Page 53 - Computer Graphics Handout

2.4.5 Text
Graphical output in applications such as data analysis and display requires annotation, such as labels on graphs. Although in
nongraphical programs textual output is the norm, text in computer graphics is problematic. In nongraphical applications, we are
usually content with a simple set of characters, always displayed in the same manner. In computer graphics, however, we often wish
to display text in a multitude of fashions by controlling type styles, sizes, colors, and other parameters. We also want to have
available a choice of fonts. Fonts are families of typefaces of a particular style, such as Times, Computer Modern, or Helvetica.
There are two forms of text: stroke and raster. Stroke text (Figure 2.19) is constructed as are other geometric objects.

We use vertices to specify line segments or curves that outline each character. If the characters are defined by closed boundaries,
we can fill them. The advantage of stroke text is that it can be defined to have all the detail of any other object, and because it is
defined in the same way as other graphical objects are, it can be manipulated by our standard transformations and viewed like any
other graphical primitive. Using transformations, we can make a stroke character bigger or rotate it, retaining its detail and
appearance. Consequently, we need to define a character only once, and we can use transformations to generate it at the desired
size and orientation.
Defining a full 128- or 256-character stroke font, however, can be complex, and the font can take up significant memory and
processing time. The standard PostScript fonts are defined by polynomial curves, and they illustrate all the advantages and
disadvantages of stroke text. The various PostScript fonts can be used for both high- and low-resolution applications. Often,
developers mitigate the problem of slow rendering of such stroke characters by putting considerable processing power in the
printer.
Raster text (Figure 2.20) is simple and fast. Characters are defined as rectangles of bits called bit blocks. Each block defines a single
character by the pattern of 0 and 1 bits in the block. A raster character can be placed in the frame buffer rapidly by a bit-block-
transfer (bitblt) operation, which moves the block of bits using a single function call. We will discuss bitblt in Chapter 7.
You can increase the size of raster characters by replicating, or duplicating, pixels, a process that gives larger characters a blocky
appearance (Figure 2.21).

Other transformations of raster characters, such as rotation, may not make sense, because the transformation may move the bits
defining the character to locations that do not correspond to the location of pixels in the frame buffer. Because stroke and bitmap
characters can be created from other primitives, OpenGL does not have a text primitive. However, the GLUT library provides a few

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