Color Spaces in Practice

Editor’s Note: This article is part of the PDF Color Learning Center

Color conversion is not always exact. There is the chance that you will have some round-off error, particularly for the more sophisticated mathematics involved in ICC color. So, you can’t always expect to end up with exactly the same color after doing a conversion and its inverse. Therefore, the fewer conversions you do the better.

Separation logic, i.e., conversion into CMYK also can lead to surprises. Even under perfect conditions you might find cases where what you believe is a single color separates onto two or more plates.

Particularly for high key images, you might find dots on the black plate when you did not expect them. This can be exacerbated by the choice of image compression. This is generally not a problem for lossless compression where you get back exactly what you started with after a compression-decompression cycle. It is more likely to happen with lossy compression, such as JPEG.

The accompanying table shows a comparison of the color spaces supported in PDF and PostScript and how they relate. For every PDF color space, there is an available PostScript color space. This also explains why PostScript is the output file format for graphic arts applications. A corresponding chart for PCL would have too many gaps.

PDF & PostScript Color Spaces
PDF Color Space Type 1st PDF Vers. 1st Acro Vers. Equivalent PostScript Color Space
DeviceGray D 1.0 1 DeviceGray
DeviceRGB D 1.0 1 DeviceRGB
DeviceCMYK D 1.0 1 DeviceCMYK
CalGray I 1.1 2 CIEBasedA
CalRGB I 1.1 2 CIEBasedABC
Lab I 1.1 2 CIEBasedABC
Separation S 1.2 3 Separation
Indexed S 1.2 3 Indexed
Pattern S 1.2 3 Pattern
DeviceN S 1.3 4 DeviceN
ICCBased I 1.3 4 See Comments (below)
Unavailable I CIEBasedDEF
Unavailable I CIEBasedDEFG

Color Space Types: D = device dependent, I = device independent, S = special

PDF & PostScript Color Spaces
PDF Color Space 1st PS Level Comments
DeviceGray 1
DeviceRGB 1
DeviceCMYK 1.x Late addition to PS 1, formally added in PS 2. Level 1 & 2 separation mechanisms are different.
CalGray 2
CalRGB 2 CIEBasedABC can cover any 3-component device independent color space, e.g., CalRGB, Lab, etc.
Lab 2
Separation 2 For spot colors. Crop marks etc. are done in Separations=All to appear on all plates.
Indexed 2 Table for a fixed number of input values. All but Pattern and Indexed may be indexed.
Pattern 2
DeviceN 3 For duotones, HiFi color seps, etc. Appears as composite color in Acrobat.
ICCBased A PostScript 2 or 3 CIE color space with the same number of components may be used.
Unavailable 3 Permits 3-component table look-up before normal CIEBasedABC processing.
Unavailable 3 Permits 3-component table look-up before normal CIEBasedABC processing.

What is the best color space for production work? No matter which you choose, you should consider what steps you will be taking downstream from Acrobat. If you are printing directly from Acrobat, life is simpler. But if you are outputting PostScript files to other downstream processes, such as trapping or imposition, then you need to verify that each of these adhere to the same assumptions.

As stated before, different color spaces may be used within a page. However, for some objects you have no color space choice. True PostScript 3 duotones, tritones and quadtones are specified in DeviceN. Spot colors are represented using Separation color space. Optionally, either of these color spaces may be printed to separate plates or converted to process color on output. This leads us to RGB versus CMYK versus ICCBased color.

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About the Author: Gary Armstrong

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