RGB to CMYK conversion for printing

[Toonwonder]

Can someone give me some tips or point me to tips regarding preparing images for commercial printing? It seems that RGB(the only color format in LW) needs to be converted to CMYK. Big difference in the two formats! When a conversion is done is Photoshop, the brightness of the colors are lost. Any wisdom regarding how to color or texture a model in RGB would be helpful as well.

Many thanks

[jumbo]

I have spent a year working towards a degree in graphic design. The problem you are facing is quite common in the area I am studying. Since you view whatever kind of document you are working on in RGB, it's usually best to work in RGB and then convert to CMYK. Photoshop will tell you when the color you are selecting is "out of gamut." This should help with the texturing.

Looking at a textbook of mine, if you want to avoid many of these problems, stay away from bright greens and blues. This book shows the gamuts of the different models and shows where RGB and CMYK overlap. This book also shows the gamut of something called hexachrome printing. It is more expensive, since it uses two more colors, but it cover almost all RGB colors.

My last suggestion is to print it in both modes from your printer and see what you like best. Hope this helps some.

[Toonwonder]

Thak you Jumbo! Where does photoshop tell you that you are "out of gammut" ?

[onimusqa]

Hellos,

Click on the foreground/background color on Photoshop's tool bar. Run your mouse across the color bar and on the color pad.. You will notice that you're off gamut when there is an exclamation point logo on the top right. (Circled in blue). You notice that the out of gamut color in the pic is red. This red is beyond the CMYK color gamut and will not appear the same when printed. It will result in a much dull red than what you see on screen. The reason is.. ink pigments used in CMYK are limited in color variations. RGB is the color spectrum of bouncing light waves and therefore has a much wider color gamut. Anything beyond CMYK's gamut will be special mixed inks, like pantones, and metallic color. There are plenty of books about color theories at your local bookstores that are very helpful. Browsing through the first few chapters will probably answer all your questions.

I've worked as a prepress technician for 4 years. I'm not an expert on colors but I have basic understanding of it. You will encounter color shifts from RGB to CMYK depending on the colors used in your image. Images with heavy saturated colors in RGB will appear dull in CMYK. There is no fix for this but you can always adjust your colors in CMYK after the conversion to get the desired results. Keep in mind that there is absolutely no way you can replicate the out of gamut colors on screen to appear in print. You might also want to check out Photoshop color correction books. Plenty of them out there.

[onimusqa]

Oh brother... I forgot to post picture.. here it is!

http://www.members.aol.com/nohnstt/colorpicker.jpg

[Broncos30]

RGB is a monitor scheme where CMYC is a printer scheme. No matter how you correct the RGB after converting it to CMYK it will be different when pulled up by a commercial printer. The commercial printer will have to make corrections. The best thing to do is to correct the colors suggested by previous posts and print a satisfactory example and show that to the printer. They will take that image and try to match it as best as they can. This is one of the problems when tring to convert something designed for the screen to be printed. When creating your 3d scenes keep in mind that they will have to be converted to CMYK and pick appropriate RGB channels that will convert.

[Toonwonder]

Wonderful Information! I also found out that there are large RGB printers out there in the commercial industry. I'm trying one of those this week with a project I have. I'm told their prices are a little more expensive. I was also asked to provide artwork at 300 dpi as well. I hope it turns out. I'll let you know.

[danilo]

Also,if you press comand/shift Y, all areas that will change when converting to CMYK will turn to gray.You can change gray to something else in preferences-Transparency&Gamut.
danilo

[gerardstrada]

KeyFrame Magazine (May-June I think) has a good article about this, written by Brad Krause. The idea is to make an appropriate conversion using the Custom CMYK panel (Edit/Color Settings) and to play with Black Generation. Besides you should set well the CMYK working space , I notice it also influences a lot the conversion process.



Gerardo

[Stephane]

Great question... some answers

First you need to "calibrate" your monitor. Set the color to 5000K. PC default factory settings are often 7500K or 9300K, 6500K on Macintosh. It's too high, too "cold". With 5000K, you're going to have a "warm" white (a little bit yellow). It's near from paper feeling. Note that is a recommandation of most print industry actors like Adobe (see Photoshop HTML help). Then you need to build an ICM profile for your monitor with a tool like Adobe Gamma (easy to setup with the built-in assistant). An ICM profile tell how a device (monitor, scanner, printer) render a color. Most of printers can't render some RGB values, so the ICM profile give that information to Photoshop with high precision when you convert or preview an image from RGB to CMYK. Cause of great render time in printed images, it's really important to be able to preview with precision. Try to find the specifications of your monitor (especially the X,Y values for each RGB channel). If you're not sure, enter a generic value (ie Trinitron). Finally set the the gamma to 1.8 (that's the natural gamma of MacOS - Windows default is 2.2 - and that's why graphics seems to be more contrasted on a PC). Perhaps you're going to think that your Lightwave is "stoned washed". But once again it's to be near from the paper look. Note that the contrast ratio of a classic monitor is at least 300:1 (to be able to represent the 256 levels of each RGB channel). "For the paper" it's only 100:1 (note that you generally set your CMYK values from 0 to 100%). So a wide range of well calibrated monitors can normally preview a printed job.

Unfortunately, there is no absolute way to anticipate the result when you convert an image from RGB to CMYK in Photoshop. A cool gradient of lighting glow can be transformed in some dirty stripes. So, as recommended in a post, keep your image in RGB to correct your colors (and keep a wide range of initial values). Preview in CMYK to avoid out of gamut (don't forget that differents ICM monitor profiles give you differents results in Photoshop when you convert from RGB to CMYK). Some professionals prefer to convert first in Lab mode (the largest color spectrum). Note there's a plugin to import FLX format in Photoshop and so working with 16bit render files.

Only experience can make the difference. You must have some CMYK color references in mind. For a nice blue sky C100 M70 Y0 K10, no more than 70 for magenta or the sky become violet. For a nice yellow C0 M15 Y100 K0, no cyan in your 3D duck or the yellow turn a little bit green. You can find samples of printed mixed CMYK colors to help you.

Note that is often necessary to export an alpha channel for each surface of your object in LW and work with a layer for each surface in Photoshop. Do it for surfs with problems. The color correction is easier like this (cause of great render time and unpredictable results if you change a light or a surf value). Try to print samples of your work as often as needed and before high def. render. It's better, if your customer can validate one or two states of the project (a rough, then the colors and the poses of the subject). It's better than to do it again from the beginning... for tomorrow!

[Toonwonder]

Wow! Stephanie, thank you! There is alot to consider. Apparently, there isn't any easy solution. I guess I'll just have to bite the bullet and do the work.

I called the RGB printer again to make sure It would work. His response was that he would adjust the printer to match my image as best as he could.

[etobiason]

This is a simple option to consider for CMYK conversions:

Take your RGB render, and instead of doing Image:Mode-->CMYK, select Image: Mode-->Convert to Profile...

Choose U.S. Web Coated (SWOP) v2 as your destination space, leave Engine and Intent as default (I think Apple Colorsync and Relative Colorimetric, respectively). Select all radio boxes except possibly "Flatten Image" (that's up to you, if you wanna flatten or if it already is).

It will preview the conversion for you, but remember, you're looking at a monitor and it should print well on paper.

That usually works for us over here.

-e

[Draven]

It would be rather nice if there was an overlay for LW's image editor to show us when something ends up out of gamut on a test render...