How to process device specific colors in PDF
The chapter about rendering in the PDF standard specifies how to transform colors from one device specific color space to another and gives hints on how to transform calibrated colors to device specific colors. But most implementations ignore those rules and use ICC profiles to characterize the output devices. But, what does this mean for the black-generation, the undercolor-removal and other functions in the graphics state?
The rules for the conversion of colors from DeviceRGB to DeviceCMYK refer to the functions BG (black-generation), UCR (undercolor-removal), BG2 and UCR2 respectively. The functions can be defined in the graphics state. However, since the device specific color spaces are characterized by ICC profiles and the color conversion process is accomplished by using a color management engine (CMS) these functions are usually ignored by PDF reader applications. In particular, the PDF/A-2 standard explicitely states to ignore them.
But, how about other functions which are used in the color conversion process such as TR (and TR2) and halftoning? The transfer functions cannot be ignored despite the fact that they should be part of the device characterization using an ICC profile. Fortunately, they are prohibited in the PDF/A standard.
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