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Question

Preserving Greyscale Dot Gain in CMYK mode

Forum|Forum|5 years ago
April 26, 2020
3 replies
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I have a question about managing dot gain in greyscale images in a CMYK workflow.

I have an RGB image that's superficially greyscale (as in, it looks like grey tones, even though it's made of rgb values). It will be placed in InD in a document that has some colour pages/images, and some black and white. The document will be exported as a PDF/X-1a with a cmyk output intent. The image in question (plus the rest of the b&w content) has to print in k black only, so I convert it to Greyscale mode in PS, adjusting levels as desired, then place it in InD, then export to PDF- in Acrobat's Output Preview, the separations read as k-values only for the image. So far so good.

The question:

1) My printer says to compensate for 25% dot gain on all images. In general, can mixed b&w and colour content in the same pdf be adequately compensated for with a single destination profile/output intent? (In this case, everything is being treated like cmyk, so will the cmyk output profile/intent be sufficient for the dot gain in the greyscale images that only have k values? Or do images using just k dots versus all four colour dots require different compensation curves, even if they're in the same "mode"?)

2) If not, where in the process can I apply a dot gain compensation to the greyscale images? I assume embedding a profile would be useless, as the PDF/X-1a conversion will strip it.

3) It's worth mentioning that, for some reason, my printer did not supply me with a profile to use, even when directly asked. I'm assuming that without a specific printing profile, the 25% dot gain instruction is actually useless? (I've just picked US Web Coated SWOP v2 as my workflow profile, since its Adobe's the standard working space and stuff does indeed come off the press looking good when we use it. But that still brings me back to 1)- is US Web Coated SWOP v2, as a cmyk output intent, sufficient for both cmyk and k only dot gain?)

Any clarification greatly appreciated!

This topic has been closed for replies.

rob day

Community Expert

InDesign doesn’t have a grayscale space—placed grayscale objects output on the document’s CMYK black plate with their output values unchanged. When Overprint Preview is turned on, the document CMYK profile handles the preview of placed grayscale objects—an embedded Gray profile will show in the Links panel but it is ignored. With Overprint Preview turned off grayscale objects soft proof as sGray (2.2 Gamma), which would be useful if you are designing for web or screen display and not print.

Photoshop does have a Gray space, and if you want the soft proof of gray values to match between Photoshop and InDesign you have to use a Black Ink profile that matches the InDesign document’s CMYK profile.

For example, if your InDesign document has US Sheetfed Coated as its CMYK assignment you would want to use Black Ink -US Sheetfed Coated as the assigned Photoshop grayscale profile. Photoshop’s Info panel Actual Color shows the gray values that will output to the plates, and the Gray profile adjusts the soft proof to show the affect of dotgain:

Black Ink-US Web Coated assumes less dot gain, so the same output values display as lighter values:

To use a Black Ink profile in Photoshop open Color Settings>Gray Working Space>Load Gray... and choose the desired CMYK profile. The Gray profile will be listed as the CMYK profile with a Black Ink - prefix:

If the printer will not provide a CMYK profile, but is expecting you to make CMYK conversions (PDF/X-1a) you are shooting in the dark for both CMYK and Gray color management.

BlueCat55Author

Known Participant

"if you use a profile that is designed for the destination press, you shouldn’t have to manually compensate because the assigned profile affects the preview."

"Black Ink-US Web Coated assumes less dot gain, so the same output values display as lighter values"

Just to clarify, I wasn't compensating to create an accurate preview- my intent was to lighten midtones and improve contrast so that the image might print better regardless of dot gain. ie. With no profile I don't know how much dot gain there'll be. But regardless, I figured that with any amount of dot gain, lightened midtones and heightened contrast will print more readable than a flat image with fewer tonal values.

But with regards to grey soft-proofing, am I correct then that Photoshop's proof is showing what the absolute k values will look like after print, in dot gain conditions simulated by the profile?

If the printer will not provide a CMYK profile, but is expecting you to make CMYK conversions (PDF/X-1a) you are shooting in the dark for both CMYK and Gray color management.

Unfortunatley this is the case. I've had to guess- but I've been lucky in that our first print job turned out well. So I've stuck with the workflow for that first project ever since.

...you would not want to convert to the default US Web Coated SWOP, and try to compensate by color correcting the numbers. At the very least you would have a total ink problem—SWOP’s 300% vs SNAP’s 220%

They did give me an ink limit- 240% for 4-colour content and 140% for 2-colour content. (Is this kind of split limit normal?) To get colour images under this limit, I have been editing them in Photoshop with levels/curves/channel mixers + Info Panel, and then checking TAC for the whole image in Acrobat or InDesign. It's time consuming, but seems to work. I'm working in US Web Coated (SWOP) v2 but never hit 300% simply because I've edited those colours out. (Unless I'm missing the point- are you saying that a given ink coverage in two different profiles entails a different, absolute volume of ink in each process being simulated? ie. is 220% in SWOP a heavier ink volume than 220% in SNAP?)

What cmyk profile would you recommend using in this situation? With this target TAC, is it better to use a profile that's over or under?

Bob_Hallam

Legend

So if I understand- even in the absence of a specific printer profile, it's better to be editing images in RGB, then converting to a cmyk newsprint profile that at least has similar characteristics to my output device and given specs, rather than use the default working space. Greyscale images can be converted to greyscale using the black ink of my chosen newsprint profile, and then converted to cmyk in the final PDF/X-1a

(I guess what gets me is the gamut differences between rgb and cmyk- I've always edited images in my chosen cmyk profile because it's the lowest common denominator, colour-wise. I've never wanted to fall in the trap of relying on bright, saturated rgb colours that can't be repoduced by my printer, so I never bothered with rgb for printed images and edited in cmyk from the start, to see what I have to work with. Yet I hear everyone saying it's best practise to work in rgb mode and convert afterwards. Can someone speak to this?)

Tangent aside- I'm going to see if it's feasible for my workplace get some test prints done, to compare this workflow to what I've been using. (Not to mention keep hounding the printer for a profile. I imagine that trying to calibrate monitors for good soft-proofing will still be guesswork without this.)

Hearing the recommendation to use a newspaper profile because the dot gain is at 26% would work, but o Ky if you're printing on newspaper. If you're not then don't use a newsprint profile. This are more reasons than dot gain alone that make a profil work for a print process. Not the least is gamut. Newspaper profiles have a very small gamut and high dot gain because of the stock and print process. It is a low quality high speed process. So if your work isn't being printed that way don't go there.

if you're printing on coated stock use a coated profile that's going to get you close to the gamut of the print process your printer is using. If they don't have web presses don't use the SWOP. For coated sheet Fed work use GraCol 2013 or CRPC-6 as the destination profile and lighten the file by 5% in the mid tones using curves. If you tried using a newspaper profile on coated sheet Fed printing the results would be very poor.

most of today's high quality work is edited in RGB. I prefer Adobe 1998 RGB for most work and supply color corrected RGB to printing companies. Those printing companies convert to the appropriate press profile and there is no disappointment.

ICC programmer and developer, Photographer, artist and color management expert, Print standards and process expert.

NB, colourmanagement

Community Expert

Hi

Basically you need an accurate ICC CMYK profile from the printer. That way no discussion on dotgain will be needed.

Many printers have adopted the sensible approach of standards based printing, that’s what ISO 12647 and GRACoL are all about.

These guys seem rather seriously out of step with current working practices

I hope this helps

if so, please "like" my reply

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 chronological order]

BlueCat55Author

Known Participant

In reply to both, this is a cold web offset press and we're printing on newsprint. Their specs are at least partially based on SNAP. The printer told me 25% dot gain, and PDF/X-1a, but didn't give a profile. (They're a large company too- it's strange)

My manager told me that in the past we'd had problems with images coming out too dark. I don't know what the previous workflow was, but I do know that when we work in US Web Coated (SWOP) v2, manually edit greyscale images in Photoshop before placing (bring up midtones, heighten contrast), and place/export them in the workflow I described, they generally come out looking good. So I've been doing something similar to what you suggested, @Bob_Hallam1- just not in any fixed, quantified way.

So would I be best off taking those images that did turn out well, and basing all future curve/levels edits on those ones?
(We aren't set up for reliable soft-proofing. The output happens to match our screens decently, but a) we can't calibrate them and b) without a profile it guess wouldn't be much use anyways.)

rob day

Community Expert

If you use a profile that is designed for the destination press, you shouldn’t have to manually compensate because the assigned profile affects the preview. You can see that here where I’m comparing Black Ink-US Newsprint with Photoshop’s Dot Gain 25% curve.

The DotGain 25% preview shows more contrast than will likely happen in the print—newsprint flattens contrast because of the absorbent paper, and the Black Ink US Newsprint profile is displaying the expected lower contrast.

Bob_Hallam

Legend

Sounds like this printer is very low tech. 26% dot gain is high as well, and it's rare (Like I have never seen one) where a printer will have equal dot gains in all colors. Also of note is it's the printers' responsibility to adjust their process to a standard. So that begs the questions, What print standard do they use and What print process is this? (ie Offset Flexo, Gravure etc)

So go through those questions and try to give us a better picture of what you're dealing with if you can.

The reason for those questions is there are 2 ways now to get to your solution.

One is to convert to CMYK with a standard profile that compensates for a "normal" amount of dot gain and lighten the files afterward using curves to go the rest of the way your printer needs. I suggest this but I'd go so far as to say that your printer must, if they have been in business for very long received file that are at print standards and uses them. They compensate with plate curves and life is good. That would be the expectation of anyone supplying files to a printer.

The other is doing a custom CMYK profile in PS where your responsibility is total for every little bit of how that conversion and print process will happen. I don't recommend this because so far at least the information received here doesn't tell us if you have any proofing capabilities. Without proofing capabilities this is a bad road to take.

ICC programmer and developer, Photographer, artist and color management expert, Print standards and process expert.

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