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Should I match rgb values to compensate after changing colour profile?

Forum|Forum|5 years ago
May 16, 2020
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My primary edits are in ProPhotoRGB

I then copy and paste into a template for export, which also changes the colour profile for printing

I noticed the eyedropper tool no longer reads the same RGB values in the greyscale spectrum, i.e, 0,0,0 > 2,2,2 and (50% grey) 128,128,128 > 152, 152, 152. etc. The same applies for all the grey values. I assume something similar would be going on in the colour values too.

- Should I apply a curves to change these values back to what they read in the ProPhoto original? Or at least change the black point back to zero?

- Is there a reason this is happening? It seems odd that even the zero black point would change in a conversion to a narrower colourspace.

Thanks,

Jonathan

This topic has been closed for replies.

Correct answer NB, colourmanagement

Hi

RGB values inevitably change as the conversion takes place and colour values are adapted to suit the printer. The printer profile describes the printer's characteristics.

After conversion leave the values alone, or you risk damaging printed colour accuracy.

For example - If you send RGB 128,128,128 it may print with a tint.

On that printer on that paper to get neutral you may need to send 128,128,120 to get neutral grey.

That’s normal & expected.

change it back to 128,128,128 it will again print with a tint

Compare this, take a French essay and translate (convert) it to English, it now suits the English speaking reader. You would not consider altering a few of the words back to French, right? Same with ICC profile conversions.

I suggest to assess the process, just make a print and check the visual appearance against your calibrated screen, don't obsess on the RGB numbers. It's great you are trying to understand this but as you have found it can be pretty confusing.

The standard (preferred) requirement for a print provider is for a customer to send files in a working colour space, say Prophoto RGB (with the ICC profile embedded of course), then they convert to their ICC profile at the time of printing.

Generally IF a printer profile is provided to customers, it's intended just for soft-proofing.

By the way, if you decided to convert to the printer ICC I hope that you're making a copy and converting that, otherwise you lose to ability to re- purpose the file later.

Yes - use Black Point Compensation always

Always flatten a layered file before converting to another colour space.

[which ideally means archiving full gamut (Prophoto in your case) layered file and making a copy before flattening. Space permitting its always better to save the layered version.]

Derek asked about Pro Photo, well, bigger is not always better, smaller working colour spaces can be easier to use.

There are instances where Prophoto [or a similar better targeted large color space such as Joseph Holmes one of DCAM spaces] will not "clip" captured colour values, whereas, say, Adobe RGB might. Unfortunately, that concept / issue takes some understanding. Yiu can read uop about it at imaging guru Joseph Holmes' website https://www.josephholmes.com/profiles

I hope this helps

if so, please "like" my reply and if you're OK now, please mark it as "correct", so that others who have similar issues can see the solution

thanks

neil barstow, colourmanagement.net :: adobe forum volunteer

[please do not use the reply button on a message in the thread, only use the one at the top of the page, to maintain the thread title and the chronological order]

jonpxlAuthor

Known Participant

Thanks Dave,

Yes, soft proof changes the preception of colours on screen when on, that I totally understood. I mentioned the change was different between the soft proof version (default settings) and the profile conversion version.

So if profile conversions aren't needed, are you recommending doing manual local gamut corrections?

Last time I tried that, I found it didn't do as good a job as asking photoshop to do a profile conversion (copy and paste). The gamut warning and soft proofing I hear are not very trustworthy.

I'm quite happy with ProPhoto, I've been using it for a while and I have a wide-gamut monitor, so although we don't have ProPhoto monitors and printers, it's a safe starting point for P3 or Adobe RGB purposes. If ever it's an issue, I can always convert them.

Thanks, much appreciated.

jonpxlAuthor

Known Participant

Thanks for such a grounded and thorough answer.

Can I check that I've understood?

Colour conversions and seperate files for each paper type are not needed in this case
The colour profiles that are provided by the printers: I should only use to soft proof, and not actually have embedded in the file I send them, instead embed the working colour spacce, which is done by deafult, in this case ProPhotoRGB
If doing conversions in future, don't adjust the RGB of a converted image as a fix

But using a profile for soft proofing yields a very different visual look to if you copy and paste into that profile, making anther layer of confusion.

Also copy and pasting into a document with the destination colour profile, often handles out-of gamut areas much better than actually trying to manually fix this in your original document (by which I mean using toggling on/off a group of adjustment layers for each intended output profile)

But if the print supplier does the conversion themselves, then I'd likelt get better gamut conversion and the correct colours if, in this particular instance, I leave that to the printing company. Therefore, I should only have one output jpg/png file, with ProPhotoRGB embedded, rather than one for each paper type.

In a nutshell - I went down a rabbit hole, only to come back to output from the original file as it is. But still very good to now hopefully have understood a lot more.

Thanks very much.

davescm

Community Expert

Colour conversions and separate files for each paper type are not needed in this case

The correct printer and paper profile are always needed , but only at the softproofing and at the printing stage. There is no need to convert the document

The colour profiles that are provided by the printers: I should only use to soft proof, and not actually have embedded in the file I send them, instead embed the working colour space, which is done by default, in this case ProPhotoRGBYes. Use the document profile for the document. Incidentally , I agree with D.Fosse's comments above regarding ProPhoto. Adobe RGB is plenty big enough for printing and will allow you to spot colour errors on a good screen. No screen made can show the ProPhoto gamut.
If doing conversions in future, don't adjust the RGB of a converted image as a fix
Absolutely - otherwise you are not converting.
But using a profile for soft proofing yields a very different visual look to if you copy and paste into that profile, making anther layer of confusion.
The idea of the soft proof is to show you where problems with out of gamut colours may lie and gives you a simulation of the print. It should look different, it is designed to show you how your colours will change. It is not perfect as it is still limited by your monitor, but it does give a feel for what will happen. There are a couple of ways to deal with those out of gamut colours. That method of dealing with them is set by the rendering intent. Relative colormetric moves out of gamut colours to the nearest in gamut colour, but does not move in gamut colours. Perceptual rendering intent moves the out of gamut colours to the nearest in gamut colour but also moves (desaturates) some in gamut colours so that the difference between colours is maintained. You can try both in soft proofing.

Dave

NB, colourmanagement

Correct answer

Community Expert

Hi

RGB values inevitably change as the conversion takes place and colour values are adapted to suit the printer. The printer profile describes the printer's characteristics.

After conversion leave the values alone, or you risk damaging printed colour accuracy.

For example - If you send RGB 128,128,128 it may print with a tint.

On that printer on that paper to get neutral you may need to send 128,128,120 to get neutral grey.

That’s normal & expected.

change it back to 128,128,128 it will again print with a tint

Compare this, take a French essay and translate (convert) it to English, it now suits the English speaking reader. You would not consider altering a few of the words back to French, right? Same with ICC profile conversions.

I suggest to assess the process, just make a print and check the visual appearance against your calibrated screen, don't obsess on the RGB numbers. It's great you are trying to understand this but as you have found it can be pretty confusing.

The standard (preferred) requirement for a print provider is for a customer to send files in a working colour space, say Prophoto RGB (with the ICC profile embedded of course), then they convert to their ICC profile at the time of printing.

Generally IF a printer profile is provided to customers, it's intended just for soft-proofing.

By the way, if you decided to convert to the printer ICC I hope that you're making a copy and converting that, otherwise you lose to ability to re- purpose the file later.

Yes - use Black Point Compensation always

Always flatten a layered file before converting to another colour space.

[which ideally means archiving full gamut (Prophoto in your case) layered file and making a copy before flattening. Space permitting its always better to save the layered version.]

Derek asked about Pro Photo, well, bigger is not always better, smaller working colour spaces can be easier to use.

There are instances where Prophoto [or a similar better targeted large color space such as Joseph Holmes one of DCAM spaces] will not "clip" captured colour values, whereas, say, Adobe RGB might. Unfortunately, that concept / issue takes some understanding. Yiu can read uop about it at imaging guru Joseph Holmes' website https://www.josephholmes.com/profiles

I hope this helps

if so, please "like" my reply and if you're OK now, please mark it as "correct", so that others who have similar issues can see the solution

thanks

neil barstow, colourmanagement.net :: adobe forum volunteer

[please do not use the reply button on a message in the thread, only use the one at the top of the page, to maintain the thread title and the chronological order]

davescm

Community Expert

Can I ask why you are converting your document to the printer & paper profile rather than just setting "Photoshop manages colours" in the print dialogue, selecting the appropriate printer & paper profile for your specific printer and paper and letting the conversion happen in the print process.?

.....and yes, black point compensation should be on.

Dave

D Fosse

Community Expert

Yes, that too 🙂

D Fosse

Community Expert

Black Point Compensation should normally always be on. There is no commonly occurring reason to turn it off. It prevents hard clipping when remapping the black point, giving a soft taper instead.

You can never print 0-0-0 black, so print profiles often partially compensate for ink density. You can see that with most CMYK profiles. You can't compensate fully because it depends on the paper characteristics and paper reflectance. To do that, you calibrate your monitor to a visually matching black point.

Anyway, this goes in the opposite direction, so I would suspect a defective Hahnemuhle profile. Yes, I'm aware it's a well-respected manufacturer, but we've seen bad Hahnemuhle profiles here before.

As for ProPhoto, my personal opinion is that it's overrated. Larger gamut is not automatically "better".

You very rarely need the huge gamut, which comes at a price. I've talked about the shadow compression before, which makes subtle shadow adjustments difficult, and very difficult to spot in the histogram. Adobe RGB is much esasier to work with. ProPhoto will also exaggerate and bring out latent problems in the color management chain.

Use it when you need it. When not, use a smaller and more manageable color space. Above all, don't use ProPhoto if you're not fully aware of the implications.

jonpxlAuthor

Known Participant

Thank you. I'm not sure it's a profile error, as mentioned this applies to other profiles as well, not just this specific manufacturer's ICC. It also occurs when moving from Adobe RGB, to any other profile, be that paper, filmstock etc, so it seems like normal behaviour.

The question is what's the correct course of action: Leave it, or compensate. And why?

Of course printers can't print pure black, but still the conversion process, doesn't link the minimum black of the image, to the minimum black of the destination. I can quite happily use the brush, select 0,0,0 and brush away a darker region than is visible in the converted image. Still within gamut of course as you're painting directly in the image output colour profile, an additional soft proofing (for what it's worth) confirms that.

Despite having been a creative professional for many years, I and many colleagues, still find colour managment quite an oddity. I do enjoy at least trying to understand these matters though.

I think the simple answer is to get sample prints with a few extra variants, expensive, but informative. I had hoped to better understand this before going that route. Thanks for all your insight.

D Fosse

Community Expert

The numbers are supposed to change when converting to a different color space. That's the whole point: the numbers are recalculated to preserve the same visual appearance. When pasting, this conversion is performed automatically.

Grayscale is subject to standard color management just like any other color space. The fact that there's only one channel, instead of three or four, changes nothing.

Different color spaces, including grayscale ones, have different tone response curves. ProPhoto has a gamma 1.8 curve, which means all grayscale values will change relative to a gamma 2.2 space, or sRGB, which has its own idiosyncratic curve.

In short - just make sure the document has a profile embedded, and everything will be correctly handled whn going from one color space to another. If the profile is missing, all bets are off and anything can happen.

jonpxlAuthor

Known Participant

Thanks, it's still RGB. It's a printer profile.

The image is colour it's not grayscale, I am simply using black, grey and white points as an example of values changing.

It seems odd that the black and white points would move away from 0 and 255 in any profile conversion though.

My images contain a lot of black, so 0 in prophoto now no longer being zero in the printer profile converted image makes me concerned. In the converted profile doc, I can select a brush with a darker value than what the darkest pixel in the image contains, this seems odd.

jonpxlAuthor

Known Participant

That 0/0/0 and 255/255/255 change to other values in another RGB Color Space would seem surprising to me but unless you answer questions it is hard to tell what the issue could be.

What are the Edit > Color Settings, in particular the Intent?

Please set the Status Bar to »Document Profile« and post meaningful screenshots.

What is the target Color Space? (Color Space is not the same as Color Mode.)

Is the »printer profile« definitely an RGB profile or maybe a CMYK profile?

Thanks very much for being on hand to help out, it's much appreciated.

In answer to your questions here are 2 screenshots that should have most of the information you've asked for. Please note that when the screenshot was taken, the eyedropper was hovering over the black 0,0,0 value. These are my test contactsheets for printing.

I have 3 paper types I'm printing to. All three have this issue, but read different numerical values

So, in a nut shell... all those grey blocks, have their numerical RGB value in prophoto written beside them. But after conversion, all these grey blocks no longer match their numbers. Still grey, i.e all channel RGB values are equal, but shifted. The white doesn't seem to shift, but the black does.

This seems odd to me, and I am trying to determine is this is an error in my conversion method / if I can manually add a curves to fix the 10 values / just fix the black point / or do i leave it how it is and just have faith without needing to know why black is no longer black.

My conversion method: Source: Select All & Copy Mergeed, Destination: Paste into existing template document with already assigned printer profile, confirm colourspace conversion.

Thanks very much for your time, I can happily provided further information if any more questions.

- Interestingly I have tested with other profiles, and these grey values always get shifted, some even tint it. So either my conversion method is wrong, or I'd quite like to know why the black changes, and if/should I change it

It seems odd to go to a narrow colourspace and not have the image minimum black level match the minimum black level of the destination. min/max you'd think would stay the same, as it's relative to the colourspace even if there's a perceptual change.

Original/Source:

Destination / For printer:

Here is a screenshot of my color settings:

Conversion Dialog Box:

c.pfaffenbichler

Community Expert

And what is the template file’s Color Space?

c.pfaffenbichler

Community Expert

Please set the Status Bar to »Document Profile« and post meaningful screenshots.

What are the Edit > Color Settings?

Do you have a basic understanding of Color Management?

It is not unexpected that pixels’ RGB values change when converting an RGB image to another RGB Color Space.

https://helpx.adobe.com/photoshop/using/understanding-color-management.html

http://www.gballard.net/psd/cmstheory.html

jonpxlAuthor

Known Participant

My document colour profile is ProPhotoRGB, the destination document has a printer profile (RGB)

Yes, I have a basic understanding, but I wasn't aware that a minimum value (0,0,0 black) would be converted to where it is no longer the minimum value (in this case 4,4,4), thereby allowing an even darker value relative to the original document to be used to darken what was already black. Odd

Thanks for your insight.

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