From the previous discussion of hardware it should be apparent that the distinguishing features of any graphics computer display are: the maximum density of the display matrix, the way text and graphics are mixed on the screen, and whether or not color is provided and, if so, how many colors. In the following sections we will examine these important features in the light of real products on the market.


Fig. 2-18. Screen output for Matrox Alt-256**2 graphics board features resolution of 256 × 256 and into S-100 8080-based computer.


Fig. 2-19. Digitized Mona Lisa using eight levels of gray per Pixel and three Matrox ALT-256**2 boards.


Perhaps no other feature is as motivating as color. Some computers, such as the Apple II, allow a screen element to be any one of 16 different colors. Contrast this with most computers that only allow black and white. The colors offered do not necessarily cover the entire spectrum (some are shades of the same color) but they do allow very interesting and stimulating effects, like the mandala pattern shown in Fig. 2-20 or the Probability Machine shown in Fig. 2-21.


Fig. 2-20. This mandala display was produced by an Apple II running in the low-resolution color mode.

Screen Density

The density of a computer display refers to the maximum number of accessible screen elements in the display matrix. Screen density is usually given as the number of horizontal elements by the number of vertical elements in the display, or H × V. Sometimes it is described as X by Y, or columns by rows. Screen density affects the kind of detail possible in graphics displays. The range of densities in graphics computers today is enormous and one has to be most careful when reading manufacturers' claims about density. For example, some manufacturers claim to offer a density of 512 by 256 elements but when looking closer we find that only an 8 by 8 portion of this matrix is accessible. In other words, if a single dot in the total display is to be changed then we must also change (or at least access) up to 63 dots around it. Other displays are not like this and allow any dot in the matrix to be accessed. There are, of course, limits to the density a standard tv will allow with rf modulator entry and this is somewhere around 512 dots horizontally and 256 lines vertically. As you get into expensive monitors, higher frequencies and hence higher densities are possible.


Fig. 2-21. The Probability Machine is another example of what can be done with color.

Typical densities for low-cost graphics computers run from 40 × 25 (PET 2001) to 280 × 193 (Apple II). In the lower-density computers, graphics characters are usually provided and these have enough detail to make the display look like the density is a lot higher. It is, however, real work to use such displays (as we shall see).

Screen Format

Screen format tells us the number of rows and columns in the text matrix and how the text characters are mixed in with the graphics. Ideally, we would like to be able to mix text anywhere on the graphics screen. In reality, totally mixed graphics coupled with very high screen densities are rare features to find together. For example, the Apple II computer provides a text scrolling window at the bottom of the graphics matrix. Up to four lines of text can be output to the window. Furthermore, the window can be relocated to places other than the bottom of the screen (such as a side column window).


Fig. 2-22. Screen output from Apple II equipped with Superchip shows mixed graphics, rotated characters, and custom character fonts.

But you cannot mix text characters right in with graphics (such as for labeling objects). There is a company, however, that sells a special ROM chip you can plug into the Apple to make it do this and other things. As shown in Fig. 2-22 the chip allows the Apple to rotate letters upside down or sideways. The chip also allows you to create your own custom characters for fancy text effects.

On the computers with lower density (PET, TRS-80, etc.) the text characters can easily be mixed in with graphics because a text character is the same size as a graphics character. However, you have to be careful in the TRS-80 because the smallest accessible screen element using SET and RESET is one-sixth the area of a text character so you could wipe out your graphics with a text character wrongly placed.

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