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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".