Not Just Decoration: quality graphics for the Web

Portable colour graphics

The problem

We have already seen that the same RGB image will display with different colours on two different monitors. To some extent this is unavoidable as some colours at the edges of one gamut are outside the other monitor's gamut, and vice verca.

Manufacturers currently take great deal of care to ensure that certain colours, for example those associated with a particular brand or with a corporate logo, are reproduced precisely in printed media. It is only a matter of time before they demand a similar fidelity from online media.

In some cases, we could produce a more accurate colour rendering, but we would need to know the CIE XYZ values of the monitor on which the original image was generated. In most cases, we do not know this.

By analogy with gamma, if this information was stored in the image file then a start could be made with increasing the colour fidelity. There are currently three image formats that hold such information. One is Kodak PhotoCD [10], another is extended TIFF, and the third is PNG.

Accurate colour

Knowing the CIE XYZ values of the current system monitor and (from the image) of the originating monitor, a colour transform for accurate display may be computed and applied to an image at minimal computational cost. In some cases, colour management functions may be provided by the underlying platform.

Accurate grey scales

Knowing chromaticity data of the originating monitor it is trivial to compute the correct colour to greyscale transformation. This gives noticeably superior results, even on an 8 bit indexed display, to the oft-quoted formula Grey = 0.30R + 0.59G + 0.11B which is only correct for the NTSC broadcast monitor, greatly atypical of modern computer monitors.

Better colour printing

Whenever people acquire a colour printer they are initially delighted, but typically soon become disillusioned as the quality of output falls far short of their expectations - a colour magazine, for example. While accurate screen-to-print colour matching is still not a fully automated process, advances have been made in recent years and modern printers with a Level 2 PostScript interpreter can take calibrated RGB data and do a better job of matching the on-screen colours than with raw RGB data.

But what of the future?

More advanced methods of generating accurate portable colour have been proposed, such as the International Color Consortium Profile [18]. A legitimate concern is whether these more complex methods will supercede the methods discussed in this paper. The answer is that they may well do, but as all such approaches are built upon the foundation, directly or indirectly,of the CIE XYZ colour space, the simpler approaches are headed in the right direction and offer a smooth upgrade path.

Notes to implementors


Attempting to produce better colours for screen display or for printing assumes that simpler issues such as the gamma correction are being handled correctly and that a truecolour display is available.


Accurate device independent colour reproduction has been spreading from high-end pre-press applications into the mainstream for the last five years or so. Web user agents which are able to make use of chromaticity information will be at an increasing market advantage in the years to come.

Next: Embeded metadata