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View Full Version : RGB to CMYK. What the......?



kiwi dave
09-10-2005, 10:48 PM
Most of my work is for print, so converting from RGB to CMYK is usually not a problem. The difference is minimal, and colours can be corrected in photoshop.

But when I was working on a logo recently, I got an extreme change when I converted colour modes - take a look at the attached files and you'll see what I mean.

Has anyone noticed this before, and if so how did you get around it.

Cheers

Dave

BeeVee
09-11-2005, 02:23 AM
Blues and greens are the hardest colours to maintain in an RGB to CMYK conversion, although it's much easier with Hexachrome. The reason is that there's only Cyan as far as blue is concerned, and there's no green at all in CMYK, so compromises have to be made. You cannot simply use an automatic conversion, you'll have to baby it through...

B

toby
09-11-2005, 02:26 AM
What my limited experience tells me is that your hotspot there is just out of range of what the cmyk can display, while being a very similar hue to the rest of the logo at the same time. Can you simply render a darker image, then color correct it?

edit -
ah, someone who knows what he's talking about ;D
don't listen to me!

09-12-2005, 09:57 AM
I hate printers (the people).
This is because they always try to pass the buck on getting the colour right.
CYMK/RGB conversion is NOT a 'constant' - like metres to feet or PAL to NTSC or even .3ds to .lwo.
It is absolutely dependant on the 'profile' of the printing inks/machines/processes to be used.
If you change your GRB output to CYMK without being absolutely sure of the profile required for the particular situation, you are shooting yourself in the artistic foot, you are giving the printer licence to seriously screw up your job and absolving them of all blame.
Simply converting in Photoshop or whatever to whatever generic profile you have loaded by default is NOT the way to go - anyway, photoshop will then simply store the image in a different format, and convert it back to RGB to display it. Sometimes (if you're lucky) it will, by default, use a different profile to the one used in the onversion, which will give you a clue that all is not well!
Yes, there are some colours (more usually reds) which are very difficult in CYMK.
Talk to your printers - let them know that getting the colour right is their responsibility, and ask how you can help, they may suggest spot colour or a little change in saturation.

etobiason
09-12-2005, 07:34 PM
Instead of Convert to CMYK try the Convert to Profile option, and set the CMYK profile for your printer.

The other thing I do to save those blues is shift them before I convert. Experiment with it.

toby
09-12-2005, 10:25 PM
Adrian, are you saying he should *not* convert the image, let the printer do it? It makes sense to me, let them see exactly how you want it and convert it according to their equipment...

etobiason
09-13-2005, 07:48 AM
That is not always an option... it entails having the printer convert, send you a proof, you view the proof under color-corrected lighting and ask for color moves here and there. It can involve several rounds of changes, and the printer charges for it.

The other option is to do the conversion yourself, for whatever printer you have on hand, and then send that to the printer as the proof. The printer will then correct the file for their printer in order to match your proof. They don't have to keep coming back to you for opinion, they know what you want because you've given them a proof.

-e

Thomas
09-13-2005, 08:18 AM
If you have a calibrated monitor, slap ctrl+Y. Then you can se the CMYK results in Pshop but still be in RGB mode.

Play around with curves and hue/saturation to try to get back the highlight. It will NEVER be the same color as in RGB but you can probably get a nicer conversion after you corrected it in RGB first.

Try Hue/Saturation with Cyans:Hue+13-18 and lightness +10. Since we're adding a bit of red there is more "color" to choose from.

Be sure you get the printers ICC-profile for the press/paper combination used!

Regards

Thomas

kiwi dave
09-13-2005, 04:16 PM
Thanks for the feedback guys. Most of what you have said re the conversion I have tried but without great success.

To get the logo right I duplicated the Red channel (it showed the highlight area with the most contrast) then converted it to CMYK, made a selection of the white areas from the Red channel, added a new layer and filled it with white.

A bit of mucking around with masks on the new layer, but in the end the file looked exactly like it should.

Dave

J. Stuart J.
09-14-2005, 12:28 AM
One trick you can try is to create a luminosity map to help recover the high-lights that are lost by the compressed by tonal range of CYMK.

1. Load the image and convert it to LAB mode.

2. Display/Click on the L channel (Luminosity), select all and copy it to the clipboard.

3. Now convert to CYMK or Desired profile.

4. Once the file is converted to CYMK, Paste the L channel copied to the clipboard into a new layer and change the blend mode to Luminosity. Then just adjust the Opacity to suit. You may still need to make a slight curves or levels adjustment, but a luminsity bump layer should help restore that lost highlight with out casting.

Hard and soft light also provide useful results.

When converting diffucult images, Its also helpful to try adjusting photoshops rendering intent from relative to perceptual. Locate in you color preferances or as a drop down choice while using convert to profile.

J. Stuart J.