Using the standard technique for drawing characters with glBitmap(), drawing each pixel of a character is an all-or-nothing affair—the pixel is either turned on or not. If you’re drawing black characters on a white background, for example, the resulting pixels are either black or white, never a shade of gray. Much smoother, higher-quality images can be achieved if intermediate colors are used when rendering characters (grays, in this example).
Assuming that you’re drawing black characters on a white background, imagine a highly magnified picture of the pixels on the screen, with a high-resolution character outline superimposed on it, as shown in the left-hand diagram in Figure 14-1.
Figure 14-1 Antialiased Characters
Notice that some of the pixels are completely enclosed by the character’s outline and should be painted black, some pixels are completely outside the outline and should be painted white, but many pixels should ideally be painted some shade of gray, where the darkness of the gray corresponds to the amount of black in the pixel. If this technique is used, the resulting image on the screen looks better.
If speed and memory usage are of no concern, each character can be drawn as a small image instead of as a bitmap. If you’re using RGBA mode, however, this method might require up to 32 bits per pixel of the character to be stored and drawn, instead of the 1 bit per pixel for a standard character. Alternatively, you could use one 8-bit index per pixel and convert these indices to RGBA by table lookup during transfer. In many cases, a compromise is possible that allows you to draw the character with a few gray levels between black and white (say, two or three), and the resulting font description requires only 2 or 3 bits per pixel of storage.
The numbers in the right-hand diagram in Figure 14-1 indicate the approximate percentage coverage of each pixel: 0 means approximately empty, 1 means approximately one-third coverage, 2 means two-thirds, and 3 means completely covered. If pixels labeled 0 are painted white, pixels labeled 3 are painted black, and pixels labeled 1 and 2 are painted one-third and two-thirds black, respectively, the resulting character looks quite good. Only 2 bits are required to store the numbers 0, 1, 2, and 3, so for 2 bits per pixel, four levels of gray can be saved.
There are basically two methods of implementing antialiased characters, depending on whether you’re in RGBA mode or color-index mode.
In RGBA mode, define three different character bitmaps, corresponding to where 1, 2, and 3 appear in Figure 14-1. Set the color to white, and clear for the background. Set the color to one-third gray (RGB = (0.666,0.666,0.666)), and draw all the pixels with a 1 in them. Then set RGB = (0.333, 0.333, 0.333), draw with the 2 bitmap, and use RGB = (0.0, 0.0, 0.0) for the 3 bitmap. What you’re doing is defining three different fonts and redrawing the string three times, where each pass fills in the bits of the appropriate color densities.
In color-index mode, you can do exactly the same thing, but if you’re willing to set up the color map correctly and use writemasks, you can get away with only two bitmaps per character and two passes per string. In the preceding example, set up one bitmap that has a 1 wherever 1 or 3 appears in the character. Set up a second bitmap that has a 1 wherever 2 or 3 appears. Load the color map so that 0 gives white. 1 gives light gray, 2 gives dark gray, and 3 gives black. Set the color to 3 (11 in binary) and the writemask to 1, and draw the first bitmap. Then change the writemask to 2. and draw the second. Where 0 appears in Figure 14-1, nothing is drawn in the framebuffer. Where 1, 2, and 3 appear, 1, 2, and 3 appear in the framebuffer.
For this example with only four gray levels, the savings are small—two passes instead of three. If eight gray levels were used instead of four, the RGBA method would require seven passes, and the color-map masking technique would require only three. With 16 gray levels, the comparison is 15 passes to four passes.
