Details
To see how this works in practice, suppose we have a particular RGB and CMYK color space, and want to convert from this RGB to that CMYK. The first step is to obtain the two ICC profiles concerned. To perform the conversion, each RGB triplet is first converted to the Profile connection space (PCS) using the RGB profile. If necessary the PCS is converted between CIELAB and CIEXYZ, a well defined transformation. Then the PCS is converted to the four values of C,M,Y,K required using the second profile.
So a profile is essentially a mapping from a color space to the PCS, and from the PCS to the color space. The profile might do this using tables of color values to be interpolated (separate tables will be needed for the conversion in each direction), or using a series of mathematical formulae.
A profile might define several mappings, according to rendering intent. These mappings allow a choice between closest possible color matching, and remapping the entire color range to allow for different gamuts.
The reference illuminant of the Profile connection space (PCS) is a 16-bit fractional approximation of D50; its white point is XYZ=(0.9642, 1.000, 0.8249). Different source/destination white points are adapted using the Bradford transform.
Another kind of profile is the device link profile. Instead of mapping between a device color space and a PCS, it maps between two specific device spaces. While this is less flexible, it allows for a more accurate or purposeful conversion of color between devices. For example, a conversion between two CMYK devices could ensure that colours using only black ink convert to target colours using only black ink.
Read more about this topic: ICC Profile
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