GCR color shifts

[Ptheobald1]

Hi Group,

I work in Prepress for a packaging printer and one of our press divisions is requesting pretty significant GCR conversion be applied to their client supplied CMYK images. I have been experimenting with using Edit/Convert to Profile/GCR/Heavy with an 85% Black Ink Limit, 300 Total Ink Amount using Photoshop CS5.
This is introducing some unwanted color shifts, especially in the magenta and yellow channels. I have also experimented with doing a LAB mode conversion then going to GCR/CMYK but it does not seem to make much difference....I'm pretty sure Photoshop goes to LAB as a conversion color space when using the Convert To Profile move.

Has anyone come up with a better way to to apply GCR to supplied CMYK without having to go back and do adjustment curves to get the appearance close to the original supplied images? The images are mostly food products, cookies, snacks, chips etc...
Thanks in advance for any suggestions,
Pat

 

[Greg_Firestone]

Have you looked into doing device link color conversions? You'd need additional software to do it but the results are better than standard GCR using traditional ICC color conversions (at least in my opinion).

Greg

 

[mglouis]

Not using Photoshop's custom ink but with ICC profiles then color shifts are negligible. For this you need an icc profile that represents your print specification and has relatively aggressive GCR. If printing to Swop or Gracol then you could play with the K generation settings in Monaco Profiler or similar program until you get the black length and width where you want it. If you like what you are getting from Photoshop, minus the color shift, then you would tweak Monaco's K-plate settings until the K-to-CMY ratio in your grayscale is similar to what you are getting out of Photoshop. Now use this profile for both source and destination with absolute colorimetric rendering.

If Gracol is appropriate for your needs then PM me your email and I'll send you a copy of our production profile.

Matt Louis

 

[Lukas Engqvist]

Try using the colour profiles at colormanagement there are alternate ink limit profile for standard colour spaces.

 

[Ptheobald1]

GCR color shifts

]Have you looked into doing device link color conversions? You'd need additional software to do it but the results are better than standard GCR using traditional ICC color conversions (at least in my opinion).

Greg

Hi Greg,
I am certainly going to look into device link conversion. I have started doing some research into it, it looks pretty promising.
Thank you,
Pat

 

[Ptheobald1]

GCR color shifts

"Try using the colour profiles at colormanagement there are alternate ink limit profile for standard colour spaces."

Lukas,
This is great, I am going to get in there and do some testing today.
Many thanks,
Pat

 

[Stephen Marsh]

"Try using the colour profiles at colormanagement there are alternate ink limit profile for standard colour spaces."

Lukas,
This is great, I am going to get in there and do some testing today.
Many thanks,
Pat

Pat, there are also higher GCR versions of standard profiles which you may also wish to test (I am sure that Lukas was referring to these as they also have reduced ink limits).

There are folks on the forum that can convert a file for you using device links, as DVL are not usually available for test installation due to license conditions. You may also be able to purchase a DVL profile rather than the software used to create the profiles if budget is a concern.


Regards,

Stephen Marsh

 

[Ptheobald1]

GCR color shifts

Hi Stephen,
When using an ICC profile approach for GCR conversion do users Mode Convert from CMYK to LAB then back to CMYK using (for example) the SWOP2006_Coated3_GCR_bas.icc I downloaded from the site Lukas referred me to?
Is this one of the higher GCR versions of standard profiles you mean?

Budget is (of course) always a concern, it's the new normal we all live with : )
Pat


Pat, there are also higher GCR versions of standard profiles which you may also wish to test (I am sure that Lukas was referring to these as they also have reduced ink limits).

There are folks on the forum that can convert a file for you using device links, as DVL are not usually available for test installation due to license conditions. You may also be able to purchase a DVL profile rather than the software used to create the profiles if budget is a concern.


Regards,

Stephen Marsh

 

[Lukas Engqvist]

I normally convert to RGB (ECI-RGB, AdobeRGB would also work if you do not have very intense yellows) and then to the desired CMYK profile. Why I don't go via Lab is that then I would need to go 16 bit, using relative conversion with BCP this approach is fairly painless.
Device link works too, and we use that in our RIP, but some times I want to be able to do the colour reduction, and teach clients how they themselves can solve ink limit issues.

If there are areas that for some reason are negatively affected, if you do CMYK RGB CMYK in PS you can "paint with history" in any area that you want to restore to original, provided you don't have too much ink there… use a soft brush and it should be seamless. (this is still using tools you have paid for).

PS does support device link in current version, but wish there was a "GCR brush tool in CMYK mode" a brush that could paint + or - GCR, have wished for it every time a new version of PS comes out.

 

[meddington]

I normally convert to RGB (ECI-RGB, AdobeRGB would also work if you do not have very intense yellows) and then to the desired CMYK profile. Why I don't go via Lab is that then I would need to go 16 bit, using relative conversion with BCP this approach is fairly painless.

Are you inferring a CMYK conversion to Lab, then convert to CMYK, of just referring to the profile connection space (Lab)? Not sure I see the benefit of adding a conversion to RGB first.

 

[Stephen Marsh]

Hi Stephen,
When using an ICC profile approach for GCR conversion do users Mode Convert from CMYK to LAB then back to CMYK using (for example) the SWOP2006_Coated3_GCR_bas.icc I downloaded from the site Lukas referred me to?
Is this one of the higher GCR versions of standard profiles you mean?

Budget is (of course) always a concern, it's the new normal we all live with : )
Pat

There should be no need in most cases* to go through an intermediate mode in Photoshop of RGB/Lab. One can convert direct from one CMYK profile to a different CMYK profile (there are of course some exceptions, however these are not the general rule).

The profiles at the site in question that have GCR in their name are the higher GCR versions - so yes. Compare the channels to a regular conversion. You can layer one conversion over the top of the other (preserve colour numbers), view an individual channel and then toggle the layer on and off, as well as use colour samplers and info palette.

Like all profiles, you will need to test them using both synthetic and natural images to see if the various profile attributes and their resulting conversions are going to be acceptable.

I am sure that a member here can convert a file for you using a device link, in order to demonstrate the result with one of your files. I am also sure that a member of the forum could license you a device link profile for a smaller fee than you purchasing the software (unless you have many of them to make).


Stephen Marsh

 

[Lukas Engqvist]

@meddington
Yes I have done CMYK to RGB, and usually in that stage I do some fixes to compensate for bad separations. If I have native files I usually keep them in RGB, and then do the conversion back to CMYK as the PDF is generated from InDesign, which is most jobs. (levels, curves, hue-saturation and shadow-highlight work better in RGB in my opinion). From InDesign I export to x1a (colour managing RGB to CMYK with rel and BPC) or PDFx4 and let the RIP do the CMM (disadvantage being some RIPs don't support BPC).
With high quality images then a device link would be preferred, but with low quality material, adding 30 seconds of enhancing in RGB while converting can really lift output of a job.

I know LAB is more theoretically correct, but since captured images are RGB I feel more comfortable with fast fixes in RGB (also i would feel that i'd need to go 16-bit if I was going Lab). It is a practical workflow rather than a perfect one.

 

[meddington]

@meddington
Yes I have done CMYK to RGB, and usually in that stage I do some fixes to compensate for bad separations....

I know LAB is more theoretically correct, but since captured images are RGB I feel more comfortable with fast fixes in RGB (also i would feel that i'd need to go 16-bit if I was going Lab).

If you intend to fix/enhance the images, then converting from CMYK to RGB may certainly have some merit, though I feel this would be more for subjective reasons based on an opinion to "improve" the image. Conversions themselves are not intended to improve the visual result. Someone on this forum once said "If you get what you think you should get how can you get better than that?"

If you're converting simply for the sake of GCR, and already content with the image visually, then the RGB conversion doesn't really add value here. Might also point out that making subjective color edits, versus fixing separation issues are two different things, and that the nitty gritty of the CMYK separation will still be determined by the destination profile (based on its gamut mapping algorithms, black generation, etc.).

Archiving the file in RGB, however, would allow a quicker transform into different destination spaces, so there is a benefit there, and this alone may make it worth your while.

As far as converting to Lab as an interim step, a non device-link profile is going to map to Lab (or XYZ) anyway, with or without an interim step of converting to RGB. And unless your file is already 16 bit, there wouldn't be much benefit gleaned from going to 16 bit for the CMYK-Lab-CMYK conversion at this point.

 

[Lukas Engqvist]

@meddington Agree with your reasoning around RGB. And when I want to reduce GCR with not touching the image I'll do it in the RIP, or in fact using PDF convert colours because RIP and Acrobat give me the option to preserve pure black (which is one of the strongest arguments for device link).

I disagree though that Lab does not require 16 bit. The reason is the size of the gamut that will give rise to banding at 8 bits. It's been some time since i did evaluative analysis though, but the maths hasn't changed, and simply looking at the histogram should make it clear.

 

[meddington]

So would you treat an interim conversion to Lab space differently than a straight CMYK>CMYK conversion, wherein the connection space is Lab? If so, I don't follow the reasoning there. If we're talking image editing in Lab mode, I would certainly recommend 16 bit, but converting from 8-bit to 16 for a straight image conversion with rel. colorimetric I don't think is giving you any benefits. Certainly not hurting anything though.

 

[Lukas Engqvist]

I meant in an interim conversion, not at CMYK CMYK PCS

 

[meddington]

I meant in an interim conversion, not at CMYK CMYK PCS

Both would use the profile's A2B table to get to Lab. Unless your planning on editing in Lab, what would be the difference?

 

[David Milisock]

I would go with device link. If not that then a conversion rom CMYK to an RGB space wide enough to encompass all the source CMYK color space and then convert to a CMYK profile designed to give you the GCR required.

 

[meddington]

I would go with device link. If not that then a conversion rom CMYK to an RGB space wide enough to encompass all the source CMYK color space and then convert to a CMYK profile designed to give you the GCR required.

I agree about the use of a device-link. I still don't see a compelling reason for an interim conversion to RGB (or Lab, or from 8 bit to 16) unless for archival or editing purposes. As you carefully point out, coverting to RGB could in fact clip the cmyk gamut unless a large enough color space is selected.

 

[David Milisock]

I agree about the use of a device-link. I still don't see a compelling reason for an interim conversion to RGB (or Lab, or from 8 bit to 16) unless for archival or editing purposes. As you carefully point out, coverting to RGB could in fact clip the cmyk gamut unless a large enough color space is selected.

The only reason I know of for any conversion from CMYK to RGB is if the supporting software does not support device link conversions. This is an issue, which is why I suggested a method of conversion that allows the user to use software at hand that may not support device link conversions but will still meet their needs.