InDesign Prepress: Transparency Flattener Magic

The longer source for this document can be found here: InDesign 2.0: Printing Output Choices and Flattener Tricks (including force Greyscale export!)

The Transparency Flattener is your colourspace conversion friend. If you take a placed EPS or PDF element that you are not sure is in a CMYK or RGB colourspace, by setting this placed element’s transparency to 99.9% [Normal], a colourspace conversion is undertaken at print time

Now when printing, this element is routed through the magic of the Transparency Flattener prior to output. It sees that your are printing “Composite CMYK” or “Composite RGB” and converts the output to that colourspace. The next question is “what happens, doesn’t this make it see through? Won’t it blend with the colours underneath?” Well, no. 0.1% is a VERY small percentage, and it rounds back to a full number (evidently, some stuff is represented as integers, so 0.1% of 255 is a 254.75, which rounds back up to 255)

You can also use this to force a document into Grayscale. Setting placed EPS/PDF elements with 99.9% transparency and printing Composite Gray results in a 100% Grayscale PDF [watch for spots!]. Good for Newsprint applications. Be warned; the grayscale colours chosen might not always be what you want at print time.

What about exporting EPS or PDF?

Yes, this same process applies.

EPS: you have a choice of CMYK, Gray or RGB. The flattener trick with 99.9% transparency works here too, as elements have to be flattened in the Postscript stream.

PDF: you have a choice of CMYK, RGB or Leave Unchanged. Again, the flattener is invoked where required.

What is the Difference Between InRIP Separations and Composite CMYK?

When printing InRIP separations you are printing Composite CMYK (as above), but InDesign adds some extra Postscript commands to the output device. This instructs the RIP to generate a page per colourant in the file. So, if there is spot colour in the document, it will be separated onto its own plate.

By the way, Acrobat Distiller 4 and 5 ignores this “separate” command, and you get a PDF from the Postscript that is the same as a Composite CMYK PDF. Except that InDesign gets a chance to apply “Application Built-in” trapping prior to creating the Postscript. (ref: InDesign Prepress: Generating Composite, Trapped PDFs)

Different RIPs have different settings for line screen ruling/angles — and in some cases override what the application outputs. Usually because “the application gets it wrong” according to prepress operators I speak to.

What is the Difference Between the two Transparency Blend Spaces?

When Flattening two objects at print time, you’ve got to do some mathematical stuff to determine how colours will mix together. The colourspace this is executed in may change the effective colour of the resulting flattened object. This is similar to the difference you see in Photoshop with some blend modes in RGB vs. CMYK. The recommendation is to set this to CMYK for printed output, and RGB when doing on-screen Acrobat 4.0 style PDFs. Acrobat 5.0 PDFs are not flattened at export time. You can also see a subtle change on screen in InDesign 2.0/CS.