2. Basics of Image Processing
2.12 Color Management and ICC Color Profiles
An old problem when processing images is, that a print-out often looks
completely different to the on-screen display. This is due to the fact, that
printers usually are not able to print as many colors as a computer monitor.
Therefore, several hardware and software companies have founded the ICC
(International Color Consortium). The goal of the ICC was and is to define
a standardized way for a uniform appearance of images and graphics and
different output devices (printer, screens, ...).
The solution of the ICC are color profiles. In the ideal case, every device
reading or displaying an image should have an associated color profile. The
color profiles are describing how a certain device interprets a given color.
Usually you proceed like this:
- A color profile provided by the monitor manufacturer is assigned to the
monitor.
- A color profile provided by the printer manufacturer is assigned to the
printer.
- If an image is scanned, the color profile of the scanner is assigned to this
image.
If all 3 points are fulfilled, a scanned image printed or displayed on-screen
looks like the original - at least in theory. In practice there are two problems:
- By using the brightness and contrast controls, the monitor can be
adjusted differently as expected by the color profile. Only test print-outs
and accordingly made changes to the contrast and brightness of the
monitor can solve this problem.
- As mentioned above, different devices and thus different color profiles
have a different color gamut. As a rule, printers cannot display as many
colors as monitors. For example, a very bright cyan is nearly never
correctly reproduced. Using a color profile can‘t extend the capabilities
of a given device. But it is possible to simulate a printer on-screen (see
chapter 7.10.1.37). This way, you get at least an impression of what the
print will look like.
To at least reduce the problem with the different color gamut, the ICC has
introduced the rendition type. There are four rendering intents:
- Perceptual
- With Perceptual, the color gamut of the source profile is reduced to fit
the color gamut of the destination profile. So no colors are lost, but the
source colors may be rendered slightly differently. This mode takes
advantage of the fact that the human eye does not recognize absolute
color values very well, but reacts better to color differences.
- Relative colorimetric
- Relative colorimetric leaves the colors of the source profile unchanged.
Colors that the target profile cannot represent are replaced by the most
similar possible color.
- Absolute colorimetric
- Here, the approach is similar to Relative colorimetric, except that the
white point of the color profiles is also taken into account during the
color conversion. This can result in white becoming a shade of yellow or
blue. This mode is mainly used to simulate the output of one device on
another.
- Saturation
- This mode does not emphasize true color reproduction. Instead, it tries to
reproduce colorful colors as colorfully as possible. It is, therefore, most
suitable for diagrams or striking graphics.
Usually, only Perceptual and Relative colorimetric are used, as they
provide the best results for normal work.
Also note that color profiles do not have to support different rendering
modes. It may be that a color profile can only support one rendering mode.
The support of color profiles in PhotoLine is described in chapter 7.6.15
"Set Color Profile", chapter 7.6.16 "Convert With Color Profile" and in
chapter 7.10.1.37 "Color Management".