Color Rendering intents



Rendering Intents: Each device has a fixed range of color that it can reproduce, dictated by the laws of physics. Your monitor can't reproduce a more saturated red than the red produced by the monitor's red phosphor. Your printer can't reproduce a cyan more saturated than the printer's cyan ink. The range of color a device can reproduce is called the color gamut.
Colors present in the source space that aren't reproducible in the destination space are called out-of-gamut colors. Since we can't reproduce those colors in the destination space, we have to replace them with some other colors. Rendering intents let you specify that intent else.
The ICC profile specification includes four different methods for handling out-of-gamut colors, called rendering intents Perceptual and saturation renderings use gamut compression, desaturating all the colors in the source space so that they fit into the destination gamut. Relative and absolute colorimetric renderings use gamut clipping, where all out-of-gamut colors simply get clipped to the closest reproducible hue.

color intents

Color Settings dialog box in Photoshop: go to

Edit>Color Settings.....

Black Point Compensation

Black Point Compensation can be used when transforming files using ICC profiles. An example would be converting from RGB to CMYK. The conversion process using ICC profiles requires a source (where is the file coming from) and a destination (where is the file going). Due to the fact that there is no standard technique in how ICC profiles map pure black from the source to the destination, there are cases where the pure black of the source profile can be a different value than the black of the destination profile. In some such cases, unacceptable results can develop when the file is output. In order to correct these possible problems, Adobe uses a feature in Photoshop called Black Point Compensation. When this option is checked, Photoshop examines the black points of both profiles to see if each will work in harmony.

Use Dither (8-bit/channel images) option controls whether to dither colors when converting 8-bit-per-channel images between color spaces. When the Use Dither option is selected, Photoshop mixes colors in the destination color space to simulate a missing color that existed in the source space, making for smoother transitions between colors.
Perceptual tries to preserve the overall color appearance by changing all the colors in the source space so that they fit inside the destination space while preserving the overall color relationships, because our eyes are much more sensitive to the relationships between colors than they are to absolute color values. It's a good choice for images that contain significant out-of-gamut colors.
In rare cases using Black Point Compensation can cause unacceptable results and the effect is usually washed out detail in the very dark regions of the final image. In our experience this problem usually rears its ugly head with some RGB output profiles. Adobe recommends, that in almost all cases, Black Point Compensation should be on when dealing with CMYK files (doing RGB to CMYK conversions or CMYK to CMYK conversions). In most cases, doing RGB to RGB conversions without Black Point Compensation will produce desirable prints. However, depending on the profile, doing a conversion from RGB to RGB with Black Point Compensation can produce poor output with washed out blacks. It appears that this problem with some RGB profiles is dependent on the software that is used to generate the profile. Apparently there is a “Black Tag” feature in ICC profiles that in some cases can be used or unused depending on the software that actually creates the profile. For this reason, there is no hard and fast rule that says we should or should not use Black Point Compensation with RGB output. Our recommendation is to turn off Black Point Compensation with RGB output profiles or if possible, try a test with Black Point Compensationon and off.
Adobe (ACE)
Uses the Adobe color management system and color engine. This is the default CMM (Color Management Module) in Photoshop.
Saturation just tries to produce vivid colors, without concerning itself with accuracy, by converting saturated colors in the source to saturated colors in the destination. It's good for pie charts and other business graphics, or for elevation maps where saturation differences in greens, browns, or blues show different altitudes or depths, but it's typically less useful when the goal is accurate color reproduction.
Recommended default
Relative colorimetric takes account of the fact that our eyes always adapt to the white of the medium we're viewing. It maps white in the source to white in the destination, so that white on output is the white of the paper rather than the white of the source space. It then reproduces all the in-gamut colors exactly, and clips out-of-gamut colors to the closest reproducible hue. It's often a better choice for images than perceptual since it preserves more of the original colors.
Absolute colorimetric differs from relative colorimetric in that it doesn't map source white to destination white. Absolute colorimetric rendering from a source with a bluish white to a destination with yellowish-white paper puts cyan ink in the white areas to simulate the white of the original. Absolute colorimetric is designed mainly for proofing, where the goal is to simulate the output of one printer (including its white point) on a second device.
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