Color Management Systems and sRGB

The popularity of the RGB color space is at odds with its fundamentally device-dependent nature. In order to address this problem, a number of manufacturers of computer-related equipment and the International Color Consortium have cooperated to define a standard RGB space to which various devices such as monitors, printers, scanners, and electronic cameras can be calibrated. This specification, known as sRGB, is expected to be approved as an international standard by the International Electrotechnical Commission (IEC) by mid-1999; it will formally be known as IEC 61966-2-1.

sRGB allows one to create a PNG image on one system and print or display it on another with full color fidelity and without ever converting to XYZ or another device-independent color space. How well it works in practice remains to be seen, but a well-specified international standard--and manufacturers' evident interest in it--will go a long way toward ensuring that future devices are compatible at the RGB level.

In addition, an image that was created under sRGB can be flagged as such with very little overhead. Only one parameter, the rendering intent, is required; it is stored as a single byte in PNG's sRGB chunk. The rendering intent, also known as ``artistic intent,'' indicates how the creator of the image wishes the colors to be mapped when the output device's color gamut (recall the discussion in the previous section) does not match that of the original device. For example, imagine that an artist creates an image on an sRGB-compliant monitor and graphics system, and when he's finished he sends it to an sRGB-compliant color printer. Because the light-emitting phosphors of the monitor and the light-reflecting inks of the printer and its paper will be able to represent somewhat different ranges of colors--ideally, mostly overlapping, but conceivably with only a little overlap--it is necessary for the artist to specify how he wishes the different color gamuts of the devices to be mapped to each other.

The simplest rendering intent (in concept) is known as absolute colorimetric. The word ``colorimetric'' means color-measuring, and this intent indicates that, for the region of overlap between source and destination gamuts, any given pixel will be measured to have identical colors on the two devices. When the output device is not capable of reproducing some of the colors of the input device (i.e., the gamut is more restricted in that region of color space), the colors are clipped to the nearest color that can be reproduced. The result is that dynamic range will be lost in some areas. For example, suppose that the image has a smoothly varying blue gradient and that the output device is restricted to only the darker blues. The output will show a smoothly varying gradient progressing from darkest blue to medium blue, but then it will saturate and render all of the remaining gradient as a constant, medium blue. Likewise, the intensity range may be clipped if the output device is incapable of rendering absolute black or the brightest shades of white. This rendering intent might be used in cases in which three or more profiles are involved--for example, when an image created on a computer display is intended for a particular typesetter but first needs to be proofed on a local printer.

A similar intent is relative colorimetric. As with the absolute flavor, RGB values correspond to precise CIE color measurements, but they are modified according to the intensity range and color cast (i.e., the white point) of the output medium. Referring to our artist again, his monitor may be capable of displaying true, 5,000K CIE white, but the paper in his printer generally will not uniformly reflect all of the wavelengths that hit it, regardless of the source.[83] To put it another way, the paper will have a different white point than the monitor. As a result, it may be desirable to sacrifice perfect color correspondence in favor of a similar dynamic range in intensities, by referencing the RGB values to whatever paper or other output medium is used. The output image may have an overall lighter or darker appearance or an overall color shift, but there will be no clipping of grayscale gradients, and the colors will appear to match--thanks to the human visual system's tendency to acclimate to an overall tint or, to put it another way, to the ``prevailing white''. The relative colorimetric intent is the ICC's default; it might be desirable for displaying and printing corporate logos.

[83] And if he's silk-screening white T-shirts, no amount of bleach will change that. There are some detergents that infuse clothing with small amounts of phosphorescent chemicals in order to make ``whites whiter''; one's clothes are no longer strictly reflective, but actually glow slightly when exposed to blue or ultraviolet light. Such detergents are generally not part of an sRGB-compliant display system.

A still more approximate intent, but one that may capture more of the personality of the original image, is the perceptual rendering intent. The idea in this case is to map the full color ranges of source and destination devices as well as possible. This may involve either expansion, compression, or shifting of the color gamut. Even colors within the region where the gamuts overlap may be modified; in other words, absolute color fidelity is less important than preserving the dynamic range in both color and intensity of the image. This is often the most appropriate intent for rendering photographs.

Finally we have the saturation-preserving rendering intent, which is similar to perceptual rendering in that it doesn't necessarily enforce completely accurate color reproduction. But rather than favor overall gamut mapping like the perceptual intent does, this rendering intent specifies that the saturation of each color should remain constant. Saturation can be thought of as the amount of gray in a color of a given hue (say, greenish-aqua) and lightness. As the saturation approaches zero, the color approaches gray; maximum saturation gives the purest shade of the given hue. Since a cheap inkjet printer might have only two-thirds of the saturation range of an expensive dye-sublimation printer, colorimetric rendering might induce another kind of clipping in the inkjet's output. Saturation-preserving rendering would avoid that, but could possibly result in changes in hue and/or lightness. It might be the preferred intent for printing business charts and graphs.

PNG's sRGB chunk encodes the rendering intent with the same values specified by the International Color Consortium for ICC profiles: that is, byte value 0 for perceptual, 1 for relative colorimetric, 2 for saturation-preserving, and 3 for absolute colorimetric.

Because the sRGB color space encompasses gamma and chromaticity information, it is not strictly necessary for a PNG image to include gAMA and cHRM chunks in addition to the sRGB chunk. But since not all applications will know how to interpret sRGB, encoders should nevertheless include a gAMA chunk that corresponds to sRGB, and possibly a cHRM chunk as well. Decoders that know how to deal with cHRM are likely to know how to deal with sRGB, too, which is why cHRM may be omitted. The proper values for the two chunks are in Table 10-3.

An sRGB-aware decoder should ignore gAMA and cHRM whenever an sRGB chunk is present; the latter takes precedence. Less sophisticated applications can use gAMA and cHRM to render the image approximately as intended, even without knowledge of the sRGB color space. But note that there is no excuse for any application written after the PNG 1.1 specification not to recognize sRGB, at least; it is now part of the core spec, and new applications should know what gamma and chromaticity values correspond to it, regardless of whether the corresponding chunks--or even conflicting chunks--are actually present in the file. As with gAMA and cHRM, only one sRGB chunk is allowed, and it must appear before any PLTE and IDAT chunks.