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

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

[ILLUSTRATION OMITTED]

From a distance the colors in the dithered image are integrated by the eye so that they appear correct. However, the fundamental problem with dither is that most dithered images are viewed up close and so the dithering pattern is noticeable in the image.

Dithering Is Key

It is important to realize that to approximate any true color value, a spatial region of the screen is required. This often leads people to say that dithering is a method that trades off spatial resolution for color resolution. However, this is misleading. Some people believe that a single-pixel object cannot be dithered. Actually a single-pixel object can be dithered. The result is that the object will be one of the two dither colors. Going back to the example above, a single-pixel red object specified as binary 01011000 (decimal 2.75) will be stored at any single pixel location as either binary 010 or 011 (decimal two or three). Taken by itself, any single pixel is not a perfect approximation of the true color. However, it is still a reasonable approximation.

The idea of being able to encode each pixel in the image independently by using dither is key to enabling color recovery to work in an interactive environment. As a historical note it should be mentioned that over the last few years several people have developed methods to bring true color capabilities to eight-bit graphics devices. However, these attempts have been based on complex multipixel encoding schemes. For the most part they have applied data compression techniques to the data stored in the frame buffer. These methods have produced high-quality images, but the encoding is so complex that the user must give up interactive performance to use them. Because of the performance problems these methods have not been widely adopted by the computer graphics community.

HP Color Recovery

The simplest explanation of HP Color Recovery is that it performs the task your eye is asked to do with an ordinary dithered system. In essence, an HP Color Recovery system takes 24-bit true color data generated by an application and dithers it down to eight bits for storage in the frame buffer. Then as the frame buffer data is scanned from the frame buffer to the display, it passes through specialized digital signal processing (DSP) hardware where the work of producing millions of colors is performed. The output of the DSP hardware is sent to the display where millions of colors can be viewed. It is important to recognize that since the data stored in the HP Color Recovery frame buffer is dithered, thousands of applications can work with it. It is also important to recognize that these applications will run at full performance in an interactive windowed environment. In other words, applications do not need to be changed to take advantage of HP Color Recovery.

The Process

HP Color Recovery is a two-part process. First, true color information generated by the application is dithered and then stored in the frame buffer. The type of application generating the true color information is immaterial. For example, true color data can be generated by a CAD application program or as part of a video sequence. The dithering may be done in a software device driver or in the hardware of a graphics controller. It is very important to note that each pixel is treated independently. This pixel independence is key to the ability to work within an interactive windowed environment. The second part of the HP Color Recovery process is to filter the dithered data. The filter is placed between the output of the frame buffer and the DACs that drive the monitor. Fig. 3 shows the HP Color Recovery process starting from when an application generates true color data to when the image appears on the screen. Note that "application" refers to any program that generates true color data for display.