Does removing an embedded cmyk profile change the output of a cmyk tiff?

JM
Posted By
James_McRobert
Jul 11, 2004
Views
277
Replies
8
Status
Closed
Hi,

If someone could help me out or point me to reference material (or a forum topic I couldn’t find) I’d be very grateful.

If I have a cmyk tiff with an embedded cmyk profile and I run it to press or film, can I expect a 20% area in the file to show in the film/output as a 20% stipple – or will the embedded cmyk profile change the values or an already separated image?

If so, does this mean in the ripping process, the image is ‘re-separated’ as instructed by the profile.

I have run into an issue with a printer where a file (cmyk tiff) was run by them twice – once supplied with a generic euro cmyk profile embedded and once without. The colour numbers / channel values were identical.

My impression was the working space colour separation is consulted (and stored as a profile) to converted the image from rgb to cmyk. Once the file is separated, the colour numbers are set in stone unless you apply a transfer function.

The documentation seems to say ‘profiling makes everything nice’ and doesn’t cover the nitty gitty of how the cmyk image data is affected.

Thanks
James

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JS
John_Slate
Jul 11, 2004
Most printers use rips that do not reseparate using the profile, so whether or not you embed makes no difference.

For one thing, single color blacks will generally end up as 4 color blacks (CMYK>Lab>CMYK).

Then again some printers use rips that do reseparate using the profile.

So you should check with your supplier.

In theory what your really need is a profile from your printer to use when converting from RGB to CMYK, in which case whether or not you embed the profile becomes moot, but good luck finding a printer that will give you one.
MO
Mike_Ornellas
Jul 11, 2004
The film should be linear.

A profile can change the output if the RIP accepts it and is set to apply said profile.

Transfer curves describe or adjust press plates for beharior.

It’s best that a profile is not applied until whomever figures out what they are doing.
MO
Mike_Ornellas
Jul 11, 2004
John,

FYI

A Brisque can accept standard ICC profiles as well as device links.
AR
Andrew Rodney
Jul 11, 2004
The profile is only a descriptor of the numbers in the document. So all things being equal, sending the same numbers with and without a profile will NOT affect the output. Now IF the RIP or color server is ICC aware and is setup to do a conversion when it detects a profile, a conversion (alteration of the numbers) can happen. So ask the service provider what they run.
JM
James_McRobert
Jul 12, 2004
Thanks very much for responding guys.

I’ve been working printing since postscript’s inception and have always relied on a rgb tiffs are colour unreliable (having to be separated before output) and tiffs once actively separated into cmyk with the GCR or UCR tools of choice were fixed and ‘colour stable’.

I’m going to have to ensure that no profiles are embedded in future (which WAS really useful as it could tell you exactly how it was separated) in case the image is treated once at separation and ‘sometimes’ again when ripping.

Its a minefield out there.

Thanks again.
James
MO
Mike_Ornellas
Jul 12, 2004
‘colour stable’. ?

um…

I think you need to do some more homework.
JM
James_McRobert
Jul 12, 2004
Bad choice of words.

Colour number stable – as in the cmyk colour numbers in the file represent the stipple value the output device will endevour to produce. It isn’t ‘re-separated’ or ‘re-evaluated’ any differently than other colour references in the layout document importing it.

Of course if you run a cmyk separated for magazine and run it in news press you’ll get dot gain and total ink issues and it’ll look like proverbial.

What I’m trying to avoid is re-interpretation of an image’s colour values when the rest of the layout document isn’t receiving the same treatment – and therefore cause colour differences within a layout.

Cheers
James
MO
Mike_Ornellas
Jul 13, 2004
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