Question about "Preserve RGB Numbers" in Proof setup

[J Isner]

In Photoshop, View -> Proof setup -> Custom... displays a dialog.  In the dialog, you specify the "device to simulate" by choosing an icc profile.  This shows you what your document will look like when you print with that profile.

There is also a checkbox labeled "Preserve RGB Numbers."  As I understand it, checking this box causes Photoshop to show you what the document will look like if you send the current R, G, and B numbers to the printer WITHOUT converting to the chosen profile (you're simply telling the printer "use these numbers").  This is useful to me because I am about to start printing using a driver with no color management.

Here's my question: Say my document profile is Gray Gamma 2.2.  In the Custom proof setup dialog, I check Preserve RGB Numbers .  But I also need to specify a "device to simulate".  What profile should I choose for "Device to simulate?"  Should I choose profile I would use if printing WITH color management?  I'm guessing the answer is "yes" because that gives Photoshop information about the printer and the kind of paper you'll be printing on. 

[D Fosse · Accepted solution]

https://forums.adobe.com/people/J+Isner  wrote

But I also need to specify a "device to simulate".  What profile should I choose for "Device to simulate?"

"Device to simulate" is the whole point of proofing - that's what you proof to. The purpose is to visualize on screen any gamut clipping that may happen in the final output, in the conversion from document color space to output (e.g. print) color space.

Proof is just an on-screen simulation. It has no effect on the file or the final output.

What you normally see on screen is the result of a standard document profile > monitor profile conversion. When you proof, an extra step is inserted in this standard color management chain. The output profile is inserted between the two, and this limits the gamut accordingly by clipping to the output profile before it reaches screen.

With "preserve numbers" unchecked, all these steps are normal "convert to profile" operations, which preserve color by recalculating the numbers. So everything displays normally, except clipped to the output profile.

With "preserve numbers" unchecked, the extra step is not a "convert", but an "assign". Assign Profile does just that - it preserves numbers into the destination color space, which means the colors will change. The numbers are not remapped into the new color space. Assign/preserve numbers only works in the same color model, obviously you can't preserve RGB numbers into a CMYK color space.

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Finally an important point that few think about: this whole thing is ultimately limited by your monitor color space, and with a standard gamut monitor this is usually the smallest of the color spaces involved. So that limits the usefulness of proofing - a lot of real gamut clipping that affects the print happens here, but it's outside monitor gamut so you can't see it anyway. The value of a wide gamut monitor is that it covers almost all real life print spaces, so proofing is very reliable if the monitor has been properly calibrated to match the output.

Note: Proof does not compensate for your calibration targets. If your screen is too bright, the proof is too bright, and the print may still come out too dark. Proof does not compensate for print black level or paper color. All of this has to be dealt with in calibration.

[D Fosse]

Oops, typo there! Preserve numbers unchecked = convert. Preserve numbers checked = assign.

And I almost forgot: If your file is grayscale (as you say), proofing is totally irrelevant and moot. Just forget it, it won't show you anything. There's obviously not going to be any gamut clipping of saturated colors here.

[G_Hoffmann]

Very good explanations, Dag.

But eventually there is one thing not yet clear. Probably the OP refers to inkjet printers

with RGB input, which are often  called – misleading – "RGB-printers".

For instance Epson 7890 can be used as RGB-printer or, together with a RIP, as a CMYK-

printer with CMYK input (what I'm doing).

Here is a typical RGB profile for such an "RGB-printer":

http://docs-hoffmann.de/Epson-RGB-Printerprofile.icc

Original EPSON, with shorter name. ICC-Profile Inspector shows clearly: RGB-Input.

Proof Setup for this printer profile and an RGB image offers indeed Preserve Numbers.

In my opinion this should be ignored – it doesn't make any sense.

Best regards --Gernot Hoffmann

P.S.: Proofing Grayscale Images is eventually useful. This shows, how an image with

defined Gamma (e.g. 2.2) or Dot Gain (e.g.20%) will look if reproduced by an output

device with different Gamma or Dot Gain.

[D Fosse]

Yes, but if Photoshop handles the printing, the grayscale profile will be converted into the (RGB) print profile. So the tone response curve is also remapped, and appearance unchanged.

Of course, as we both know, grayscale support outside Photoshop is very sketchy and mostly non-existent. So any other software involved in the printing will likely mess things up. Then it's useful to know how the file looks with other gamma or dot gain output.

[J Isner]

Thank you for a clear and thorough explanation.  I see now why I was confused.

In your follow-up post, you say "if your file is grayscale ... proofing is totally irrelevant and moot ... it won't show you anything. There's obviously not going to be any gamut clipping of saturated colors here."

If I were going to soft proof in the normal way (that is, without Preserve numbers) I would do it in Lightroom.  In fact, I have been soft proofing in Lightroom for several years (it's so much easier!).  I am only now being forced to do it in Photoshop because I need the Preserve Numbers feature.

In Lightroom, when I soft proof a grayscale image for printing on matte paper (which accounts for 99% of my printing), tonal adjustments are almost always needed due to the reduced contrast range of the paper.  Soft proofing allows me see where those tonal adjustments are needed, so I wouldn't say "it won't show you anything."  If I soft proof in Photoshop with "Preserve numbers" I assume the same holds true: I can make needed tonal adjustments, and these adjustments will change the underlying numbers. 

[D Fosse]

I have to confess I usually avoid grayscale, mainly because very few applications properly support it. Grayscale profiles are mostly ignored, even by Adobe applications like Illustrator and InDesign. Then you're at the mercy of output dot gain or gamma, as Gernot mentioned.

To my knowledge Photoshop and Acrobat are the only applications that handle grayscale correctly and reliably - meaning they remap tone response curves from source to destination profiles, which is what correct color management is all about. Even if there's no color, color management is still required.

Anyway. When you say,

tonal adjustments are almost always needed due to the reduced contrast range of the paper

that's correct - but, grayscale color management aside, this is in the realm of setting the proper calibration targets. The standard procedure is to set monitor white point to visually match paper white, and monitor black point to match max ink density for the paper. With this set, and assuming correct color management to the output profile, what you see is what you get. At this point you would proof to check for gamut clipping. In the case of grayscale, there obviously won't be any.

Granted, switching calibration targets constantly to accommodate different papers isn't straightforward in most calibration software. But in high-end software like NEC Spectraview and Eizo ColorNavigator you can do just that. This is one of the major returns on investment with these rather expensive monitors. In terms of matching screen to print, these units make life considerably more predictable.