RGB vs CMYK

slush11

Well-known member
I have asked this question in a different forum and am looking for even more feedback from here. Apparently, some printers require RGB files instead of CMYK. I've never heard of this and am hungry for knowledge. My questions are:
1. Can any of you comment on your type of printer if you do accept RGB files and the quality it provides vs. CMYK?
2. How on earth does a printer that uses CMYK inks accurately print an RGB file?
 

Stephen Marsh

Well-known member
1. In theory, it should not matter on the type of print process (digital or offset for example)*
2. A conversion to device CMYK, or device CcMmYK, or device CMYK+++++ etc still takes place

Take a look at the PDF/X standards – X-1a is “early binding” converting to CMYK early in the process, no RGB allowed. X-3 and X-4 allow RGB content, this is a “late binding” workflow.

This reply will likely take more than post to cover, it is a deep topic.

When a user converts RGB to CMYK, they decide on the gamut compression or gamut clipping, black point compensation, contrast etc – how the RGB renders/converts to CMYK. There are no surprises, they get to see how well or poorly the RGB original converts to CMYK. If the user has an ICC profile that accurately represents the output condition, then they are pretty much good to go**.

The issue is that many convert to CMYK without knowing what exact CMYK “flavour” they should be converting to. This often results in incorrect tonality, or gray balance being incorrect, or incorrect total ink limts etc.

_______

* Although it should not matter if the process is digital (toner or inkjet) or offset, to me it does matter. For a digital process that has good process control, profiling and colour management – then supply of neutrally balanced RGB content in a working space profile (not device) should deliver the best results. Let colour management correctly handle the mapping of colours, gray balance in digital devices is not the same as standard press CMYK. For CMYK work, I personally prefer to hand off CMYK if the output condition is known, even more so if the image or content is critical or the service provider was unknown.

**Different images can get better/worse results depending on the profile and conversion settings, even if the different profiles all describe the same output condition. Over and above the initial conversion to CMYK, one can elect to make edits to the file in CMYK which may make it subjectively “better” than with no edits.



Stephen Marsh
 

gordo

Well-known member
I have asked this question in a different forum and am looking for even more feedback from here. Apparently, some printers require RGB files instead of CMYK. I've never heard of this and am hungry for knowledge. My questions are:
1. Can any of you comment on your type of printer if you do accept RGB files and the quality it provides vs. CMYK?
2. How on earth does a printer that uses CMYK inks accurately print an RGB file?


Adding to what Stephen wrote - This is routinely done in newspaper and magazine publishing, i.e. receive RGB files (mixed in with CMYK files). The conversion to CMYK happens in the RIP. The upside is that the printer controls the appropriateness of the CMYK makeup. The downside is that the image colors may change drastically. But, I don't think that happens as much as it once might have and besides it should be caught in proofing.. For digital printing I believe the most common file format is still sRGB.
I sometimes do image rescue/fixing for a quarterly home magazine published by our local daily paper. They receive image files from a wide variety of sources. I never knew there were so many flavors of RGB and CMYK. LOL I offered to teach their staff photographers how to set up their files correctly so at least those images would have some commonality. But they would have none of that. I take all the variety of images and bring them to RGB Adobe 1998 and they then get placed in the magazine and are shipped off across the country to the printers. No hard proofs are used. Haven't had a complaint yet.
 
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Simon Ivarsson

Well-known member
I have asked this question in a different forum and am looking for even more feedback from here. Apparently, some printers require RGB files instead of CMYK. I've never heard of this and am hungry for knowledge. My questions are:
1. Can any of you comment on your type of printer if you do accept RGB files and the quality it provides vs. CMYK?
2. How on earth does a printer that uses CMYK inks accurately print an RGB file?

The printer just knows best how to convert to CMYK.
Google for "late binding"
 

Stephen Marsh

Well-known member
The printer just knows best how to convert to CMYK.
Google for "late binding"

I would qualify that Simon, the printer sometimes knows best, sometimes they know less than their customers do. It is one of those great printing industry myths that one should always hand off RGB and leave it to the printer as they know better. Traditionally colour was handled by a prepress house, the printer simply made or used plates with no knowledge of the separation process. Now prepress is in-house for general/commercial print, I am not aware of any prepress houses that exist today servicing this sector. In general/commercial, the prepress skill set has mostly been lost. There is no guarantee of a knowledgeable or caring operator. Workflow software is often setup in a “set and forget” manner and mindless automation rules. Exceptions of course exist, some still care and know their craft.

In packaging (flexo and gravure), due to the expense and complexity of the work, the traditional model still survives, there are successful and profitable prepress houses providing prepress services and plates to converters that only print and have no prepress in-house.


Stephen Marsh
 

slush11

Well-known member
Adding to what Stephen wrote - This is routinely done in newspaper and magazine publishing, i.e. receive RGB files (mixed in with CMYK files). The conversion to CMYK happens in the RIP. The upside is that the printer controls the appropriateness of the CMYK makeup. The downside is that the image colors may change drastically. But, I don't think that happens as much as it once might have and besides it should be caught in proofing.. For digital printing I believe the most common file format is still sRGB.
I sometimes do image rescue/fixing for a quarterly home magazine published by our local daily paper. They receive image files from a wide variety of sources. I never knew there were so many flavors of RGB and CMYK. LOL I offered to teach their staff photographers how to set up their files correctly so at least those images would have some commonality. But they would have none of that. I take all the variety of images and bring them to RGB Adobe 1998 and they then get placed in the magazine and are shipped off across the country to the printers. No hard proofs are used. Haven't had a complaint yet.


OK so I work in large format and have worked with 4 different brands and types of large format printers now, all which require CMYK. I've spoken to our digital and offset pressmen and all of them say they require CMYK as well, as do our small toner based copiers/printers. We are one of the biggest printers in our city...
So...is it just that we don't have profiles set up for this?
 

gordo

Well-known member
OK so I work in large format and have worked with 4 different brands and types of large format printers now, all which require CMYK. I've spoken to our digital and offset pressmen and all of them say they require CMYK as well, as do our small toner based copiers/printers. We are one of the biggest printers in our city...
So...is it just that we don't have profiles set up for this?


I'm a bit confused (not unusual ;-)

Here are some scenarios of what I think happens::

Offset:
RGB file in - RIP converts RGB to CMYK - CMYK separations out to plates
CMYK file in - RIP passes CMYK through - CMYK separations out to plates
CMYK file in - RIP reseparates CMYK to CMYK (device link) - CMYK out to plates

Digital
RGB file in - RIP converts RGB to CMYK - CMYK separations out to plotter
RGB file in - RIP converts CMYK to C/LC, M/LM, Y, K/LK - C/LC, M/LM, Y, K/LK out to plotter
CMYK file in - RIP converts CMYK - CMYK separations out to plotter (secret sauce may be applied)
CMYK file in - RIP converts CMYK to C/LC, M/LM, Y, K/LK - C/LC, M/LM, Y, K/LK out to plotter (secret sauce may be applied)

For digital, it would be interesting to take the same image and render it as sRGB, Adobe 1998 RGB, and Adobe 1998 CMYK then send to the RIP and see what difference there is in the output. In the limited work that I've done with inkjet plotters, feeding them sRGB has always given the best results. I'd love to see what you get with your set up.
 
D

Deleted member 16349

Guest
I'd love to see what you get with your set up.

Of course your statement is directed at the specific issue posted but it is also a general theme that seems to govern prepress.

Due to the methods used in prepress, there tends to be the uncertainty of what is going to be seen in the final output. It seems people hope for a good result but they don't know for sure what the result will be. IMO, this is a characteristic of a process or processes that are not capable.

Predictable colour is another interesting topic I think needs rethinking. I am still in the "collecting ideas" phase on this one. :)
 

oxburger

Well-known member
Answer to #2:
 

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mwc

Well-known member
OK so I work in large format and have worked with 4 different brands and types of large format printers now, all which require CMYK. I've spoken to our digital and offset pressmen and all of them say they require CMYK as well, as do our small toner based copiers/printers. We are one of the biggest printers in our city...
So...is it just that we don't have profiles set up for this?


You need to understand a little color theory here for this to make sense to you...once you learn it, it all will make a little more sense.
Quick summary : On PAPER = CMYK makes ALL color* (subtractive color/reflective) , On SCREEN = RGB makes ALL color* (additive color/emitted)

ALL/EVERY single Digital and Offset printers require CMYK* to print...(if you go to Staples to get Ink for your printer, you never see RGB ink, do you?)

The conversion of SCREEN RGB to PAPER CMYK is where all the fun begins...
(just to add to the confusion - Screen RGB can show the colors CMY(K), and Paper CMY(K) can show the colors RGB)

Really, just read the following article for the 'ah-ha' moment.....
http://www.rgbworld.com/color.html

*blah,blah this is a simplistic statement, I didn't stay at a Holiday Inn last night....
 

gordo

Well-known member
[SNIP]It seems people hope for a good result but they don't know for sure what the result will be. IMO, this is a characteristic of a process or processes that are not capable.

It may not be that the process or processes are not capable. It may be the people using it, coupled with poorly written standards, specifications, and documentations that are not capable.
 
D

Deleted member 16349

Guest
It may not be that the process or processes are not capable. It may be the people using it, coupled with poorly written standards, specifications, and documentations that are not capable.

Possible, but I tend to follow Deming. People are usually only 5% of the problem in his view. I would also say that poor standards and specifications etc. are partly the fault of processes that are not capable and therefore hide the fundamental causes of issues and cloud the best approaches to deal with colour reproduction.

So I tend to think it is most important to fix the processes even before training the people. But after that would be done, I am sure there will be lots of opportunities for some people to mess it up. :)
 

slush11

Well-known member
You need to understand a little color theory here for this to make sense to you...once you learn it, it all will make a little more sense.
Quick summary : On PAPER = CMYK makes ALL color* (subtractive color/reflective) , On SCREEN = RGB makes ALL color* (additive color/emitted)

ALL/EVERY single Digital and Offset printers require CMYK* to print...(if you go to Staples to get Ink for your printer, you never see RGB ink, do you?)

The conversion of SCREEN RGB to PAPER CMYK is where all the fun begins...
(just to add to the confusion - Screen RGB can show the colors CMY(K), and Paper CMY(K) can show the colors RGB)

Really, just read the following article for the 'ah-ha' moment.....
http://www.rgbworld.com/color.html

*blah,blah this is a simplistic statement, I didn't stay at a Holiday Inn last night....


Hey mwc, I do actually know color theory and all that you said already. I had a look at the article and I don't really feel like it answered my question about how an RGB file can accurately be printed with CMYK inks. It gets into it a little bit at the end but I'm not really understanding it. :(
 

gordo

Well-known member
Hey mwc, I do actually know color theory and all that you said already. I had a look at the article and I don't really feel like it answered my question about how an RGB file can accurately be printed with CMYK inks. It gets into it a little bit at the end but I'm not really understanding it. :(


The terminology might be confusing you.

I hope you're sitting down...

The chromatic colors RGB and CMY are the same thing.

Think in terms of numbers...

+1 and -1 are the same in the sense that they're both "1"

Betweeen +1 and -1 is a "0" in the sense that "0" is neither a +1 nor a -1. I.e. "0" is neutral, neither biased towards being +1 nor -1.

So, you could say that the chromatic colors are actually:

+R, +G, +B and -R, -G, -B.

But instead of saying that, convention, for the sake of convenience, replaces the designation of -R, -G, -B with the letters/words C, M, Y

When light is added to a reflective surface we use the terms RGB - but it's really +R, +G, +B because we're adding light.

When light is subtracted from a reflective surface we use the terms CMY - but it's really -R, -G, -B

The "0" in this scenario is the neutral, or grey, between, for example +R and -R (a.k.a.Cyan) or between +G and -G (a.k.a.Magenta) or between +B and -B (a.k.a.Yellow)

Does that help?
 

mwc

Well-known member
Slush11,
I never assume who knows what, just trying to understand what the real question is...and hope I know enough to explain what's in my head....(right or wrong)

RGB has a larger gamut than CMYK, meaning that certain RGB colors just can't be reproduced faithfully in CMYK....look at a gamut mapping and you'll see pure reds, greens, and blues get 'clipped' in CMYK colorspace.

How can RGB be printed in CMYK?....color separation...it's all about profiles, and what gets converted, and when, in your workflow.
If you are supplied CMYK, the conversions have already be done for you, end of story. (unless you use device links to convert CMYK to 'adjusted' CMYK for your shop)
If you are supplied RGB, then you have MANY places where the RGB to CMYK conversion can take place (photoshop, Acrobat/Pitstop, making a PDF, in your workflow color conversion setting....)
What settings get honored, and where the conversion happens is up to you. In the end, you are always printing CMYK to plate or digital devices.

A basic conversion:
0,0,0 RGB (BLACK) --->Convert/Separate ---> 0,0,0,100 CMYK
255,255,255 (WHITE) --->Convert/Separate ---> 0,0,0,0,CMYK
0,255,255 RGB (CYAN) --->C/S--> 100,0,0,0 CMYK
And so on....

Now on to "how an RGB file can accurately be printed with CMYK inks?" it's all in that Convert/Separate stage....what is the source profile of the RGB file, what is the destination of the CMYK file...the 'magic' conversion box steps up to the plate and makes a transformation based on the details selected.

The actual profiles, methodology, and options selected for your conversions depends on what tools you have to use...and where you want the conversions to happen
 

slush11

Well-known member
The terminology might be confusing you.

I hope you're sitting down...

The chromatic colors RGB and CMY are the same thing.

Think in terms of numbers...

+1 and -1 are the same in the sense that they're both "1"

Betweeen +1 and -1 is a "0" in the sense that "0" is neither a +1 nor a -1. I.e. "0" is neutral, neither biased towards being +1 nor -1.

So, you could say that the chromatic colors are actually:

+R, +G, +B and -R, -G, -B.

But instead of saying that, convention, for the sake of convenience, replaces the designation of -R, -G, -B with the letters/words C, M, Y

When light is added to a reflective surface we use the terms RGB - but it's really +R, +G, +B because we're adding light.

When light is subtracted from a reflective surface we use the terms CMY - but it's really -R, -G, -B

The "0" in this scenario is the neutral, or grey, between, for example +R and -R (a.k.a.Cyan) or between +G and -G (a.k.a.Magenta) or between +B and -B (a.k.a.Yellow)

Does that help?


I'll have to read that a few more dozen times before it really sinks in. But I'm getting there lol
 

gordo

Well-known member
I'll have to read that a few more dozen times before it really sinks in. But I'm getting there lol

That's OK - I doubt you've ever heard it described that way LOL.

But if you can understand that then it helps to answer your second question:

2. How on earth does a printer that uses CMYK inks accurately print an RGB file?

The thing is that it can't. Although it seems that way based on it all being RGB but "+" or "-" there are a couple of problems with the "-" RGB side of things.
The problems are contamination of the hues of the -R,-G,-B (a.k.a. CMY) by the substrate, loss of the viewing light brightness/luminance by the substrate, impurities in the pigments of the -R,-G,-B (a.k.a. CMY), and the spectral inefficiency of the pigments especially when combined.

So, you can't actually use CMYK (-R,-G,-B) inks accurately print an RGB file. Instead, what you hope to do is to preserve the visual appearance of the RGB image using CMYK (-R,-G,-B) inks/colorants. The famous blue sky/green grass/white clouds/healthy skin appearance. The color is not accurately reproduced but the appearance is.
 

Stephen Marsh

Well-known member
From a colour management perspective, if the original RGB values are “in gamut” for the CMYK destination space, then the L*a*b* appearance of the before/after is exactly the same. This theory can be demonstrated in Photoshop with ease. For colours that are “out of gamut” then they can’t be reproduced and colour science then becomes art/craft.

Stephen Marsh
 
D

Deleted member 16349

Guest
From a colour management perspective, if the original RGB values are “in gamut” for the CMYK destination space, then the L*a*b* appearance of the before/after is exactly the same. This theory can be demonstrated in Photoshop with ease. For colours that are “out of gamut” then they can’t be reproduced and colour science then becomes art/craft.

Stephen Marsh

IMO even if a colour is in gamut, it does not mean that it can be reproduced. In an abstract way of looking at colour spaces, a colour that is in gamut should be reproduceable but with colours made from real pigments, it is my view that all colours within the gamut are not reproduceable.

Let's take for example the gamut for a CMY space. One would think that black and dark grays, which are within the gamut, should be reproduceable but they are not. If one tries to make a black or dark gray from some CMY ink sets, one tends to get muddy browns. When one tries to get a CMY patch to look gray, it usually seems to be a slight bit off the neutral gray. Black is no different than any other colour and therefore there can be many other specific colours that can not quite be reproduced too. Anyhow for practical purposes, we use CMYK so that the K can make nice blacks and not mud. :)

In my view this is due to the fact that colours are built up with combinations of inks and screens but the individual inks are affecting the whole spectrum and therefore it is not possible to for these inks filter out specific ranges of the spectrum to fine tune a colour.

I am sure this situation gives some printers lots of frustration since they try to hit a specific colour but it just does not quite get to where they want it to be, no matter what they do.

I hope there are or will be some studies that can confirm this issue.
 
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