Color Management setup

These are practical notes to issue a PDF for printer, using RGB images, and having to issue a CMYK PDF.

You are supposed to have at least a very basic knowledge of : - what is CMYK, what is RGB - what is a color profile

Preamble
Color Management deals with how color is managed both in PDF output files and in image input files.

Setup is spread across three places:
 * Scribus preferences and document setup
 * Color pane in the export to PDF dialog
 * Little "color management" button in status bar

Scribus and Document Setup
These are the preferences for the PDF output file.

Beware ! When a document is created, it features the Scribus defaults ICC profiles. If later changing the default Scribus ICC profile, the document's own ICC profile dont change and the profile previously set through default scribus setup.

Color management settings
These preferences can be set both in the "Application preferences" and in a specific "Document settings", in the "Color management" tab.


 * "Activate colour management" should be checked when you want colour management


 * "RGB Images" : this is the default profile for RGB images, ie the profile which is used if an RGB image has no embedded profile.


 * "CMYK Images" : This is the default profile for CMYK images, ie the profile which is used if a CMYK image has no embedded profile.


 * "RGB Solid Colors" : This is the profile used for RGB solid colors, ie named colors that have been defined using "Edit > Colors and Fills menu" (in 1.5.x).


 * "CMYK Solid Colors" : This is the profile used for CMYK solid colors, ie named colors that have been defined using "Edit > Colors and Fills menu" (in 1.5.x).

Note that Solid Colors are not related to spot colors which have their own color space.


 * "Monitor" : profile for your monitor when you got one.


 * "Printer" : RIP color profile. Ask your printer.


 * "Rendering intent" : see scribus help for details


 * "Simulate Printer on the screen" : gives visual hints on what the settings and document will produces. All simulation settings have no effect on the produced PDF, only on the display.


 * "Convert all colors to printer space" option has only an effect if the default profiles for CMYK colors or images is different from the selected printer profile.

When "Convert all colors to printer space" option is unchecked, the simulation supposes the CMYK values as defined in the document stay unchanged, are passed as is to the RIP then printed. This is the most common workflow.

When "Convert all colors to printer space" option is checked, the simulation suppose a completely color managed workflow, ie the ideal case for PDF/X. In this case the simulation supposes the CMYK values as defined in the document are tagged with ICC profiles, *converted* to the printer/press colorspace, passed to the RIP then printed.

So that's two printer output simulations for two different workflows.


 * "blackpoint compensation" : when checked, it maps the black point of the input color space to the black point of output colorspace. When input colorspace has a much darker black point than the output colorspace, this is generally the case when converting RGB colors to CMYK, the black point compensation avoid dark colors to be clipped prematurely and consequent loss of details in shadows.

There is no white point compensation because ICC workflows basically suppose the white points of input and output colorspaces are mapped to each other. This comes more or less from the way how device independent colorspaces used during calculation of ICC profiles, CIELab or CIECAM02 usually, are defined.


 * "Mark colour out of gamut" : when checked, colours that are out of gamut are marked green.

Color pane in the export to PDF dialog
First option is to specify the output type : either RGB (web) or CMYK (printer)

Then the "united colors" and "images" title options are to specify how to read images : input setup.

Color management button down in the statusbar
This controls all !



Whenever it is not pushed, no CM will occur.

PDF output
The PDF file format has several versions: 1.3, 1.4, 1.5, X-3 (and X-1, which is available in Scribus 1.5 svn).

Some versions of the format, such as X1, only include CMYK image files; other versions, like X3, accept mixes of RGB and CMYK.


 * PDF/X-1a: Blind exchange in CMYK + Spot Colors, based on PDF 1.3
 * PDF/X-3: Allows CMYK, Spot, Calibrated (managed) RGB, CIELAB, with ICC Profile, based on PDF 1.3.

When creating a pdf that only accepts CMYK, like PDFX1a, Scribus converts all RGB images to CMYK, using the provided color profile.

To create a CMYK-only PDF1.4 or PDF1.5 file, one has to:
 * Set up the correct ICC profiles in the document setup
 * Enable Color Management through CM button in the status bar
 * In the Color pane of the PDF setup dialog, select "printer" as output, and leave everything unchecked in that pane, particularly do not embed profiles for images.

PDF Readers
In Ubuntu, CMYK images are displayed poorly by both Scribus (on screen) and Acrobat Reader: Scribus display is too dull and dark, whereas Acrobat Reader display is too clear and bright. Evince displays CMYK colors correctly.

to be confirmed / investigated
In Ubuntu :

- why does pure CMYK yellow appear brownish when in font, but not when in images ? (and not in windows ?). Might be related to screen profile but why do text differs images ? - why is screen display different when color management button is ON, even when option "display on screen as color managed" is not clicked in setup ?