Tracey Salazar 5:15pm, 11 November 2005
I'm piggybacking from another thread in another group.

I don't have a printer. None. At all. Wish I did, but I don't want to buy one yet. I'm holding out until I know more about what I need and want and until I can afford something on the high-end of "decent." But even so, when I finally get into LOTS of printing (keeping fingers crossed on that one), it makes more sense to me--at least for a good while--to send the jobs to a real professional.

Both things said, that I do eventually want a quality home printer AND to be able to send jobs to a pro, my questions relate to figuring out the printable range of an image regardless of the printer in question.

I am of the understanding, but admit that I could be very mistaken, that CMYK mode is the way to go for top-quality printing (I'm talking pro jobs here). Yet many, many digital printing shops actually seem to want files in RGB.

Question 1: So just what is the difference between CMYK and RGB modes? And why would I have a preference?

I also am curious about the 0 - 255 range of RGB. Mostly, I wonder if there is a saturation point we need to be aware of, a magic number at either end of the scale (for example: nothing below 10 and nothing above 250) in order to get the best possible prints.

So...

Question 2: What are the printable tonal ranges for digital images? (And are they the same/similar for both RGB and CMYK?)
nicolai_g 13 years ago
This is a really complex set of questions, but I'll give it a shot:

No output device can reproduce all of the colors that the human eye can see. The set of colors that a device can reproduce is called a gamut.

Actually, just read this tutorial, it looks pretty good.

Hopefully, the following will answer a few questions that don't appear to be addressed in the above link:

CMYK (Cyan, Magenta, Yellow, blacK) is the standard color model used on offset printing presses (magazines, color books, brochures, etc.) and is often referred to as "four color process". Offset printing involves making a plate (basically a high quality stamp) for each of the colors and the image gets built up as the paper runs through the press, getting printed with each color.

RGB is what most digital output (and input) devices use. Some use an expanded variant, for example Epson's higher-end inkjets use 7 colors, but it's still derived from RGB.

The 0-255 range isn't inherent to RGB, it's just that that's the range of values available in 8 bits (per channel; it's sometimes referred to as 24 bit, which is 8 bits times 3 channels: 8 for each R, G, and B).

As far as min and max values goes, with CMYK printing on a press, there are max combined values that you don't want to exceed, usually epressed as a percentage of ink coverage. It's not a max per-ink percentage, but a total. For example, you can have C and Y each at 100% to make solid red, and that's fine, but you'll run into problems if you have a large area covered by 100% of all four colors because the paper gets too wet with ink.

Similarly, with a photographic image, every channel bening a the max (255 for an 8-bit channel) means that you're just getting the white of the paper; there's no information there. This means that the value is "clipped", that the value is too high to record in 8 bits. The value might have been 256 or 256,000, when your max value is 255, all the values in your image that are supposed to be over that get clipped off at 255. This is a blown out highlight, which most people generally want to avoid unless you're doing it for creative effect. (With noon-photographic images, of course there will probably be plenty of "just paper" showing through as in a book with black type on the background of the paper.)
Tracey Salazar Posted 13 years ago. Edited by Tracey Salazar (member) 13 years ago
nicolai: Thanks for your response.

I am familiar with where blow-outs occur in RGB on the monitor, but I'm wondering what is an acceptible value for print. Similarly I'd like to know where excessive shading (all black) occurs--again "for print." A dark image is all well and good, but for printing, at what point does it all just become black and not beautiful toning?

I also understand that CMYK refers to color values on a printing press and that RGB refers to digital output (like a monitor).

My question was a little more geared to the fact that some pro-printers, even high quality shops are asking for images in RGB rather than CMYK...does this mean the printer really isn't as high quality as we think? Perhaps I worded my question strangely in my previous post.

Regarding monitor viewing, I understand that above 250, some say 245, is all overblown highlight.

But on PAPER, in RGB mode, is there a generally accepted "do not go over this value" for printing? Similarly, is there are "do not go under this value" for excessive shadow?

And in CMYK mode, for printing of course, are there a similar magic numbers (assuming, that is, those magic numbers actually do exist in RGB)?

For now, I'm off to take a look at your tutorial link...
TangoPango PRO 13 years ago
The point where shadows close into a solid is exactly printer (print engine, ink and media) dependent.

With an offset press utilizing a traditional half-tone dot, look for it to close up at around 95% or higher tint value. However a digital printer might be able to hold shadow detail to a 99% value.

On another point . . . When a printer or print service provider asks for an RGB file, it's because they want to have the full amount of color data so that their application preferences, working space, RIP device (in other words their entire color-managed work flow) can have a crack at the RGB to CMYK conversion while preserving the maximum amount of colors.
I work for HP as a technical product specialist on some very high end printers, and I get to answer that question every day.
One thing that you may also not be considering is that many, many printers are not just CMYK.
Many are CMYKcmk (meaning additional lighter shades of cyan, magenta and black).
Those sorts of color translations are much better handled by the agency/color specialist than just converting it in a desktop application - even a good one like Adobe Photoshop.
What lots of folks don't know when they convert their RGB color into CMYK in Photoshop is that they are throwing away color information as the gamut of RGB is so significantly larger than that of CMYK. Once it's thrown away, it's never able to be regenerated. If a RIP or printer driver doesn't need information contained in a file - O.K., nothing lost. If the RIP/driver/printer combination could use the information and it's missing, image quality is compromised.

Lastly, guidelines for standard numbers to use as plug-in values depends on what your plan is for the image. If it's going to press - you need to have color value EVERYWHERE even in highlights - don't take your brightest highlights to a point where there won't be any ink.

When I work in Photoshop, adjusting images for Flickr . . . I set my levels usually at 5-9 range for shadows and 245-255 for highlights. Keeping that in mind, if the purpose of an image is to be viewed electronically (like Flickr) . . . if I prefer another look, I do what I like, being liberated from consequence.
nicolai_g 13 years ago
The point where shadows close into a solid is exactly printer (print engine, ink and media) dependent.

Just re-stressing this. Every device is different, and the ink and paper make a tremendous difference as well.
Tracey Salazar 13 years ago
nicolai: just popping in to thank you for the link and to recommend it to anyone who has similar confusion. GREAT INFO!
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colorcritical 13 years ago
wow - I can't wait to read all of this. I have been on vacation this week...but I will definately get to this post. Thanks Tracey!
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colorcritical 13 years ago
Let's see if I can give a stab at this as well:

nicolai - great post btw...

Tango - kudos on a great response as well.

Here is a recap of Tracey's questions:

My question was a little more geared to the fact that some pro-printers, even high quality shops are asking for images in RGB rather than CMYK...does this mean the printer really isn't as high quality as we think? Perhaps I worded my question strangely in my previous post.

Regarding monitor viewing, I understand that above 250, some say 245, is all overblown highlight.

But on PAPER, in RGB mode, is there a generally accepted "do not go over this value" for printing? Similarly, is there are "do not go under this value" for excessive shadow?

Here is my take:

There is a lot of confusion in the industry over terms. The reason is that most concepts have several contextual meanings. I want to answer your direct questions but then post a new thread defining terms. The reason why some pro printers want RGB has to do not only with what Tango said but also becasue the professional photographic print systems today say for instance an Oce Lightjet, a Durst Lambda, ZBE Chromira, Fuji Pictrography, Fuji Frontier, etc all utilize a RGB to RGB conversion due to the systems themsleves use a LED to image onto a substrate. If one where to give a CMYK file to a printer who uses a Lightjet the file would need to be converted back to RGB when printed. This itself causes out of gamut colors to be compressed or even clipped depending on the rendering intent.

As far as a RGB value not to exceed, I like 245 for highlight detail and not under 10 for shadow detail.

CMYK has it's limitations as well all depending on the print technology, Offset, Web, Inkjet, etc. The conversion to CMYK from RGB has these dot limits built into the profile for conversion. I am sure you noticed this in Photoshop and may have used SWOP v2 which is a conversion for 300% total ink dot gain 20%.

I really recommend a book called Mastering Digital Printing by harold Johnson. This book is a great reference and explains these concepts in detail.

Cheers,

Marc
Phil Nesmith 13 years ago
I have the Mastering Digital Printing book and have found it a great resource!
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colorcritical 13 years ago
Yes This is a great book. The aurthor, Harold Johnson is a great resource.
koaflashboy PRO 12 years ago
Properly separating an RGB file (which of course is always done - either manually or by a RIP or printer driver) into CMYK is a complex ordeal that requires an extensive knowledge of the CMYK printing process along with ICC profiles.

In a typical "consumer" workflow, this conversion is handled by the printer driver, though not always in the best manner. And someone has no control over how any given RGB value is converted to CMYK for printing by the driver. You were asking why "high-end" printers may be requesting an RGB file - I would think that, in addition to the reason colorcritical mentioned above, is that they want to do the separation themselves - rather than getting a poorly-separated CMYK file that they would have to reseparate after converting back to RGB (which would result in image degradation).

Basically - trust the professionals or be prepared to learn a fair amount about what I mentioned above. Or obtain a decent profile for the media/printer combination you're using and apply it correctly in Photoshop before sending the file to the printer. Incidentally, unless printing specifically on a web offset press using SWOP-standard inks, separating an RGB file with the US Web Coated SWOP v2 profile is a poor choice because it does not generate - without tweaking UCR and GCR values - a very useful black separation. Obtain a media/printer profile from either the printer of media manufacturer and try that instead - checking your total ink limit in the 3/4 and shadow densities.

Sorry if that wasn't much help, but the topic of CMYK conversions is really quite complex
koaflashboy PRO 12 years ago
Oh - and incidentally - to answer this thread's original question - choose View/Proof Setup and either select your working CMYK profile or a custom setup, then turn on the View/Gamut Warning to see what colors will have to be remapped in order to print from RGB -> CYMK, remembering that even an RGB file is always printed in CMYK (they will be highlighted in whatever color you've chosen in the Preferences - by default, a bright magenta).

In File/Print with Options - this is where you select the rendering intent for how you want those out-of-gamut colors to be remapped. Without getting into the science of them, I would highly recommend choosing "relative colorimetric," though others use "perceptual" - in my experience, the first handles out-of-gamut colors more accurately. Of course, if you just select "Print" and let the printer driver handle the conversion, you have no idea how it's doing that and what colors will be remapped or how that will be accomplished.

And no - CMYK has a smaller gamut than RGB.
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