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Internet

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Device-Dependent Color Spaces Most color spaces are device dependent. Even though a particular device, such as a monitor, may use the RGB color space, the colors it renders for given RGB values are often slightly different than all other types of monitors. It is precisely for this reason that the ICM 2.0 color management system is so useful. All color spaces have a white point, which is defined as the whitest white that can be produced in that color space. Since each device is different, its white point is also different. All colors that a device can produce are relative to its white point. Therefore, a color management system must be able to map the white point of one color space into another and preserve the relative locations of all colors. It must also be able to map a color in one color space to its closest match in another color space regardless of the differences in the white points. ICM 2.0 is able to accomplish both of these tasks. The RGB color space is often used on computer monitors. As such, it is device dependent. Printers typically use CMYK. Each printer implements its own version of the CMYK color space. Digital images may not actually store colors in them. They may store index numbers into a palette of colors. The result is that it is very hard for individual application developers to use or provide a standardized method of moving color images from one device to another. Imaging professionals commonly experience the frustration of creating a graphic image on a computer screen and having it turn out very differently when it is printed. ICM 2.0 addresses these imaging needs. Device-Independent Color Spaces Recognizing the need for standard, device-independent color measurements, the Commission International de l'Eclairage (International Commission on Illumination), or CIE, created a color space based on "imaginary" primary colors. No actual device is expected to produce colors in this color space. It is used as a means of converting colors from one color space to another. The primary colors in this color space are the abstract colors X, Y, and Z. The CIE XYZ color space is widely used as the basis for color space conversion. With the rise of the Internet, however, bandwidth considerations have made the XYZ color space unwieldy. The exchange of images over the limited bandwidth of the Internet necessitates a more compact color model. As a result, Hewlett-Packard and Microsoft have proposed the adoption of a standard predefined RGB color space known as sRGB, so as to allow accurate color management with very little data overhead. A white paper discussing the technical issues involved in sRGB is available on the Internet at: http://www.color.org/contrib/sRGB.html A help-file version of the white paper, sRGB.HLP, is also available in the \Help folder of the ICM 2.0 Programmer's Reference in the Platform SDK. Different file formats may use or add a flag to specify that the image is in sRGB color space. In the Windows device-independent bitmap (DIB) format, setting the bV5CSType member of the BITMAPV5HEADER structure to LCS_sRGB specifies that the DIB colors are in the sRGB color space.

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