Color Approximations and Dithering

Although an application can use color without regard to the color capabilities of the device, the resulting output may not be as informative and pleasing as output for which color is carefully chosen. Few, if any, devices guarantee an exact match for every possible color value; therefore, if an application requests a color that the device cannot generate, Windows approximates that color by using a color that the device can generate. For example, if an application attempts to create a red pen for a black and white printer, it will receive a black pen instead Windows uses black as the approximation for red.

An application can discover whether Windows will approximate a given color by using the GetNearestColor function. The function takes a color value and returns the color value of the closest matching color the device can generate. The method Windows uses to determine this approximation depends on the device driver and its color capabilities. In most cases, the approximated color's overall intensity is closest to that of the requested color.

When an application creates a pen or sets the color for text, Windows always approximates a color if no exact match exists. When an application creates a solid brush, Windows may attempt to simulate the requested color by dithering. Dithering simulates a color by alternating two or more colors in a pattern. For example, different shades of pink can be simulated by alternating different combinations of red and white. Depending on the colors and the pattern, dithering can produce reasonable simulations. It is most useful for monochrome devices, because it expands the number of available "colors" well beyond simple black and white.

The method used to create dithered colors depends on the device driver. Most device drivers use a standard dithering algorithm, which generates a pattern based on the intensity values of the requested red, green, and blue colors. In general, any requested color that cannot be generated by the device is subject to simulation, but an application is not notified when Windows simulates a color. Furthermore, an application cannot modify or change the dithering algorithm of the device driver. An application, however, can bypass the algorithm by creating and using pattern brushes. In this way, the application creates its own dithered colors by combining solid colors in the bitmap that it uses to create the brush.

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