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CMYK conversion question

Craig_Lamson

New member
Ok, I know cmyk conversion questions are a can of worms but I need some guidance.

I'm producing files for a marine brochure, all interior images shot on my 1DsMKII.

The printer wanted the agency to send in camera raw files and I refused. They said ok send us an rgb file and we will see what you have. I still refused as I prefer to ship cmyk. So the printer sends me the profile for thier proofer (good move...match the proof on press). I send them two files, one a colormatch rgb file and the other a cmyk file (relco no PBC).

A few days passes and I get a call from the printer...how did you convert this file they ask. I tell them and then the ask if they can swing by the studio with the proofs.

Seems my conversion is better than what they are getting and they want to know how I got such good color. We compare conversion process setups and they are the same. Both proof are very close and are a very good match to my monitor when viewed in a 5000k viewing booth. The differerence are subtle, my file has just a bit more black.

So heres my question. My files were processed on a PC using Adobe Ace engine and the printers file was processed on a Mac. Can this explain the difference in the conversions?

Mind you I'm very happy with the proof outputs and they assure me they will run very well on press and are more tean happy to accept my conversions (not to mention wanting to offer up my name when a customer is looking for a photographer). I am just curious.
 
Craig_Lamson said:
So heres my question. My files were processed on a PC using Adobe Ace engine and the printers file was processed on a Mac. Can this explain the difference in the conversions?
One would hope that if both used ACE that they would process identically (IEEE standard arithmetic on the CPU through the same algorithms). So, before we go and blame Adobe, Intel, Apple, Freescale (Motorola's spun of PowerPC CPU fab divisions), Metrowerks or Microsoft (the OS-X and XP compiler writers that Adobe uses last I read), and etcetera let us first ensure you are using exactly identical CMYK conversion settings.

In CS2 open an image and go to Edit->Convert To Profile and select Custom CMYK from the drop down menu. You should see the following (minus my shameless plug for my snuff photos [a European Paper Wasp taking down some arachnid]).

CMYK_conversion.jpg


Were all your parameters for Ink Colors, Dot Gain, Separation Type, Black Gneration, Black Ink Limit, Total Ink Limit, and UCA Amount absolutely identical?

If so, then something is wrong and I seriously suggest contacting Adobe. If not, then the chosen default CMYK conversion settings differ and that is likely the cause.

This post is just to double check you checked the right thing. If you did, then I very seriously would like to know what you learn from Adobe as a result.

enjoy your day,

Sean
 

Craig_Lamson

New member
I'll check that on Monday. In the mean time I'm going to assume (ok I know...really bad thing to do) that all the custom settings were the same since the profile used was supplied by the printer.

I'll let you know what I find next week.

Thanks
 

Andrew Rodney

New member
-->So heres my question. My files were processed on a PC using Adobe Ace engine and the printers file was processed on a Mac. Can this explain the difference in the conversions?

Using the same profile and settings in Photoshop, no. The conversions would be identical. Something else had to have happened.

Sean, you're showing the Classic CMYK engine which is bad news and should be avoided plus it's NOT using ICC profiles nor ACE which the original poster said he used. He's using a totally different (and better) conversion process.
 
Andrew Rodney said:
Sean, you're showing the Classic CMYK engine which is bad news and should be avoided plus it's NOT using ICC profiles nor ACE which the original poster said he used. He's using a totally different (and better) conversion process.

Andrew,

This is just a subdialog of Edit->Convert To Profile and ACE was selected. How is it not using ACE? You get the same result* whether the input image is sRGB or ProPhoto RGB which implies that the color space is being handled. Albeit, using the built in US Web Coated (SWOP) V2 profile yields a different conversion, it still yields the same results regardless of input color space.

It was my understanding that the Custom CMYK dialog simply defined the conversion parameters (shape of the null space of the transformation). Is this wrong?

As to avoid the Custom CMYK dialog. That is horrible advice for post processing. That dialog is the only way I know of the control the shape of the null space in the RGB->CMYK transform which can be very helpful in automatically generating masks to isolate features for various post processing work. But then, I consider any advice that circumvents thought and understanding is horrible advice. Giving up a tool in one's toolbox is just plain silly. Especially such a powerful tool.

thanks,

Sean



* Within numerical tolerance.
 

Andrew Rodney

New member
-->It was my understanding that the Custom CMYK dialog simply defined the conversion parameters (shape of the null space of the transformation). Is this wrong?

Yes. The classic CMYK engine (around since about version 2) has nothing to do with the engine that is used by ICC profiles, accesses ACE and Black Point Compensation etc. It's a flaky, buggy way to handle CMYK conversions. You can build a conversion and save out an ICC profile then use that with the more modern conversion engine by Photoshop (you get all the nice bugs applied in the saved profile). But if you use the classic CMYK engine, you're not using ICC profiles, ACE etc and the ink models which this old engine is based is outdated and incorrect.

Do this: Load SWOP Coated in the classic engine and do a conversion from RGB. Now do the same on a duplicate using U.S. Web Coated (SWOP) v2 ICC profile. Not the same, not by a long shot.
 
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