Rendering out for print - seeking advice CMYK

[rizon]

i do this alot, there's not much to be afraid of... just render your piece at a high resolution, open it in photoshop. Change image size to 300dpi (make sure that your pixels scale along), change rgb to cmyk, and make if necessary some color adjustments.

[station75]

Hi I made several works for print and I have one big tip to you - render it normally via rgb and after all postprocessing work use PhotoShop or other programm like this to convert your work to cmyk. In that way you have full control over colors and levels/histograms of your work and by using color managment profile you can get right colors in print.

below are links to two of my works for print - the

http://www.station75.com/portfolio/s75_abs_01.jpg

http://www.station75.com/portfolio/s75_abs_02.jpg

[vesalus]

once rendered in the appropriate size and dpi, open your render in photoshop and convert first your render in lab and then in cmyk, the conversion will be better than rgb to cmyk...

[dan22]

once rendered in the appropriate size and dpi, open your render in photoshop and convert first your render in lab and then in cmyk, the conversion will be better than rgb to cmyk...

That's interesting - not heard this before.

Could you kindly explain why this is, plus whether it has a bearing on final printed colour gamut, and how it works?

Cheers,

D.

[imashination]

once rendered in the appropriate size and dpi, open your render in photoshop and convert first your render in lab and then in cmyk, the conversion will be better than rgb to cmyk...

What makes you think that?

[lllab]

i heard that too quite often, but cannot confirm if it is really true.

cheers
stefan

[Chrissyboy]

I'm pretty certain this is nonsense, Photoshop uses LAB as its internal transfer format, ie all mode conversions go first to LAB and then to the destined colourspace. Going via LAB manually is a waste of time, the only way it would make any difference is if you've set up you conversion profiles wrongly.

The key is to keep RGB until you've finished all colour correction, comping work, and convert to CMYK only in the finished, flattened file. I've recently been working with 16-bit renders too, the extra lee-way you get for brightening your image is fantastic.

Lastly check your black plate, it should look like a high-contrast skeleton of the finished image, very bright with dark shadows. Feel free to email/upload the CMYK file if you'd like me to have a look at it.

Seeyou - Chris

once rendered in the appropriate size and dpi, open your render in photoshop and convert first your render in lab and then in cmyk, the conversion will be better than rgb to cmyk...

[mecha]

Try to avoid using very bright and flourescent colours in the render, they will go very dull when converted to CMYK and can be nearly impossible to recreate.

Also, in Photoshop convert to CMYK as soon as possible because the last thing you want to do is Render & Comp, then have CMYK destroy your work. In fact, if your computer allows it (and you dont need to use too many PS filters) work in CMYK mode all the way!

I know I disagree with Chrissyboy, but my experiences have taught me to work in CMYK and avoid drastic colour correction at the end.

Dont know about the Lab thing, I think thats more to do with black and white photography.

mecha

[Rantin Al]

Chris,
My understanding from reading books by Blatner & Fraser, Photoshop uses Lab as an internal reference to build a look-up table for RGB > CMYK conversion. It does not actually do a rgb>lab>cmyk conversion. Monitor set up profiles come into this equation. Unless everything is spot on with calibration, inaccuracies will be introduced.
The reason for the single conversion is a trade off between time factor versus accuracy.

Using RGB>Lab conversion manually is device independent and bases the conversion on the actual file RGB profile.

RGB>Lab>CMYK will need two actual colour-space conversions but will give a more accurate conversion.

Tip: RGB > Lab and ditch the ab channels gives the best result for converting to greyscale.

If sending to a print burea for printing, ask them what format they need.
'Usually' they will have a RIP calibrated for their workflow and will be able to handle the conversion best suited to their print process.

HTH. Cheers, Al.

[tonyg3d]

Depends what your print destination will be.

I did some 3d for use on a 48 sheet billboard. I rendered out RGB (obviously) as
a multi-pass psd file. When I converted to CMYK still layered, the specular / reflection
layers went haywire. I also set up a colour profile converter to convert from RGB to CMYK
that takes ink limits into consideration, which also screws your gorgeous RGB colours up
even more! Just something to be aware of. Just be prepared to play around heavily with
the levels and colour saturation once converted.

I'd also suggest supplying your client with a pre-converted CMYK proof. This way, your
client (and you) won't be so dissappointed with the flattened colours your glorious RGB
image promised! I've started doing this!

This might help to, I have it taped to my monitor for reference;

print sizing - printsize (inches) X DPI = pixel render size.

so, for a 200mm X 160mm image @ a decent dpi, say 300 dpi

200mm (7.87 inch) X 300 = 2361 pixels
160mm (6.3 inch) X 300 = 1890 pixels

there you go. render at 2361 X 1890 pixels for a print quality render at 200mm X 160mm

Hope that's some help, :hmm:

TonyG.

[flingster]

ok i'm curious about conversion from 16bit down to 8bit?

essentially should this be done prior to sending to the print shop?
when i do convert down...i see visually on my monitor any type of gradient effects tend to have more linear blocks to them rather than smooth gradients...will this show on final prints or is it just a visual thing? i just noticed this last week so not had time to look into it but popped into mind when i saw this post....essentially the gradients tend to look blocky and ringed in areas...if i remember rightly zooming in i couldn't see the blocks...is it anything i should worry about...since i don't particularly want a blocky looking gradient...want them smooth transitions.


edit: also is it necessary sending converting all this extra data...i'm looking for quality but is this something that a print place will consider in the first place..all advice accepted as always guys thanks for your help.

[Finster]

I've been converting straight RGB>CMYK for years (packaging printing). I'll have to look into the LAB intermediate step. For standard commercial work RGB>CMYK should be fine, sometimes with some additional color tweaking. Just check your color settings to make sure your CMYK isn't set to anything funky. I am not sure what the default is these days, but we typically use the SWOP standard.

If you know the printer/service bureau (and you trust them), ask them what they need. Some might prefer RGB so that they can do the CMYK conversion themselves. It gives them options to run hexachrome printing (not likely) or to change the GCR (gray component replacement). But then again, you are trusting someone else with your images.

In packaging, we will often increase the GCR in images that have a lot of light gray tones (especially product shots of plastics). This will replace more of the CMY in the highlight with black, which makes it easy to hold that neutral gray. Improper GCR here might lead to your products looking pink or blue or yellow through a press run. It is more difficult to hold a balance on press between 15% cyan - 15% yellow - 15% magenta, than it is to run 5% cyan, 5% magenta, 5% yellow, and 15% black (example, not actual conversion).

[jph]

ask your printer what kind of color scheme he is using to print the job, than choose that colorscheme from the photoshop color settings... (atatched jpg) e.g. euroscale uncoated...

as it was said before try to do all the photowork in rgb, you might have to adjust some colors after converting though (cmyk color space is smalller than rgb)
instead of converting it in the beginning there is also a preview state,the 'Proof setup'
Menue: view/ 'proof setup' and 'proof colors'....

are you working on a flatscreen?, if you are, you will have the problem that printjobs shouldn´t be done on a flat screen since they don´t show color as acurate as an old cathode-ray tube...

before it goes into final print you really should color proof it, and check if the colors come out right! cause if they don´t you might end up having to pay the printjob, since you delivered bad data... (this sums up) .
Show the colorproof to the client, have him sign it, prooving that this is what he wants. Hand the proof to the printer as a reference. This way your ass is covered, and you did everything to deliver good data!

later jan

[Chrissyboy]

are you working on a flatscreen?, if you are, you will have the problem that printjobs shouldn´t be done on a flat screen since they don´t show color as acurate as an old cathode-ray tube...

This was good advice around three years ago, but these days decent LCD screens are now every bit as good for colour work as CRTs, we use them exclusively here, you can often hold up the printed page to the screen and barely perceive the difference. The clarity of a good LCD is also superior, we use Eizo 21" which are pretty reasonable but are great for colour.

CMYK really isn't as scary as it sounds. There's a few gotcha's - deep blue skies can easily turn purple, and oranges can turn to mud if you make it too deep, but generally it's pretty straightforward.

Seeyou - Chris

[jph]

This was good advice around three years ago, but these days decent LCD screens are now every bit as good for colour work as CRTs, we use them exclusively here, you can often hold up the printed page to the screen and barely perceive the difference. The clarity of a good LCD is also superior, we use Eizo 21" which are pretty reasonable but are great for colour.

Seeyou - Chris

....time to do some investments for me then...

[segart]

RGB profile is also very important. I usually work with RGB too (layers blend better I think) with sRGB profile, wich is more accurate for translating some difficult colors, but it depends of the dominant color in my image, some other RGB profiles are better for some other situations. There is no need to use only one RGB profile always and testing is required for choosing the right one for your image. And I always use CMYK preview (ctrl-Y), we really don't need to see RGB colors when working for print.

[belushy]

for colormangement and conversions i would read
material and files provided by The European Color Initiative (ECI) www.eci.org

rui's plugin
just lets you select cmyk values or their eual RGB counterparts but does not effect the output in any way

For conversions i would suggest not to convert the files before the last job in photoshop is done since the CMYK colorspace is much smaller than rgb
another new radical way is to leave the converison to the rip (tip from a litho studioprof. *vienna Paint*) and deliver RGBs with eciRGB workspace since it is even bigger than sRGB
to the printer
a very new approach

to control the conversion in the old way just use the proofing funktion of photoshop in the view menue

cheers belushy

[ernia]

Lastly check your black plate, it should look like a high-contrast skeleton of the finished image, very bright with dark shadows. Feel free to email/upload the CMYK file if you'd like me to have a look at it.

Seeyou - Chris

This may be all right for some look or style, Chrissboy, but that black plate is very important for the final look in print, as I'm sure you already know. Depending on the press capabilities and paper stock, the black plate that looks as you suggest will be plugging up all over, creating a high contrast look and not printing very well. Keep in mind that the inks are designed to lay on top of each other. So I would suggest, depending on the stock--which is why you should have your printer prepare the image--that you keep your black ink dot percentages in the high eighties to low nineties. At least this is how was when I was bangin' a few years back.

ernia

[segart]

Thanks, a lot of info there! And a very nice ICC profile for RGB.

I also like the DonRGB color space for saturated and bright colors (as a saturated RGB green for example).

http://www.hutchcolor.com/Don.html Some RGB color spaces in the free section.

[awrieger]

Yes, change from rgb to lab first before changing to cmyk. Here's a spectrum:

http://www.nec-display-solutions.co.uk/coremedia/generator/Internet/Subsidiaries/United_20Kingdom/Content/DynamicNavigationTeasers/Home/Home/Images/WideClolourPic2__en,property=ThumbNail.jpg

The roundish blob of colour is what is visible to the human eye. This is approximate to LAB colour.
The larger triangle is the Adobe RGB colour space.
The smaller triangle is the standard sRGB colour space most people have Photoshop set on.
The white area represents the area of colour that can be printed in cmyk.

What happens when you convert straight from sRGB to CMYK is that you clip off the blues and greens outside the left side of the sRGB triangle. It simply goes *poof!* and it's gone forever. It's why bright green grass and bright blue skies go muddy in cmyk. Ditto a bit of yellow and orange. CMYK can print these colours, but more often than not they are simply thrown away so there's nothing for CMYK to print. So convert to LAB first to keep this colour, and then to CMYK.

The Adobe colour space was obviously designed to cover the cmyk printable colours are. You can of course work in the Adobe RGB space (via Photoshop colour settings) which includes this area, but you can't see what you're doing because LCD monitors cannot show anything outside the sRGB space, which is why sRGB is the standard. Some CRT monitors can show the Adobe space, and so far as I know only two LCDs can show it too the EIZO ColorEdge CG220 24" and the NEC 2180UX WG (the WG stands for Wide Gamut, ie more colour, ie Adobe colour).

You can also work in 16 bit camera RAW format in the ProPhoto RGB colour space which is almost as full as LAB colour, as much as eye-visible colour, but that's something I'm not sure about. Photographers are the main people who use this.

NB. I'm not an expert at all. This is all just fresh in my mind from researching a new monitor for exact colour proofing and I'm leaning towards buying an EIZO 220 (and sell the house to pay for it) which has 99%+ colour accuracy and 99%+ uniformity across it's entire wide screen. Based on my own exhaustive research, it's simply the best monitor in the world for colour reproduction and proofing. And they price it accordingly. It uses a Hitachi LCD. Only one other company in the world, a small German outfit also specializing in ultra high-end visual production and colour proofing called Qato Graphics, uses the same screen in their monitor production.


CAVEAT: Perhaps I'm wrong about the colours being lost when you convert to CMYK. Perhaps that colour data is already discarded when you open the file in PS if you have PS set to sRGB. I'm not sure if PS retains that data, because it seems to work better converting to cmyk via LAB. As I said, I'm not an expert on how these colour spaces technically work, so perhaps someone more knowledgeable can explain it more accurately

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