Gamma correction basically refers to the ability to correct for differences in how computers (and especially computer monitors) interpret color values. Web authors in particular are probably aware that Macintosh-generated images tend to look too dark on PCs, and PC-generated images tend to look too light and washed out on Macs. An image that looks good on an SGI workstation won't look right on either a Macintosh or a PC, and even a PC-created image won't look right on all PCs.

Gamma information is a partial solution. It's a means of associating a single number with a computer display system, in an attempt to characterize the tricky physics lurking within a graphics card's digital-to-analog converter (RAMDAC) and within a monitor's high-voltage electron gun and display phosphors. Gamma is only a first approximation that accounts for overall ``brightness,'' but it is generally sufficient for casual users. More demanding users will additionally want to adjust for differences in the individual red, green, and blue channels--the so-called chromaticity values, which are also supported by PNG. Even this is merely a second approximation, however.

The absolute best solution currently available is to use a complete color management system, which allows one to take into account things like the viewing environment (a ``dim surround,'' for example) and its interaction with the human visual system. The International Color Consortium has defined a profile format that describes the relationship between an input color space (say, a digital camera or scanner) and the output color space that the user sees. This is the most general way to account for cross-platform differences (and, of course, PNG supports it via the iCCP chunk), but its flexibility comes at a cost: it tends to add at least 250 bytes and often 2,000 bytes or more to every image.

Fortunately, a new proposal for operating systems and physical devices avoids the overhead of a complete ICC profile. Called sRGB, for Standard RGB color space, it defines just that: a standard, unified color space that devices can support, thereby allowing true color management with minimal file overhead and no need for the user to wade through a complicated end-to-end calibration procedure. As of January 1999, the sRGB proposal was in ``Committee Draft for Voting,'' and it should be approved as an international standard[3] by mid-1999; conformant devices should start appearing shortly thereafter. PNG supports sRGB via a chunk called, logically enough, sRGB.

[3] sRGB is Part 2 of IEC 61966 (Colour Measurement and Management in Multimedia Systems and Equipment), a proposed standard of Technical Committee 100 of the International Electrotechnical Commission. The IEC is a standards body similar to the International Organization for Standardization (ISO); in fact, international standards such as MPEG, VRML97, and the Latin-1 character set are all joint ISO/IEC standards, and PNG is on track to join them.

Gamma, chromaticity, and color management are described in more detail in Chapter 10, "Gamma Correction and Precision Color"; PNG's basic structure, including the means by which it can be officially or unofficially extended, is covered in Chapter 8, "PNG Basics" and Chapter 11, "PNG Options and Extensions".