RGB to CMYK - different values

[lightbulb]

Hi

A customer has supplied artwork (in Word Format) for a letterhead that is to be offset printed. We use a trade offset printer who requires (obviously) a PDF file with all images CMYK (no RGB). This word document has an logo in it that is RGB.

I am creating a PDF by printing to Adobe PDF from Word. To convert images to CMYK in the PDF, I then run the "convert to CMYK only" preflight in Acrobat.

What I'm then doing is a quick digital proof to make sure all looks ok before I send the file out to our offset printer. When I rip the original PDF (with RGB images), the rip converts the RGB to CMYK and for a particular blue I get a value of 88C 76M 0Y 0K. This is different to the CMYK value that say, the convert to CMYK only (SWOP) preflight gives me in Acrobat, which is 100C 67M 2Y 0K.

What I would like to know is how can I convert the RGB in the PDF to CMYK so that it gives me the same values (or very close to) as what our RIP converts them to? Or should I be converting to CMYK another way? The colour that I get from the RIP converting to CMYK is "better" (lighter and more like their previous jobs) than the colour I get from using the preflight (too dark).

I understand this post doesn't demonstrate a best practice approach (i.e. having artwork supplied in Word format for an offset job), but in the real world for a small shop it's not always possible to go back to a client and explain certain things or have them pay us for our designer to recreate the job in InDesign etc.

 

[Bloodsaler]

This convert depends on what ICC you choose to convert RGB to CMYK,and it also related to the what kind of convert intension you choose.Usually there are 4 kind of intensions.

 

[Stephen Marsh]

One could use the TouchUp Object tool (instead of a global preflight) to select the RGB Photoshop logo and then perform a manual colour conversion on the Color tab, selecting an output profile that is similar or the same as on your RIP. Obviously, the input RGB profile will also affect the conversion, so Acrobat would need to use the same RGB and CMYK profiles as the RIP.

Another option would be to use the Convert Colours command. If you have RGB black text that should be 0cmy100k then select the Convert Options - Preserve Black feature (presuming that the RGB text was solid black to begin with).

Yet another option would be to select the TouchUp Object tool and to use the "Edit in Photoshop" command, where you will have fine control of the exact colour values in the file.

EDIT: As bloodsaler points out, the rendering intent and the CMM also play a role, in addition to the input and output profiles.


Best,

Stephen Marsh

 

[mattbeals]

Yup, it's in the profiles, CMM and rendering intent. If you're working with a Word file then you're dealing with RGB that is "sRGB". If you're converting to CMYK in Distiller (by way of printing to Acrobat Distiller with "press quality" settings) you are converting to SWOP v2. I think the default rendering intent for PDF is Relative.

But you may get more desirable results converting with perceptual when coming from Office. Whether you use Acrobat Pro or Distiller. In Acrobat Pro you can specify the rendering intent for various object types (depending on which version of Acrobat Pro you have).

 

[lightbulb]

Thank you very much for the information, I am learning quite a bit.

I suppose the next question is, if it's not asking too much, is what files/settings do I need to copy from the RIP to my computer in order to use the same profiles so that when I convert to CMYK using one of the methods above I will get the same colour as on the rip?

The Rip is a Fiery System 6 running on Windows XP. My computer is running Acrobat 9 Pro Extended on Windows 7.

Here are screenshots from CWS5 showing the colourwise expert settings tabs

ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting
ImageShack® - Online Photo and Video Hosting

Thanks again for all of your help and information.

 

[Stephen Marsh]

You may not hit the exact same values, however you would need to try the following:

1. Install the EFIRGB profile from the RIP onto your machine that runs Acrobat.

2. Install the Fiery Coated profile from the RIP onto your machine that runs Acrobat.

3. Convert from the EFI to the Fiery profile using Saturation or Perceptual/Image rendering intent (not sure on the CMM)

It would probably just be faster and or easier to use the TouchUp Object tool and to use the "Edit in Photoshop" command, where one would use Photoshop tools such as channel mixer, selective colour, curves or whatnot to hit the required target values.


Best,

Stephen Marsh

 

[lightbulb]

Thanks very much for your information Stephen. I will give this a go.

 

[killer1loop]

Hi there,

Doing rgb to cmyk conversion is quite a dangerous thing if not following any principle. Be aware that in any of the conversion you are trying, all text blacks will be CMYK composite blacks, which is not a good thing.

Let me know if I can be of further help.

 

[Stephen Marsh]

Be aware that in any of the conversion you are trying, all text blacks will be CMYK composite blacks, which is not a good thing.

As I mentioned in post #3 regarding Acrobat Pro Convert Colours command:

"Another option would be to use the Convert Colours command. If you have RGB black text that should be 0cmy100k then select the Convert Options - Preserve Black feature (presuming that the RGB text was solid black to begin with)."


Best,

Stephen Marsh

 

[mazengh]

First figure out how and what the Rip is using as a rendering intent and profile to convert Images and line art from RGb to CMYK, then use those same settings in your preflight, and this should yield the same result. That is if the rip software is not doing any GCR manipulations.