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HP Color Recovery technology - for low-cost color image display - Technical

Hewlett-Packard Journal, April, 1995 by Anthony C. Barkans

For many years the only practical way to display high-quality true color images was on a computer with a graphics subsystem providing at least 24 color planes (see the definition of true color on page 52). However, because of the high cost of color graphics devices with 24 planes, many users chose 8-plane systems. Unfortunately, using these 8-plane systems required giving up some color capabilities to save cost.

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HP has developed a technique called HP Color Recovery which provides a method for displaying millions of colors within the cost constraints of an 8-plane system. For an example of the image quality provided by HP Color Recovery consider Fig. 1. Fig. 1a shows a close up of a jet plane stored as a full 24-bit-per-pixel true color image. Fig. 1b shows the same jet plane displayed using a traditional 8-bit-per-pixel system. Finally, Fig. 1c shows how the jet plane will be displayed when using HP Color Recovery in an 8-bit-per-pixel mode on the HCRX-8 graphics device.

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Of course, pretty pictures aren't enough. Therefore, one of the primary design goals for HP Color Recovery was to supply the additional color capabilities without giving up interactive performance. Another goal was to be able to work with all types of applications running in a windowed environment such as the X Window System and HP VUE. The implementation of HP Color Recovery used in current HP workstations meets these goals.

Traditional Eight-Plane Systems

Traditional eight-plane systems can display only 256 colors. Two approaches have been employed to get the best results with limited colors. The first is called either pseudo color or indexed color. This method selects a set of 256 colors and then limits the application to using only that fixed set of colors. For many applications, such as word processing and business graphics, this approach works reasonably well. This is because the resultant images are made up of very few colors. However, when an application needs more than 256 colors, such as realistically shaded MCAD (mechanical computer-aided design) images or human faces in video sequences, then another approach is needed. Since more than 256 colors are required for these applications, a technique to simulate more colors is used. For these applications a technique called dithering is employed. The idea of dither is to approximate a single color by displaying two other colors at intermixed pixel locations. For example, a grid of black and white pixels can be displayed to simulate gray. Such a grid of black and white pixels will indeed look gray when viewed from a distance. The primary problem with dithering is that since most people tend to work close to the display, dithered images are viewed as having a grainy or textured appearance (see Fig. 1b).

Color Theory and Dither

Before discussing the details of how HP Color Recovery works, an overview of color theory as it relates to computer generated images and dither should be helpful. This overview describes how the human eye is tricked into seeing color, color precision in graphics, and a dithering method.

Tricking the Human Eye

It is often noted that computer monitors use red, green, and blue (RGB) to produce true color images. A reasonable question to ask is: "Why use these particular colors?" If one examines the spectrum of visible light, it can be seen that red is at the end of the spectrum with the longest wavelengths that the human eye can see while blue is at the other end. Note that green is in about the middle. Also note that white is a mix of all colors. Therefore by mixing varying amounts of red, green, and blue any color can be created. For example, forcing both the red and the green CRT beams to be on at any single location will result in a dot that appears yellow to the human eye.

Thus, one can create the visual appearance of any color by mixing the red, green, and blue components at any pixel location. However, it is interesting to note that the human eye can also perceive a new color when the component colors are mixed spatially. For example, a checkerboard of red and green pixels will be perceived as yellow when viewed from a distance. It is this spatial mixing of color to form a new color that is exploited by dither.

Color Precision

In most systems that deal with true color, color is specified to eight bits for each of the three color components: red, green, and blue. The choice of eight bits is based on two factors. First, the human eye cannot distinguish an infinite number of shades because the dynamic range of the eye is limited. For the most part shaded surfaces rendered with eight bits per color appear smooth with the underlying quantization not readily apparent to the viewer. The second factor that works in favor of using eight bits per color component as a standard is that eight-bit bytes are very convenient to work with in a computer system.

Simple Dithering

When using a 24-bit color system, any displayable color component can be specified using eight bits. For example, consider the red component. When there is no red in a pixel the red component is specified with a binary value of 00000000, which is a decimal 0. A full bright red is specified as a binary number 11111111, which is decimal 255. Of course, high-end display systems, such as the HCRX-48Z, use 24 bits to store and display true color information. The visual quality of these high-end displays is shown in Fig. 1a. However, since low-cost systems typically have a total of only eight bits per pixel to store the color information, an approximation to the true color image is made. The most common method is dither using three bits each for the red and green components. This leaves two bits for blue. Using fewer bits for blue is based on the fact that the human eye has less sensitivity to blue. With fewer bits available per color component, the quantization of the colors becomes apparent to the viewer. The effects of using a limited number of bits for each color can be seen in Fig. 1b.

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HP Color Recovery technology - for low-cost color image display - Technical

Hewlett-Packard Journal, April, 1995 by Anthony C. Barkans

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