Custom CMYK and rendering intents

[Guest]

Hi,

Apparently, a custom CMYK profile generated in PS (CS5) should not support rendering intents. I tried generating a profile with rich black 78c63m64y50k (Light GCR) and converting to it from sRGB. I actually get the choice of perceptual intent that compresses the Magenta and Yellow channels to get a over-saturated conversion while relcol does it job fine and get a washed out separation I can then balance pushing the black channel.

The perceptual rendering is not what I expect. Yet, I was wondering If I could generate a custom cmyk profile intended for editing with other software.

Thanks

[redcrown]

Re: Custom CMYK and rendering intents

Take your question over to LuLa. That's where the color experts are. No color gurus here.

http://forum.luminous-landscape.com/index.php?board=45.0

[Shoku]

Re: Custom CMYK and rendering intents

Hi,

Apparently, a custom CMYK profile generated in PS (CS5) should not support rendering intents. I tried generating a profile with rich black 78c63m64y50k (Light GCR) and converting to it from sRGB. I actually get the choice of perceptual intent that compresses the Magenta and Yellow channels to get a over-saturated conversion while relcol does it job fine and get a washed out separation I can then balance pushing the black channel.

The perceptual rendering is not what I expect. Yet, I was wondering If I could generate a custom cmyk profile intended for editing with other software.

A little more info would be helpful.
1. What application is using the Custom Profile?
2. What is your ultimate intent? -Be more specific.

In Photoshop the user can choose which Rendering Intent to use with any profile, whether a canned profile like U.S. Web coated SWOP v2, or custom profiles made with other software.

[Guest]

Re: Custom CMYK and rendering intents

A little more info would be helpful.
1. What application is using the Custom Profile?
2. What is your ultimate intent? -Be more specific.

In Photoshop the user can choose which Rendering Intent to use with any profile, whether a canned profile like U.S. Web coated SWOP v2, or custom profiles made with other software.

1. The custom profile is intended for editing so PS is generating it and using it.
2. Take advantage of the CMYK structure to correct a file.

Anyway, all other things being equal, a light GCR custom profile will convert very bad with perceptual intent, while UCR will be just fine. I'll stick with RelCol.

[Shoku]

Re: Custom CMYK and rendering intents

1. The custom profile is intended for editing so PS is generating it and using it.
2. Take advantage of the CMYK structure to correct a file.

Anyway, all other things being equal, a light GCR custom profile will convert very bad with perceptual intent, while UCR will be just fine. I'll stick with RelCol.

So if the problem is Perceptual Intent used during conversion, use Relative Colorimetric intent (we recommend this intent for the majority of conversions). Always use Black point compensation. Dither is also recommend when converting from RGB to CMYK.

You may also want to use a custom black generation curve. - see attached example image.

[Guest]

Re: Custom CMYK and rendering intents

Ok. Since the custom profile is intended for editing I'll go with a separation where black ink is added to the shadows and neutrals are made from "balanced" CMY combinations.

Thanks

[Shoku]

Re: Custom CMYK and rendering intents

Ok. Since the custom profile is intended for editing I'll go with a separation where black ink is added to the shadows and neutrals are made from "balanced" CMY combinations.

Thanks

The curve in your attached image shows black in the highlights and only color in the shadows. CMYK curves have the highlights at bottom left and shadows at top right - the opposite of RGB.

[Guest]

Re: Custom CMYK and rendering intents

I see. Yet, I need skeleton black and your profile makes a skeleton CMY. If I keep that custom GCR curve and lower the total black ink (70%) I get grey ramp similar to my custom profile.

The way I read the grey ramp in my profile is that I will get rich black around 78c64m63y50k.

[Shoku]

Re: Custom CMYK and rendering intents

OK, Now I understand what you are trying to do.

FYI: This may just be semantics (how we each understand the term) but Rich Black in the Pre-Press world is a process that puts the maximum amount of black in the file, with a neutral combination of CMY to create richer blacks at press than a flat one channel black.

Some Printers use a combination similar to 75c 60m 60y 100k for a 295 total ink density for their rich black. Others use 60c 40m 40y 100k for less total ink. We use 90c 80m 80y 100k for a total ink of 350 with great results.

A light skeleton rich black would be something like 30c 20m 20y 90k, with mostly black in the file.

Your profile would be good for toned images (Like sepia) where there will still be mostly CMY in the file.

We use a 50GCR profile for toned images.

[Shoku]

Re: Custom CMYK and rendering intents

Here's an example from a recent catalog. We converted the image using a Rich Black profile to keep mostly black in the file to help preserve neutrality at press. See the info pallet for the values in the dress and wall. this does not impact the skin tone.

[Guest]

Re: Custom CMYK and rendering intents

I understand the use of mostly black in the file to help preserve neutrality at press. Also, I must say that the separation I intend (78c64m63y50k) is to have a black channel easy to tweak and expand. For editing, I am separating below the output conditions.

[Guest]

Re: Custom CMYK and rendering intents

We use 90c 80m 80y 100k for a total ink of 350 with great results.

What if you are working on a job that demands being identical across multiple media and the client is insisting that the final products match and allow for CMYK limitations?

Thanks

[skoobey]

Re: Custom CMYK and rendering intents

You hire a pre press printing specialist and everything is handled with those profiles in mind, as a retoucher your job is to work within limitations, there are no limitless files that will work everywhere.

You are, yet again, overthinking it. Custom CMYK profiles are pretty much made to suit the printing process of a specific printer, and creating your own only creates additional clipping as there is no way your custom narrowed gamut will suddenly magically be wide again to accommodate for additional clipping of the actual CMYK profile used at the presses.

Think less, practice more.

[Guest]

Re: Custom CMYK and rendering intents

If a job demands control of the entire process from creative to prepress (not relying on a service to prepare your RGB files for print), does it make sense to work on a custom CMYK and know how to go the extra mile?

Also, the custom narrowed gamut does not have to be wide again.

[skoobey]

Re: Custom CMYK and rendering intents

Do you own the print shop? You will have to handle prepress with them no matter how much of a precision nerd you are, and then there are various issues with ink, and offset and all sorts of things.

It makes sense do go the extra mile and work within the confines of a SPECIFIC profile that is based on the media it'll be printed on. It makes no sense to create a profile that will "look good" on your screen when that can be completely unachievable on the actual print. It needs to be wide again, because your CMYK profile can be completely unrelated to the print process, and then you'll clip things in your work, then clip even more for the print.

Don't overthink, please.