concepts; if you replicate this drawing on a Voodoo Graphics screen, the results may be different in detail.

Figure 4.6 shows an angled line segment one pixel wide, superimposed on a pixel grid. Some pixels are almost completely covered by the line, while others have only a small corner involved. Glide’s anti-aliasing routines compute a coverage value for each pixel and uses that in combination with the source and destination alpha values to blend the pixel color.

Figure 4.6 Pixel coverage and lines.

Glide draws anti-aliased points, lines, triangles, and polygons by setting up the alpha iterator so that it represents pixel coverage. You must correctly configure the alpha combine unit (discussed in detail in Chapter 6) and enable alpha blending before using any of the anti-aliased drawing commands. The code segment in details the proper sequence of Glide commands that must precede the actual anti-aliased drawing commands. Briefly, you must

Set the alpha combine unit to produce iterated alpha.

Set the alpha blending function. Blending functions are specified for source and destination color components and for source and destination alpha values, and the choice of function depends on whether the scene will be rendered front to back or back to front.

Set the alpha value for each vertex. The chosen alpha value should represent the transparency of the object being rendered, with opaque objects setting alpha to 255. This alpha value will be multiplied by the pixel coverage to obtain the final alpha value used for alpha blending.

Call a grAADraw or guAADraw function.
The six functions are as shown below.

• grAADrawPoint() renders the point as four pixels, each blended according to the computed pixel coverage.
• Lines drawn with grAADrawLine() will be somewhat “fatter” than expected.
• grAADrawTriangle() has three more arguments than its aliased counterpart grDrawTriangle(). The arguments, aaAB, aaBC, and aaBC are Boolean values that allow to selectively anti-alias the edges of the triangle.
• grAADrawPolygon() and grAADrawPolygonVertexList() draw convex polygons with anti-aliased edges.
• guAADrawTriangleWithClip() performs 2D clipping on the specified triangle, and draws the resultant polygon with grAADrawPolygonVertexList(). All edges of the clipped triangle will be anti-aliased.

Example . Drawing an anti-aliased triangle.

The alpha combine unit must be configured to produce an iterated alpha value in order to use the Glide anti-aliasing drawing functions.
Consider the following code segment a recipe for success in this chapter; the alpha combine unit, alpha buffering, and alpha blending are the subject of Chapter 6. The objects in the picture must be pre-sorted on depth. The alpha blending factors depend on whether the scene is drawn front to back or back to front.
The first code shows the alpha blending factors is the scene is drawn front to back.