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

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

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Hardware details. The filtering logic, which was shown in a systems context in Fig. 3, is expanded in Fig. 5. As the frame buffer is scanned, each pixel in the display is sequentially sent to the logic shown in Fig. 5. The left side of the figure shows the path taken as the data for each pixel read from the frame buffer enters the filtering logic. The data is sent both to a pipeline register for immediate use, and to a scan line buffer for use when the next scan line is being evaluated. The 32 registers shown in Fig. 5 store the data for the 2 x 16 region being evaluated. These registers are clocked at the pixel clock rate. Note that the data for each pixel on the display will pass through the location marked with the X. When a pixel is at the location X, it is called the pixel being evaluated. This means that the results of applying equation 1 are assigned to the display at the screen address of X.

The 32 pixels stored in the pipeline registers shown in Fig. 5 are sent through blocks of logic that perform the inner loop evaluation of equation 1. This inner loop is essentially an edge detector. The logic shown in Fig. 6 allows only pixels that have similar numeric values to the pixel being evaluated to be included in the summation. The summation logic is simply an adder tree that sums the results of the pixels passing the edge compare. The filter function is performed in parallel for all the pixels within the filter region.

Given the complexity of the function being performed in the filter circuit, the circuit is surprisingly small. The entire filter circuit is made up of approximately 35,000 transistors. Compared to the number of transistors required to increase the number of color planes, this is very small. For example increasing the number of color planes from 8 to 16 on a typical SVGA (Super VGA) system (1024 x 768-pixel resolution) requires over 8,000,000 transistors, which is 1M bytes of additional frame buffer memory. Because of the small size of the HP Color Recovery circuit, it is inexpensive enough to be included in entry-level graphics systems.

Questions and Answers

Thus far the concepts behind HP Color Recovery have been discussed. It has been shown that HP Color Recovery can supply additional color capabilities to low-end graphics systems while maintaining an interactive windowed environment. The following are answers to the most frequently asked questions about the practical use of HP Color Recovery.

* Question: Is there a difference between a 24-bit true color image and one displayed using HP Color Recovery?

Answer: Yes. If you view a 24-bit image and an HP Color Recovery image side by side there are differences. For example, the back edge of the wing in Fig. 1c has some artifacts in it. At normal size the artifacts can be found but are less noticeable than in Fig. 1c.

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* Question: How many colors are reproducible with HP Color Recovery?

Answer: In the best case HP Color Recovery can provide up to 23 bits of accuracy. However, in typical images about four million colors can be reproduced.