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The simplest type of color graphics display adapter requires 3 bits per pixel. The pixels could be encoded like this with 1 bit per primary color:

Bits

Color

000

Black

001

Blue

010

Green

011

Cyan

100

Red

101

Magenta

110

Yellow

111

White

But such a scheme would be suitable only for simple cartoonlike images. Most real-world colors are combinations of various levels of red, green, and blue. If you were willing to devote 2 bytes per pixel, you could allocate 5 bits for each primary color (with 1 bit left over). That gives you 32 levels of red, green, and blue and a total of 32,768 different colors. This scheme is often referred to as high color or thousands of colors.

The next step is to use 3 bytes per pixel, or 1 byte for each primary. This encoding scheme results in 256 levels of red, green, and blue for a total of 16,777,216 different colors, often referred to as full color or millions of colors. If the resolution of the video display is 640 pixels horizontally by 480 pixels vertically, the total amount of memory required is 921,600 bytes, or nearly a megabyte.

The number of bits per pixel is sometimes referred to as the color depth or color resolution. The number of different colors is related to the number of bits per pixel in this way:

Number of colors = 2Number of bits per pixel

A video adapter board has only a certain amount of memory, so it's limited in the combinations of resolutions and color depths that are possible. For example, a video adapter board that has a megabyte of memory can do a 640-by-480 resolution with 3 bytes per pixel. But if you want to use a resolution of 800 by 600, there's not enough memory for 3 bytes per pixel. Instead, you'll need to use 2 bytes per pixel.

Although raster displays seem very natural to us now, in the early days they were not quite practical because they required what was then a great deal of memory. Instead, the SAGE video displays were vector displays, more like an oscilloscope than a TV. The electron gun could be electrically positioned to point to any part of the display and draw lines and curves directly. The persistence of the image on the screen allowed assembling these lines and curves into rudimentary pictures.

The SAGE computers also supported light pens that let the operators alter images on the display. Light pens are peculiar devices that look like a stylus with a wire attached to one end. If the proper software is running, the computer can detect where the light pen is pointing on the screen and alter an image in response to the pen's movements.

How does this work? Even technological sophisticates are sometimes puzzled when they first encounter a light pen. The key is that a light pen doesn't emit light—it detects light. The circuitry that controls the movements of the electron gun in the CRT (regardless of whether a raster or vector display is used) can also determine when the light from the electron gun hits the light pen and hence where the light pen is pointing on the screen.

One of the first people to envision a new era of interactive computing was Ivan Sutherland (born 1938), who in 1963 demonstrated a revolutionary graphics program he had developed for the SAGE computers named Sketchpad. Sketchpad could store image descriptions in memory and display the images on the video display. In addition, you could use the light pen to draw images on the display and change them, and the computer would keep track of it all.

Another early visionary of interactive computing was Douglas Engelbart (born 1925), who read Vannevar Bush's article "As We May Think" when it was published in 1945 and five years later began a lifetime of work developing new ideas in computer interfaces. In the mid-1960s, while at the Sanford Research Institute, Engelbart completely rethought input devices and came up with a five-pronged keyboard for entering commands (which never caught on) and a smaller device with wheels and a button that he called a mouse. The