Color management and profile conversion

What is color management

At any work dealing with color, it is very important that the same image looks in the same color on any display or on any prints. In reality, however, the colors often look different depending on the device. For example, colors on a standard display and on a high color gamut display may look different, or colors on a screen and on a print may look different. Simply adjusting the gamma and the white balance of the display is not enough to prevent this. It should be necessary to convert the color data appropriately according to the difference in the color space of the device and pass it to each other. Maintaining colors (or color matching) in different environments is called color management.

Color management is generally performed using an ICC (International Color Consortium) profile conforming to the ICC format. An ICC profile (also referred to as a color profile, or simply a profile) is a numerical representation of the color space information of a device such as a display, camera, or a printer. It includes the coordinates of the primary colors, the coordinates of the white point, and the tone response curve (RGB Gamma correction curve or CMYK decomposition curve). As well as the profiles for typical color spaces of sRGB and Adobe RGB, profiles for various CMYK inks (such as Japan Color 2001) are available. Printer dealers also provide a color profile for a printer and distribute it with the printer driver. Furthermore, you can create a color profile for a display or a printer using a measuring device. As for image data, full-color formats such as JPEG, EPS, TIFF, PNG, and Photoshop PSD support color profiles, and it is common that the color space information of the image is embedded when the image is saved.

Recent computers support color management at the OS level. Mac OS X includes a color engine called ColorSync, which reads image profiles and processes color conversion according to the device. Windows has a color engine called WCS (Windows Color System). Many applications such as browsers read the color profile of an image and process color conversion using the OS's color engine. The images below originate from the same photo but have color profiles for different color spaces. If all images look the same in color, your browser supports ICC profile. On the other hand, saturation of the images will look different if your browser doesn't support profiles. As of 2012, the latest versions of major browsers such as IE, Firefox, Chrome, and Safari support profiles. However, the colors of images without embedded profiles, as well as background and text colors seem to depend on the browser.

Images for profile check

ProPhoto RGB	Adobe RGB	sRGB	MacBook LCD


When viewed with a profile compatible browser	When viewed with a profile incompatible browser

Color management is based on ICC profiles. For example, when displaying an image with an Adobe RGB profile on an sRGB display, the RGB values of the image are first converted to XYZ or L*a*b* coordinates, which represent device-independent absolute color, and then converted to sRGB values to be displayed. In this case, colors that do not fall inside the color gamut of the destination color space are replaced with approximate colors inside color gamut. In this way, it is possible to show an image somehow even on a display having a color space different from the original image. The XYZ or L*a*b* color space used in profile conversion is called a profile connection space because it connects the two profiles. The XYZ and L*a*b* are practically the same because they can be easily converted to each other. If color management is not performed, an Adobe RGB image shown on an sRGB display will look dull because the RGB values will be interpreted as those of sRGB and the saturation is reduced. Conversely, if an sRGB image is shown on an Adobe RGB display, the RGB values are assigned to the wide Adobe RGB color gamut, so the image may look too vivid.

original color space
e.g.
Adobe RGB

→

profile connection space
XYZ or
L*a*b*

→

destination color space
sRGB,
CMY , etc.

The same is true when printing an image with a printer or converting the mode of an image from RGB to CMYK. In this case, the RGB values of the image are first converted to XYZ or L*a*b* and then converted to the CMYK values of ink. Again, colors that fall outside the gamut of the destination color space are replaced with approximate colors inside gamut. The figure below shows the change of colors outside the destination color gamut, viewed in L*a*b* space. The color coordinates before and after the profile conversion are examined using Photoshop. In the right figure, the profile for Japan Color 2001 coated paper, which is widely used in Japan, is used as a CMYK profile.


Example of color management in Lab space: Adobe RGB → sRGB. Colors within the destination gamut remain unchanged, but colors outside the destination are replaced with colors inside the destination (light circle → white point)	Example of color management in Lab space: Adobe RGB → CMYK. Colors outside the destination gamut are replaced with colors inside the destination (light circle → white point)

Example of color profile

I will refer to profiles on Mac and Windows. On Mac OS X, we can see the details of the profiles installed on the system with the ColorSync Utility, which is included in the utility folder. The figure on the left shows the ColorSync utility displaying the list of profiles. When we open a profile, its details are displayed as shown on the right. On Windows, we can see which profiles are installed on the system using the Color Manager control panel, but details of the profiles are not shown.


ColorSync Utility. We can check installed profiles	Details are displayed when opening the profile. This is the profile of Adobe RGB (1998).

Color settings of Photoshop

Adobe products, such as Photoshop, have their own color engine (ACE = Adobe Color Engine) and support color management at the level of software. The setting can be made with "Color settings" in the edit menu. The figure shows an example of a typical setting (Prepress-Japan2) for printing materials.

Photoshop color settings

In the settings on the right, Adobe RGB is assumed as the working color space. Adobe RGB is the standard color space when dealing with printed material, and a wide color gamut display is required to take advantage of this setting. When editing an Adobe RGB image on a normal display, color space is converted to sRGB, so colors outside the sRGB gamut must be substituted with approximate colors. If you use Photoshop mainly to edit web images, you may want to set the working color space to sRGB. This is because sRGB images are usually used on the web so that colors look correct even in the environment that does not support color profiles.

As for CMYK space, Japan Color 2001 Coated seems to be common in Japan. Considering that the print manuscript will be converted to CMYK and passed, it will be better to set CMYK as shown in the figure.

The setting of "Rendering Intent" specifies the policy of profile conversion, and can be selected from Perceptual, Saturation, Relative Colorimetric, and Absolute Colorimetric. It is common to set it to Perceptual or Relative Colorimetric. This setting is used as the default for profile conversion when changing the image mode (RGB color, CMYK color, Lab color, etc.). On the other hand, when an image is converted using "Profile Conversion" menu, the rendering intent can be specified each time. Details will be described later.

It is recommended to check "Use Black Point Compensation" option when converting an RGB image to CMYK . In general, printed black is not as black as a display, so if black on the screen is reproduced with CMYK, dark colors will be blackened out. Black point compensation is an option to prevent this black out and to make dark colors look natural. "Use Dither" is an option for whether or not to represent intermediate colors with dither when the converted numeric value (RGB value or CMYK value) is not an integer in the 8-bit/channel mode image. When this option is on, color difference in gradation (band phenomenon) is less noticeable, but the color looks slightly grainy. If you worry about the roughness, you can turn it off.

In the "Advanced Controls", "Desaturate Monitor Colors" is the option to edit images having a wide gamut such as Adobe RGB on a usual sRGB display. If this option is off, colors outside the gamut of the display are replaced with approximate colors. If this option is on, however, the saturation of the entire image will be reduced so that colors outside the color gamut will be distinguishable somehow.

With the standard settings of "Color Management Policies" shown above, you will be warned if the color profile embedded in the image being opened is different from the working color space. In such a case, the embedded profile should be used. Photoshop will pass converted colors to the display having the working color space, with preserving the RGB values of the image. When the profile is not embedded in the image (often the case of images on the Web), if you select "No Color Management", the image will be shown with the RGB values of the working space. If the colors look strange without color management, you can force to assign the color space (usually sRGB or Adobe RGB) when opening the image or with "Assign Profile" button.

Printing with Photoshop

Photoshop (and other Adobe products) allows you to choose whether the printer or Photoshop manages colors in printing. In the Print window, switch the "Output" to "Color Management", and in the "Color processing" column select "Printer Manages Colors" or "Color management by Photoshop" (right figure).
If "Printer Manages Colors" is selected, Photoshop passes image data and an image profile to the printer, and the printer driver converts it to the printer's color space and prints it. For "Photoshop Manages Colors", specify the printer model and paper type in "Printer Profile". (If an inexpensive printer does not come with a printer profile, you cannot proceed.) Then Photoshop will convert the color space of the image to that of the printer and passes the data to the printer. In this case, the setting on the printer side must be set to "No Color Correction". If you leave the printer setting to "Automatic", the profile conversion will be performed twice, resulting in strange colors. The printer settings can be checked with the "Print Settings" or "Page Settings" button on the print screen. If "Auto" is set, turn it to "No Color Correction".
On the print screen, there are "Document" and "Proof" options. Normally (when the printer is the final output), select "Document". "Proof" is used for proof printing. If "Proof" is selected, Photoshop will first convert the RGB image data to the CMYK data for a print shop, then convert colors for your printer and pass the image data to the printer. Because no printer driver is used, you can get a print that is close to the image by a print shop.

Color change in profile conversion

Converting the profile of an image to another profile is called profile conversion. In Photoshop, this can be done by "Profile Conversion" or by changing the color mode. By profile conversion of an image, RGB values change to new values or RGB values are converted to CMYK values, depending on the destination color space. Throughout profile conversion, the original colors (colors in absolute color space such as XYZ or L*a*b*) are maintained as much as possible, except colors that do not fall in the destination color gamut. The policy of profile conversion can be selected from the following four rendering intents:

Perceptual ... This converts colors to look as natural as possible
Saturation ... This converts colors to suppress changes in saturation. Useful for business graphics.
Relative Colorimetric ... This maintains colors as much as possible. If the white points are different, conversion is performed so that the white point does not change. Colors inside the destination color gamut remain unchanged.
Absolute Colorimetric ... This maintains absolute colors as much as possible. When the white points or the black points are different (usually the white point on paper is not white and the black point on paper is not black also), the conversion is performed so that absolute colors including paper color remain unchanged. Useful for proof printing.

Adobe RGB → sRGB

Here are the results of experiments on profile conversion with various rendering intents using Photoshop. First experiment is conversion from Adobe RGB to sRGB. The top row of the figure on the left is the original image, where L* is constant, b* = 0, and only a* varies within the width of Adobe RGB color gamut. The lower four images are obtained by converting the profile of the original image to sRGB, and the white dots correspond to the boundaries of sRGB color gamut. As can be seen from the figure, the colors inside the destination gamut remain unchanged, but the colors outside the color gamut are slightly changed in lightness and saturation. There is no difference between rendering intents at all. The figure on the right shows the color change due to profile conversion in a cross-section of the L*a*b* color space ( ○ before conversion, • after conversion). It can be seen that the colors outside the destination color gamut have been replaced with colors near the gamut boundary (actually values of b* also change slightly, though they are not shown).
(Note) In the figure below, the saturation has been reduced so that the color difference can be seen even on the sRGB display.

On the web, images with the sRGB profile are used often because most display have sRGB gamut and also some browsers do not support color management. If you would like to make an image for the web from an Adobe RGB image, it is recommended to convert the profile from Adobe RGB to sRGB using "Profile Conversion" of Photoshop. Generally, if a profile is converted to a narrower color space, colors outside the destination gamut are pushed into the destination. Even if you try to convert the image back to the original wider color space, all the colors will remain unchanged. Lost colors never come back.


Example of profile conversion (Adobe RGB → sRGB). White points indicate the boundaries of the destination.	Example of profile conversion (Adobe RGB → sRGB). Cross section of Lab color space (the La plane) is shown

Adobe RGB → CMYK

The next experiment is conversion from Adobe RGB to CMYK. Japan Color 2001 Coated was used for the CMYK profile. Black point compensation was turned on. The top row of the figure on the left shows the original image, where L* is constant, b* = 0, and only a* varies within the width of Adobe RGB color gamut. The lower four are obtained by converting the profile of the original image to CMYK, and the white dots correspond to the boundary of CMYK color gamut.
In the "Relative Colorimetric", it can be seen that colors in the destination color gamut remain unchanged, and colors outside the destination are replaced with approximate colors.
In "Perceptual", not only the colors outside the destination color gamut but also the colors inside the destination change both in lightness and saturation, but the smoothness of the original image seems to remain almost undamaged. "Saturation" is similar to "Perceptual", but the saturation is slightly enhanced over all.
In the "Absolute Colorimetric", the entire area is bright to reproduce the screen colors on paper.

The difference between rendering intents of the profile conversion can be clearly seen in the cross section of the L*a*b* color space. In the "Relative Colorimetric", the colors in the color gamut have not changed at all. In "Perceptual", the saturation is slightly reduced, but the overall change is smooth. In "Saturation", colors generally have higher saturation than "Perceptual".
(Note) In the figure below, the saturation has been reduced so that the color difference can be seen even on the sRGB display.


Example of profile conversion (Adobe RGB → CMYK). White points indicate the boundaries of the destination.	Example of profile conversion (Adobe RGB → CMYK). "Relative Colorimetric"


Example of profile conversion (Adobe RGB → CMYK). "Perceptual"	Example of profile conversion (Adobe RGB → CMYK). "Saturation"

When a print shop accepts RGB images, simply hand the data (typically EPS format for photos) with the embedded profile, and leave the profile conversion to the print shop. On the other hand, when CMYK images are required by a print shop, profile conversion of images to CMYK is needed. Moreover, since image data of Illustrator usually use the CMYK data, the photos for the Illustrator should be converted to CMYK.
When converting RGB image to CMYK, it is common in Japan to use the Japan Color 2001 Coated profile, turn on black point correction, and use "Perceptual" as the rendering intent. However, as can be seen from the above experiment, saturation and lightness tend to be reduced in "Perceptual" because the smoothness of color change is prioritized. If the image is not so saturated and does not contain colors outside the CMYK gamut, a better result may be obtained by setting the rendering intent to "Relative Colorimetric" because the colors are preserved. In fact, in Europe and the United States, "Relative Colorimetric" is set as the default in "Color Settings." To check the area whose color is out of the CMYK gamut, it may useful to turn on the "Gamut Warning" in "View" menu (gray parts in the figure have out-of-gamut colors).

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T. Fujiwara