Digital Color Management
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A "reasonable" conversion - the rendering intent |
The colors that a device can produce are only some, and not all, of the colors that the human eye can see.
In terms of the profile of such a device, this means that all the coordinates of the device colors have corresponding colorimetric coordinates, whilst not all colorimetric coordinates can have corresponding device coordinates. And this has consequences in the conversion of colors between devices.
As we have seen, the method used to convert color coordinates from the origin device to the destination device consists of two steps:
- finding the device color coordinates in the origin profile, and determining the corresponding colorimetric coordinates.
- finding the colorimetric coordinates from in step 1 in the destination profile and determining the corresponding device color coordinates.
Clearly, in step one it will always be possible to find the colorimetric coordinates, whilst in step 2, it is possible that these colorimetric coordinates are not to be found in the profile table (or, if the profile contains data for an algorithm or method, that this algorithm cannot produce the corresponding device coordinates because they do not exist).
For example, given the two profiles below, with what CMYK values could one print the color produced on the monitor by the coordinates RGB (255, 255, 255)? This color (white) has colorimetric coordinates Lab (100, 0, 0), but this row does not exist in the second table (because the monitor’s white cannot be printed by this printer).
| Monitor | Printer | ||||||||||||||
| R | G | B | L | a | b | L | a | b | C | M | Y | K | |||
| 255 | 255 | 255 | 100 | 0 | 0 | 100 | -0.5 | 3 | 0 | 0 | 0 | 0 | |||
| 255 | 255 | 223 | 99 | -3 | 11 | 99 | -1 | 6 | 0 | 0 | 6 | 0 | |||
| ... | ... | ... | ... | ... | ... | 99 | -3 | 11 | 0 | 0 | 12.5 | 0 | |||
| 0 | 0 | 31 | 1 | 12 | -30 | ... | ... | ... | ... | ... | ... | ... | |||
| 0 | 0 | 0 | 1 | 0 | 0 | 1 | 0 | 0 | 100 | 100 | 100 | 0 | |||
In short, some of the colors we can see on a particular monitor can be printed on a particular printer, whilst others cannot be printed as they do not exist in that printer.
What can be done in this case? Obviously, if we insist on an accurate solution, the problem cannot be resolved. But all is not lost; it is possible to attempt to reproduce a reasonable approximation to the original color when no perfect match can be found.
The meaning of "reasonable" depends on the effect we wish to achieve. Four effects, known as rendering intents, have been identified which can be used in typical situations.