I have preped TIF images as grayscale. If I include these images in a PDF, the images are transformed to CMYK images.

This is my simple config:

@page {
  prince-pdf-page-colorspace: gray;
}    

@prince-pdf {
  prince-pdf-color-options: use-true-black;
  prince-pdf-output-intent: url("http://www.color.org/registry/profiles/PSOuncoated_v3_FOGRA52.icc");
  prince-pdf-profile: "PDF/X-1a:2003";
  prince-pdf-color-conversion: none;
}

What do I wrong?

It's not being converted for me (the image object has /ColorSpace /DeviceGray). Can you attach a TIFF image and the resulting PDF?

Edit: if the greyscale image has an ICC profile then it will be converted to match the destination profile. I'd still like to see if that's the case for you.

Edited 22 Jan 2018 by wangp

the image has an icc profile (PSO_Uncoated_ISO12647_eci.icc). What can I do? What is your advice? Is the PDF profile the problem or should we produce CMYK images with only K-information?

Edited 24 Jan 2018 by maddesigns

I assume you want to print the image using only black?

PDF/X-1a requires all colors in the document to be in the same color space, so Prince has to convert the greyscale image into the output intent color space (PSOuncoated_v3_FOGRA52). However, the conversion introduces non-zero CMY values. Prince cannot convert images into K-only CMYK at this time. (The use-true-black option only affects CSS RGB values).

To bypass the color conversion, you can produce a greyscale or K-only CMYK image in the output intent color space, and use the property "prince-image-magic: ignore-icc-profile" to convince Prince that the image is already in the correct color space. Alternatively, just don't embed an ICC profile in the input image.

Otherwise, you may need to generate a normal PDF then post-process it using a tool like Acrobat Pro that can give you greater control over color conversions.

'prince-image-magic: ignore-icc-profile' was the key. Now it works as expected.

Thank you for the help!