Luminous Landscape Forum

Raw & Post Processing, Printing => Colour Management => Topic started by: smilem on August 25, 2013, 07:37:48 pm

Title: cmyk neutral grayscale values ?
Post by: smilem on August 25, 2013, 07:37:48 pm
Hello, I have question about cmyk grayscale values or should I say neutral grayscale because I'm looking to use all channels.

In RGB it's easy you just send a file with equal amount of R G B like 119,119,119 produces gray patch when printed equal to LAB L50 a0 b0

To produce the same in CMYK I need to use C50 M40 Y40 K27 or LAB L50 a0 b0 right?

Second question is why photoshop colorpicker or should I say color selector is wrong all the time? If I set my color to be C50 M40 Y40 K27, then LAB should be L50 a0 b0 but it's not.
LAB is something like L47 a0 b-1 the HSB is also not H0 S0.

If I set the zeros by hand in the fields I said then paint with this color when you take colorpicker and check what you painted its all wrong.

I painted LAB L0 a0 b0 PS said it's CMYK C88 M79 Y65 K93
I got LAB L1 a0 b-1 PS said it's CMYK C88 M78 Y65 K93

The same happens all the time with other combinations required for neutral gray patches too, so I had to use a plugin for color selector to over come this nonsense.

Nobody noticed this? Photoshop Math functions are very bad here, color angles in HSB is a clear evidence for this, the angle should be the same for neutral LAB colors but the angle changes all the time even if you set LAB by hand to be zeros for a and b.
Title: Re: cmyk neutral grayscale values ?
Post by: digitaldog on August 25, 2013, 07:41:07 pm
Quote from: smilem on August 25, 2013, 07:37:48 pm
To produce the same in CMYK I need to use C50 M40 Y40 K27 or LAB L50 a0 b0 right?

Only for the CMYK profile you have loaded to provide this recipe. IOW, every CMYK device is different and will have (can have) a completely different mix of CMY&K to produce a neutral.
Title: Re: cmyk neutral grayscale values ?
Post by: MarkM on August 25, 2013, 08:34:07 pm
Quote from: smilem on August 25, 2013, 07:37:48 pm
Photoshop Math functions are very bad here, color angles in HSB is a clear evidence for this, the angle should be the same for neutral LAB colors but the angle changes all the time even if you set LAB by hand to be zeros for a and b.

Hue angles is indeterminate for neutral colors—you're directly in the center of the space so all the angles are the same. Kind of like being at the north pole and asking what your longitude is. Also keep in mind HSB is not an absolute color space—it's just a very simple transformation of RGB and is device dependent.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on August 26, 2013, 03:47:46 pm
Quote from: digitaldog on August 25, 2013, 07:41:07 pm
Only for the CMYK profile you have loaded to provide this recipe. IOW, every CMYK device is different and will have (can have) a completely different mix of CMY&K to produce a neutral.

I'm talking about device independent color spaces such as isocoated, gracol2006, swop etc.
The correct CMYK recipe is a ICC profile job.

But I must send correct color request to get ICC profile make grayscale, so I'm asking does every device independent color space listed above requires a different CMYK values to request printer profile to make grayscale ?
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on August 26, 2013, 04:35:50 pm
These are CMYK color spaces of an offset press "device", so they're device depended by definition ;)

There are countless CMYK values that will build a neutral grayscale - depending on GCR/UCR curves used while creation of CMYK ICC profile.

Quote from: smilem on August 26, 2013, 03:47:46 pm
I'm talking about device independent color spaces such as isocoated, gracol2006, swop etc.
The correct CMYK recipe is a ICC profile job.

But I must send correct color request to get ICC profile make grayscale, so I'm asking does every device independent color space listed above requires a different CMYK values to request printer profile to make grayscale ?
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on August 27, 2013, 06:17:17 am
So to create say a target for RGB grayscale you just use patches with equal amounts of RGB. And then let printer firmware/chip combo to do the conversion.

How do I proceed with the same task for native CMYK printer that mat use ISOcoated, SWOP, gracol etc?
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on August 27, 2013, 08:26:55 am
1) You leave the content in RGB and let the RIP do the conversion

2) You prepare the content in RGB and convert it to ISO Coated v2 (etc.) in a final step - many CMYK printers (like solvent printers, digital presses) automatically simulate ISO Coated v2 (etc.)

Personally I prefer the first workflow - ISO Coated v2 has smaller gamut than most digital presses, solvent/pigment printers, so you can get richer colours when converting RGB content directly to native CMYK profile of such device (it's good to have such profile to soft proof the effect). The second workflow may be useful when you prepare the content for an offset press, or want to maintain colours from offset in prints realised in other printing techniques
Title: Re: cmyk neutral grayscale values ?
Post by: MarkM on August 27, 2013, 04:27:21 pm
The fact that you can get neutral gray colors with all channels equal in RGB is an artifact of the way RGB working spaces are designed. They are mathematical constructs that are designed specifically to be well-behaved in this way. It's not necessarily a quality of RGB devices—if you look at RGB printer profiles and even some monitor profiles you'll sometimes find that to get grey involves sending unequal RGB values. In general monitors get a lot of this straightened out by calibrating the LUTs and curves on the card so may not notice it by the time you are profiling.

Since CMYK involves the real world with all it's messiness we can't control things as well. Imagine someone handing you four colors of paint and telling you to mix a perfectly neutral gray. One strategy may be to take equal amounts of each paint, but the odds of that working are astronomically small. A better strategy would be to have some sort of lookup table where you can specify the color you want in a more abstract color space and then use the table to find the right recipe for the real world. This is pretty much what profiles and color management do for you.
Title: Re: cmyk neutral grayscale values ?
Post by: digitaldog on August 27, 2013, 05:36:02 pm
Quote from: smilem on August 27, 2013, 06:17:17 am
How do I proceed with the same task for native CMYK printer that mat use ISOcoated, SWOP, gracol etc?

You must have the ICC profile that defines that process and hope it's accurately describing neutral gray balance.
Title: Re: cmyk neutral grayscale values ?
Post by: Tim Lookingbill on August 27, 2013, 11:15:51 pm
I have a prepress "Color Process Manual" copyright 2000, used by graphic designers and bought off Amazon that's sheetfed printed. I created a 128RGB gray patch in ProPhotoRGB and noted PS's Info Palette CMYK readings converting this patch to US Sheetfed Coated v2 profile and wrote the numbers down as C40%,M30%,Y30%,K1%.

I located the CMYK combo patch of this gray on my "Color Process Manual" and isolated it with the provided crop viewer and took a picture of it lit under my 5000K daylight flotubes. See below.

I've also included a picture of the CMYK ink purity ramp between two completely different printing companies using their own ink formulation, commercial sheetfed press and coated paper. The Ole No Moire color target was printed on a Komori 300LPI press at Copy Craft in Odessa Texas and the "Color Process Manual" was printed in Hong Kong.

I know the Copy Craft color target was converted to regular US SWOP in PS5 as indicated by the company's online instructions on how to prep files for their presses.

The "Color Process Manual" 24,000 CMYK combo color swatches were printed on a Heidelberg Speedmaster w/direct to plate technology using Japan ink & glossy coated paper with these additional specs...

10% dot gain, 175LPI...

Ink densities: Black 1.8-2.0, Cyan 1.6-1.7, Magenta 1.4-1.5, Yellow 1.3-1.4 AND printed in that order.

From the manual's Photoshop 5 Color Picker screengrabs I'm assuming they converted to default PS 5 US SWOP.

It doesn't seem exact CMYK grayscale numbers are all that necessary. Derive your own conclusions.
Title: Re: cmyk neutral grayscale values ?
Post by: MarkM on August 27, 2013, 11:21:41 pm
Quote from: Tim Lookingbill on August 27, 2013, 11:15:51 pm
It doesn't seem exact CMYK grayscale numbers isn't all that necessary. Derive your own conclusions.

The double negative leaves me a little unsure about your conclusion. Does that mean exact numbers are necessary?
Title: Re: cmyk neutral grayscale values ?
Post by: Tim Lookingbill on August 27, 2013, 11:29:18 pm
Quote from: MarkM on August 27, 2013, 11:21:41 pm
The double negative leaves me a little unsure about your conclusion. Does that mean exact numbers are necessary?

I caught it and changed it, Mark, before you posted.

Anything else you found valuable to comment on my contribution here?
Title: Re: cmyk neutral grayscale values ?
Post by: MarkM on August 28, 2013, 02:07:23 am
Quote from: Tim Lookingbill on August 27, 2013, 11:29:18 pm
Anything else you found valuable to comment on my contribution here?

Tim, I'm honestly not sure what I'm supposed to take away from all this. You've taken photos of various grey swatches printed on different devices. Maybe you can be a bit more explicit about what you are demonstrating (it's also possible I'm just being a little dense and could just use a little more explanation).
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on August 28, 2013, 03:45:41 am
Quote from: Tim Lookingbill on August 27, 2013, 11:15:51 pm
It doesn't seem exact CMYK grayscale numbers are all that necessary. Derive your own conclusions.

40/30/30 will give (an almost) neutral result when printed with european, US or japanese standarized offset press on type 1 or 2 paper - you can check it in Photoshop in a few seconds, the whole experiment wasn't all that necessary. You can also get virtually the same result with less CMY and more K combination like 35/25/25/10, or pure K 45.

Still the conclusion is that it's easier to work in RGB and let Photoshop find the magic combination of CMYK values for a given neutral shade.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on August 28, 2013, 06:49:53 am
The commercial associations that backing up the printing standards should then provide the neutral gray recipes for their working spaces like SWOP, Gracol, ECI etc. I'm not talking about gray with only K channel as that is self explanatory and good gradations is a problem when using K only in photographic print.

I think the most precise way to convert to CMYK is by using photoshop convert to profile not just use colorpicker as that does not use any profiles for converting you color data.

The correct CMYK ink values on device side is managed by ICC profile for that device and I don't care how it's doing the magic to make neutrak gray, I however do care that the color recipe sent to the printer would be correct else the gray will be bad.

In other words lets suppose I want to test certain printers grayscale and I'm making like 20 patch gray test target.
The target must be made for 3 standards like SWOP, Gracol, ECI.
I convert the neutral gray patches from RGB to the profiles above in Photoshop.

I see that the CMYK recipes are all over the place they not very close to each other and then when I look at LAB values I see they thay are not even gray. If I enter correct lab values for a and b the color changes to more gray looking then the Photoshop was able to make by using the ICC profile conversion.

So in the end I still do not understand what to use....    as LAB is used in every case from RGB to CMYK then I should correct the CMYK values by altering LAB in Photoshop after conversion to SWOP, Gracol, ECI right?

Then if I feed this to a CMYK printer that opens the file as SWOP, Gracol, ECI colorspace with neutral gray it feeds the data to internal CMM/RIP to do the conversion to actual CMYK values to put on paper by profile for that paper.
Title: Re: cmyk neutral grayscale values ?
Post by: digitaldog on August 28, 2013, 07:17:31 am
Quote from: smilem on August 28, 2013, 06:49:53 am
I see that the CMYK recipes are all over the place they not very close to each other and then when I look at LAB values I see they thay are not even gray. If I enter correct lab values for a and b the color changes to more gray looking then the Photoshop was able to make by using the ICC profile conversion.

So in the end I still do not understand what to use....    as LAB is used in every case from RGB to CMYK then I should correct the CMYK values by altering LAB in Photoshop after conversion to SWOP, Gracol, ECI right?

Lab is device independent. CMYK isn't even close (just the opposite). RGB working spaces are Quasi-Device Independent. IOW, RGB isn't device independent but because the RGB working spaces are theoretical constructs. What you want (CMYK that in call cases has set's of values that are neutral) require a good ICC profile for the specific process that will convert RGB and send CMYK to that device.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on August 28, 2013, 07:34:01 am
So what happens if I send LAB to CMYK printer? The printer profile avoids the RGB conversion to LAB side by making the conversion more precise?

I agree that to test a RGB->CMYK devicelink profile it's best to send RGB data.
Title: Re: cmyk neutral grayscale values ?
Post by: digitaldog on August 28, 2013, 07:38:28 am
Quote from: smilem on August 28, 2013, 07:34:01 am
So what happens if I send LAB to CMYK printer? The printer profile avoids the RGB conversion to LAB side by making the conversion more precise?
I agree that to test a RGB->CMYK devicelink profile it's best to send RGB data.

A device link is no different here. It's a recipe like the ICC profile. If it's right, you get what you expect (in this context a neutral). If not, you don't. You can send RGB and the next person in the workflow either has or hasn't got the right recipe. There is no such thing as a Lab printer. A conversion still has to take place if you send Lab or tagged RGB. Lab doesn’t solve this problem one bit.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on August 28, 2013, 10:34:56 am
So to create say a target that is true gray I have to select proper working space like SWOP, Gracol, ECI.

Then I have to create the patches from scratch by entering LAB values and let Photoshop calculate proper CMYK right?
And I have to do this for every working space all over again.

So the CMYK recipe calculated by photoshop is the best or there are other options? Like I tried the Color Composer plugin and IMO it's better, more easy to use, math seems to wander less. But it's old and I don't expect 64bit version anytime soon.
Title: Re: cmyk neutral grayscale values ?
Post by: MarkM on August 28, 2013, 01:42:00 pm
Quote from: smilem on August 28, 2013, 06:49:53 am
I think the most precise way to convert to CMYK is by using photoshop convert to profile not just use colorpicker as that does not use any profiles for converting you color data.

The color picker does use a profile. It's the CMYK profile set in photoshop's color settings.

It might help if you tell us what you are trying to accomplish.  I can't think of any reason why you would want to send a bunch of different grays from different spaces to a printer. If you just want to be sure you are getting neutral gray, a better solution is to talk to the printer and ask what profile to use. Certainly quicker and cheaper than guess & check. Then if you have a problem with a proof you can say, look, your profile says this should be neutral. This is what most people do, and it works pretty well (if you have a good printer—but that's a whole different can of worms).

If you really want to dig into the specifications (probably a good idea before you start criticizing them), a good place to start might be IdeAlliance: http://www.idealliance.org/specifications/g7 but be warned, it's aimed at printing professionals, not photographers. This is a good overview of the problem of neutral colors in commercial printing: http://www.npes.org/Portals/0/standards/pdf/ANSI-CGATS-TR015-2013.pdf
Title: Re: cmyk neutral grayscale values ?
Post by: Tim Lookingbill on August 28, 2013, 06:01:30 pm
Quote
So the CMYK recipe calculated by photoshop is the best or there are other options?

Yes, you can work with any hungry printer with a commercial press who'll guarantee neutral CMYK grayscale with no color crossover errors and provide the instructions for you to aid in helping them reach YOUR goal. Have them sign an agreement indicating what your goals are and if they don't agree, go seek another hungry printer who will. It's not like commercial press technology is in its infancy.

These guys know how to get the job done as long as you pay them and from the list of printers available shown in Mark's linked PDF those and many others will want your business and bend over backwards to reach your goal of a CMYK neutral grayscale by any means necessary.

The "Process Color Manual" (corrected the name) result I posted above is sort of like a canary in the grayscale coal mine of printing "Standards" in that you can check the Photoshop numbers against the swatches to see how much variances you get from a random sampling off a commercial press that didn't come up with the swatches using an ICC profile.

Those swatches are pretty much like a "Raw" dump of CMYK color number combinations that present a neutral appearance comparing against a Photoshop ICC based recipe. The fact that one mid gray swatch located out of 24,000 from a Photoshop ICC profile number recipe would look neutral not knowing the variables (which you seem to be trying to compensate for unnecessarily) that generated it, shows "Standards" are pretty easy to attain by the industry that's been printing CMYK color over 50 years. You'ld think these guys would know how to produce a decent CMYK grayscale and they do, so you shouldn't be so concerned about it on the front end.

Just work with a printer who'll listen to your concerns and sign an agreement indicating what you want.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 02, 2013, 03:40:09 pm
@MarkM
@Tim Lookingbill

I agree that commercial print shops knows what they are doing (most of them anyway), but to print correctly for them means to send correct data for me.

I checked the idealliance Proof Verifier V1.5 and they seem change the reference values for SWOP, gracol etc. That confirms you must send different data for each colorspace to get neutral gray, or lets say color if we are talking about color patches.

So there seems to be 2 ways:

1st. You must compensate your reference values for SWOP, Gracol like idealliance does by adjusting reference values since the test image is the same.

2nd. You must send different values to begin with. For every standard SWOP, Gracol you make different values that leads to same expected reference values.

I wonder why idealliance preferred the 1st. way of doing things since preparing neutral gray for a given color space as we came to this conclusion is rather easy, you use photoshop for this with CMYK working space set to the one you want to make grayscale for.

@digitaldog

I know LAB is not sullution since most commercial print shops will not accept LAB data files to begin with, but if a certain RIP or Photoshop for that matter opens LAB file it converts it CMYK AFAIK. The file is never converted to RGB.

Title: Re: cmyk neutral grayscale values ?
Post by: digitaldog on September 02, 2013, 03:43:29 pm
Quote from: smilem on September 02, 2013, 03:40:09 pm
I know LAB is not sullution since most commercial print shops will not accept LAB data files to begin with, but if a certain RIP or Photoshop for that matter opens LAB file it converts it CMYK AFAIK. The file is never converted to RGB.

Lab isn't defining an output space value(s) without the output profile. IOW, Lab is not a destination, no RIP, Photoshop or other software deciphers the output without a descriptor from Lab to output. Again, Lab doesn't help you at all.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 03, 2013, 12:46:25 am
Quote from: digitaldog on September 02, 2013, 03:43:29 pm
Lab isn't defining an output space value(s) without the output profile. IOW, Lab is not a destination, no RIP, Photoshop or other software deciphers the output without a descriptor from Lab to output. Again, Lab doesn't help you at all.

But LAB makes me have only one file for data not 3 for every CMYK color space. I understand that the output is handled by ICC profile for that printer/ink/paper.
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on September 03, 2013, 04:28:06 am
Quote from: smilem on September 03, 2013, 12:46:25 am
But LAB makes me have only one file for data not 3 for every CMYK color space. I understand that the output is handled by ICC profile for that printer/ink/paper.

So is RGB. Why bother to convert RGB to L*a*b? Unlike L*a*b, synthetic RGB is a working space, it's supported by all applications like PS, ID, Ai, easy to edit and so on.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 07, 2013, 10:35:14 am
Anyone knows proper custom photoshop CMYK colorspace settings to convert LAB to CMY like
ECI bvdm Gray Control Strip 2012 does ?

I tried to use Eurostandard Ink colors with K set to zero and I get L60 instead of L80 for 30% patch when entering C27 M19 Y20 K0? Any ideas? I'm looking to make 20 patch (every5%) chart with CMY inks that would be the same as K only, and the ECI has only 3 patches at 30%/50%/70% ???

Perhaps such chart already exists?
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on September 07, 2013, 12:39:40 pm
Here's my F39L based profile created with no K
https://dl.dropboxusercontent.com/u/19059944/FOGRA39L.icc
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 08, 2013, 02:16:33 pm
Thank you. Your profile is closer:

your profile input C27 M19 Y20 K0 --> L79 A0 B-1 should be L80 A0 B-1

L80 A0 B-1 --> C26 M18 Y19 K0 should be C27 M19 Y20
L80 A0 B0 --> C25 M19 Y20 K0 should be C27

is it possible to adjust your profile so that it would match with ECI? Seems the color error is non linear.
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on September 08, 2013, 03:17:23 pm
ECI ISO Coated v2 profile is build on the same data set (F39L), but with different profiler (Heidelberg Print Open) - my CMY profile was created with i1Profiler, so there might be a slight difference caused by different colour engines, rounding errors and so on.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 09, 2013, 04:25:41 am
How did you create no K profile from forgra39? MY friend has heidelberg color toolbox (don't know what version) would that work for creating better profile (I don't know anyone with printopen :( )?
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on September 09, 2013, 05:57:27 am
Quote from: smilem on September 09, 2013, 04:25:41 am
How did you create no K profile from forgra39? MY friend has heidelberg color toolbox (don't know what version) would that work for creating better profile (I don't know anyone with printopen :( )?
I don't know Color Toolbox, but usually there's some option to cut off K:
(https://dl.dropboxusercontent.com/u/19059944/i1profiler)

I'm not sure which of PrintOpen and i1Profiler created profiles is "better", but quite sure that the difference is so small, it virtually doesn't matter
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 10, 2013, 06:45:01 am
I decided why not email ECI to ask for fogra39L based no K profile. After all they should have one.

After emailing them at  info@eci.org I got a reply that:

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain eci.org by mxlb.ispgateway.de. [80.67.18.126].

The error that the other server returned was:
554 Sorry, no mailbox here by that name.

ECI doesn't even have mailbox configured at that address.  Why bother listing one then  ??? ??? ???

I wrote to another contact on their page: Public relations: Karl Michael Meinecke asking them:

Quote
Hello, you provide only only 3 patches 30/50/70

I would like to create my own test strip with different values. Could you send me the ICC profile (with no K ink) so that I could calculate values for other patches?

Thank you.

Is my english that bad  :-\ The reply is:

Quote
If  you go to the download area on ECI website, you can get the complete package, including documentation
http://www.eci.org/_media/downloads/eci_bvdm_gray_control_strip_2012/eci_bvdm_gray_control_strip_2012.zip

Seems like in Germany nobody understands English, even sellers from china reply better. I tried to explain myself better:

Quote
I have downloaded the color strip, but I want to make more patches in CMY that maches patches with K ink. For that I need fogra 39L based ICC profiler created with heidelberg print open (as this is what is used for fogra profile creation right?)

I tried to create a profile with i1profiler but the numbers doesn't match as it uses different engine.

If you could just send me a ICC profiler that was used to calculate CMY patches for the test strip it is all that I need. Thank you.

Reply is even more strange:

Quote
Hi,
you can Download the caracterisation data on Fogra.org and the ECI profiles on eci.org . This is all you need. The values of ECI/bvdm graycon patches are taken from the respective caracterisation data set patches.

What next email other ECI contact like Vice chairman, Chairman do they even know their employees are not communicating with each other because it looks like the response is from incompetent employee. All other companies at lest forward email to the right person, not the case with ECI.

I can't contact Board: Darrian Young, Color & Image Consulting as no email is listed  ???
Title: Re: cmyk neutral grayscale values ?
Post by: Czornyj on September 10, 2013, 09:32:25 am
Try converting with BPC off instead, it's almost there when you turn it off.

ECI ISO/PSO profiles were created by Heidelberg, that is a member of ECI. It's unlikely that any other ECI member has/will send you "no K" variant of such profile.
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 18, 2013, 08:22:37 am
Well, after sending a spreadsheed to the ECI they understood what I want. They sent me instruction to just use Isocoated v2 profile with Xrite colorpicker then set zero for K and uncheck it. This way no special profiles are required.

I tried this but since colorpicker uses logo engine the results were not optimal. I got a CMY profile created by a friend. Then we made some color conversions and behold, here is a list of calculated colors !

I hope no mistakes (there could be most of the numbers were entered by hand), so feel free so post errors so I can correct it.
I also attach pixels.tif for target creation.

A also want to ask how to create CGATS? do I enter absolute data or relative colorimetric as that is what PS uses by defaut?
Title: Re: cmyk neutral grayscale values ?
Post by: smilem on September 29, 2013, 11:52:59 am
So can anyone advise me on CGTAS data ?
Do I need 3 different files each for rendering intent, Absolute, Perceptual, Relative?