Stochastic Screening - Color Correction

In my last column I presented an overview of Calamus SL. There is one ability of Calamus which remains obscure. This aspect of the program is color correction and the related topic of raster screening. My next two columns will address these questions. First a little digression about stochastic screening. I do this because I love the Calamus implementation and discussing its module (Star Screening) will lead into color correction. Now down to some brass tacks.

One of my favorite Calamus modules is the Star Screening module. Star Screening Lite comes with the standard Calamus package, in other words, free. This can be updated to Star Screening Pro. Well, don't bother upgrading. Everything the Pro version can do can also be done with the Lite version, although image conversion takes a few more steps. Now after this loaded introduction let me explain what all this is about.

There are basically three ways or forms which can be used for printing. Raster screening, dithered printing and stochastic screening. The first has been around for quite a while and is the standard method used by printing presses. Dithered screening, or as I call it, "Poor man's" stochastic screening, was relatively rare. Its use is now basically limited to the personal printers owned by computer users. The same is true of stochastic screening. Both dithered and stochastic screening produce better results on the home printer than rastered screening.

Rastered screening is the creation of "raster points" meaning the individual dots a printer is capable of printing are grouped together to form raster points. These points are combinations of the various printing colors, which fool the eye into seeing the full spectrum of color. The pattern created is easily photographed by a pre-press machine and transferred through a number of steps to a press. This is done because there are limitations to the transfer technology and the larger raster points are much easier to photograph than the individual dots themselves. So for example a line screen of 100 means there is a capability of squeezing in 100 of these raster points per horizontal inch. This should not be confused with the term "DPI". DPI (Dots Per Inch) refers to the capability of the printer. While a 2,400 DPI Lino-type machine can print out many more dots than a 720 DPI printer, both can print out a 100 LPI photograph. But the Lino can cram many more dots into each raster point than your handy home Epson.

Dithered printing or ordered dither is simply the conversion of the printed target (photograph, work of art...) into individual dots which match the printer's capabilities. It is often the default method which programs like Photoshop use. With some printers, because of the printer driver software, it is the only method you can print with. Dithered printing is a straight conversion. Your photograph which consists of individual pixels of color (24-bit printing means the capability of 16,777,216 colors) is converted mechanically into the four (or six) printing colors).

Stochastic screening (also called FM (Frequency Modulation) screening) takes dithering a step further. Instead of simply converting the underlying colors to CYMK (Cyan, Yellow, Magenta and blacK), stochastic screening uses an intelligent algorithm to make this conversion. Your 24-bit photograph consists of pixels, areas of colors, which roughly correspond to the DPI of the image. Thus the software examines the first pixel of your image and makes decisions about which color dots to apply. It then makes the next decision keeping its previous choice in mind. The resulting image has certain advantages and disadvantages over conventional screening (rastered) and dithered screening.

On low resolution printers (anything under 2,400 DPI) the advantages are obvious to the eye. Much sharper images result, much more detail is evident in the image. Another advantage is the moiré patterns sometimes created by raster screening is not a problem. Moiré patterns result when an image is not properly sized to create the raster pattern. A dithered or stochastically screened image can be of any size, even dramatically distorted, without any hint of a moiré pattern. There are also a number of disadvantages. One is that extremely light areas of the image may appear grainy but the overall effect is one of extreme clarity and detail. This is not a problem with either raster or dithered screening. Another disadvantage is that once an image is converted it can no longer be resized or manipulated. It becomes fixed at the size you've screened it.

Aside from the above neither dithered or stochastically screened images can be sent to a press. They are both good for individual prints. The dots are too compact to be photographed for a conventional press, though this might change as technology advances. Finally, the process of creating stochastically screened images is time-consuming, unlike either of the other two methods. Still when one compares print-outs where all else is the same, the stochastically screened image appears sharper, sometimes dramatically so compared to raster screening and better than dithered screening as well. Take a look at the three photographs below for examples of all three screening methods. Notice the detail or lack of same in the crops of these images.

With Calamus, stochastic screening goes under the name of Star Screening. Other companies use different algorithms and use different trade names, "Crystal Screening" et al. The design used by Calamus is excellent in its results and its big advantage over other software is that with Calamus you can Star Screen one image without affecting any other aspect of the page. The Star Screening module (once again not to be confused with dithering) is far cheaper than any other source of this technology. That goes for even the Pro version. In some cases stochastic screening on other platforms is a hardware solution. Some companies sell dithering as stochastic screening which is really a pointless exercise, since most high-end programs have this as an option. The basic Calamus package cannot convert images to dithered format. This lack is meaningless since Calamus can set dithering as the screening method and your image will print out as a dithered photograph. Or the optional filters module contains a filter to convert images to dithered format. These last questions are unimportant.

Two last points in this introduction. Star Screening is memory-intensive. To Star Screen a 16x20, 24-bit color image requires something in the order of 200 MB of RAM! And it's slow. For example, on a Hades 060 you can expect this task to take 25 to 30 minutes. This assumes you have that kind of memory. It will take longer if using the virtual memory option. On a G3 Mac at 400 MHz it takes 12 to 15 minutes, once again assuming you have the RAM (for a black and white image divide by four). I like the results so much that this is my preferred method of screening for individual prints, but as the above indicates it takes time. Naturally, smaller images can be screened quite a bit faster. I should also add that laser copiers actually do a pretty good job of copying these print-outs so already there are some commercial possibilities.

How are images prepared? RGB to CYMK conversion
Now to tackle the most difficult aspect of desktop publishing and the only area of the Calamus manual which is not up to par. I'm going to be tossing around the expression, "color correction." The normal assumption, when people hear this term, is going to your color look-up table (CLUT) and increasing the contrast, or increasing blue until your picture looks right. This is not what I am talking about. Color correction is the conversion of RGB to CYMK for the actual printing process. It does not mean correcting an image so it looks good on your screen, but rather what your photograph looks like when it prints to hard copy.

Calamus has no automatic way to create corrected color prints. Photoshop, for example, has a number of methods for color correcting your prints. On the other hand Calamus has extensive methods of manually correcting your prints while programs like Photoshop have almost none. In Photoshop you select between a small number of options and you are happy. You are happy because color correction is a very difficult thing to accomplish and here someone is reaching out of the computer monitor to hold your hand. Quite a few printer drivers available on Mac and Windows machines also have a number of choices for creating good output. There are even a number of slider bars for correcting color tone and depth. These methods work fairly well and eventually you will get prints that roughly correspond to what you want. Do you care that your deep blue sky has too much cyan? Who's going to know the difference as long as it looks good? Calamus provides only manual controls but these in turn can lead to an exact reproduction of what you want or for that matter the ability to add a color cast to the exact degree you want. This month's article is going to concentrate on color correction so consider this half of what you need to know and it's the important half. My next article will deal with raster screening.

Color correction
On-screen images are presented to the viewer as RGB (Red, Green, Blue). This is the way a monitor creates the spectrum of color. This is done by light passing through the image. No RGB color and your monitor looks black. Your common ink-jet printer (or press) uses CYMK. We actually see color on paper as light being reflected from the image. No CYMK and we see white. When you do the physical action of hitting the print command you are also giving a command to make this conversion. At this time in technological history there is no absolutely fool-proof method for doing this. This is because while it would seem that a formula could be concocted, either by engineers in a lab or by witches screaming incantations over a cauldron, the fact is no method works with all images. Is the yellow you see on the screen a patina or a concentrated glop? Cyan and magenta make red. What is the intensity of the red? And so on. There's another problem. Green is the product of mixing yellow and cyan. But dark green has black added to the spectrum. While theoretical mixing CYM would produce black, actually it produces a very deep brown.

The printing trade basically uses two forms of conversion. There are more, but these two are the basics you really should know. If you ever wander down to a service bureau, filled with ignorant people anxious and willing to totally screw up your job it pays to know how to get them to do what you want. We don't need their help, we can screw it up on our own. UCR (Under Color Removal) also called Chromatic Separation and GCR (Gray Component Replacement) also called Achromatic Separation. These are the two basic methods. Both have unlimited adjustment capability.

Before I go any further let me say this is all not that difficult! It only sounds that way because we are learning a new language, but hey there are only a limited number of words to learn!

Essentially when you give that print command you are creating four color plates which your printer then methodically prints, one on top of the other. With UCR we are going to make some of that black plate with a combination of CYM. With GCR we are going to be using black for this black plate. Now the weaker the under color removal, meaning the weaker the use of CYM to make black UCR fades into GCR. Although these methods (UCR and GCR) are referred to separately in the literature, this is a semantic convenience although the differences are real. If you are using a strong UCR method you will also be using a lot of ink. This is because black in such a print-out is a combination of CYM. Everywhere in the print-out three colors are being printed on top of each other to create black. So such a method would be inadvisable for a newspaper. This paper cannot handle this amount of ink. Also, the more ink you use the greater the problems of "trapping." Trapping is the effect of ink being printed one color on top of another. With a printing press this can sometimes be a real problem. If you are doing color prints of paintings in a high quality book, with high quality paper, UCR will work just fine. It would be inadvisable to remove all the black, but in a strong UCR print true black is used only to create contrast. Flesh tones and areas of light color might very well look better with a UCR print. At the same time areas of light gray, like stone or concrete, will look better with a GCR method. This is because you are using real black to form these areas of gray. How is all this implemented in Calamus.

The UCR dialog
The dialog for creating these CYMK separations is referred to as the UCR dialog. It could just as well be called the GCR dialog, but never mind, here is where the separations are done. Let us pause for the rest of this article. Let us dispense with these cruel and unnatural terms. Let us say that inside Calamus is a dialog in which we can make smooth transformations of our photographs on the monitor into smooth and beautiful prints on our paper. To start with this relatively easy (once you know it) process is made more difficult by one big screw-up. The dialog, which resembles the CLUT controls, works in the opposite manner. By this I mean raising the line decreases the intensity instead of increasing it. Keep this in mind or you will eventually wind up homeless and insane, picking up used soda bottles for the five cent deposits.

Tests
We are faced with seven control lines, conveniently of different colors. They represent the four linearity (intensity) colors of CYMK. With these four lines you can increase or decrease the amount of the respective color, just as you do in the CLUT controls (but backwards, remember backwards). The next three lines are the black from cyan, yellow and magenta respectively. By this I mean the amount of these three colors that are being used to create black. These three lines determine how much of the ink from CYM is going to be used, not to produce the color itself, but to produce black. Green, as an example, is produced by mixing cyan and yellow. What about dark green? With dark green we have the addition of black. It can be true black or it can be CYM black. Either way you need black.

Now load in a color photograph. Go to the raster screening module while in the printer dialog and choose ordered dither. Now print it out. Looks terrible. While I don't have your photograph in hand my prediction is a good one. From the printer dialog go to the color separation dialog (you can access these dialogs from the page module but this is more convenient). You will notice all the lines are in a neutral position. There is no CYM black being printed.
 

This leads us to our first step in creating a control line for your printer. Create or steal a greyscale fountain. You can steal it from the example image in this article. Have this ready as a file. Load it into Calamus, go into the color separation dialog and delete the black linearity line. Yes, you move it all the way to the top. When you try to print, nothing will print. In the neutral position these lines eliminate all CYM black and you just deleted true black - nothing will print! To create color separation lines to match our printer all we have to do is print out a grayscale fountain using just CYM! As you near perfection, and like trying to get up to light speed, it never quite gels, add true black. You are done. Your photographs will print out astoundingly beautiful!

A few tricks
Your monitor is not calibrated. There are too many variables to create a perfectly calibrated monitor. Invers Software sells a module to do just that. Unfortunately this is one of the few modules I don't own and the demo version just demonstrates how the module works. One of the things I did was after embedding my UCR control lines in an image I used the built-in conversion options in Calamus to convert RGB to CYMK. This will not show an exact reproduction of the print-out but it helps. Make a copy of your fountain and after embedding your control lines - convert to CYMK. You might then want to print out the result and try to move your monitor adjustments to closer match the print-out.

Franklin
The full version of the curve editing module, Franklin, is now standard in Calamus. Franklin can be used in any aspect of Calamus where control lines are being edited or adjusted. Furthermore all the created lines can be saved in Franklin's own internal format, CKD. So when you are editing in the UCR dialog, the UCR format is CK7. Your test files should also be saved in the CKD format so that changes can be made without losing the ability to modify these lines. So, when you've finished editing save as a "test" CKD file and export as a CK7 file. The CK7 file is the color separation format and CKD is Franklin's internal format where Bézier control points are stored.

A few warnings
If you are moving cyan out of black you are creating extra cyan. Remember this cyan was being used to make black, now it's just hanging around mucking up the works. So as you move cyan out of black you must also adjust the cyan linearity line to decrease the amount of cyan. The same goes for the other colors. Make sure to save all these lines you are creating! Create temporary folders and keep track of the results. Once you are getting near the results you want make notes of whether the image is perfect but with a slight color cast. You might want to keep these less than perfect lines for when you want a color cast. For example I keep about four of them around. One is my "perfect" line the rest tint my subject very, very slightly in the direction of cyan, magenta and yellow respectively. This is in case a print-out needs more yellow despite using my perfect line or for that matter, if I want to provide a yellow cast to the image. The other bad news is you'll need different versions of these lines for the three methods of printing. The good news is they don't differ by too much. You can even avoid a separate line for stochastic screening. You can simply reduce the intensity of the target photograph and use the lines you created for raster prints. This little trick will not work with ordered dither but the raster control lines form a good starting point.

Finally we come back to stochastic screening. There is so much ink being laid onto the page by using this method that one further step is necessary. In this case you can make separate lines from the UCR dialog for stochastically screened images or you can use the real CLUT controls to lighten the image. I prefer making my UCR lines, but either method will work.

This is a two-part article. As I said the next part will deal with rastered screening. If an image is not color corrected the result of printing will look awful. Much of the ink being used will not be set down in the right place. Users of Calamus who always got good results with black and white and grayscale images suddenly find they can't print out the simplest photograph. I will discuss this question in much greater depth but you will find that once an image is color corrected the default screen provided by Calamus and quite a few others will work flawlessly and your results will thrill you.

The above images represent an experiment. I tried scanning my print-outs but the scanner picked up too much of the relatively large dots produced by my printer. I've just purchased a new digital camera, the Nikon D1X, and this machine did an incredible job. This camera has a resolution of 3,000x2,000 and by photographing the images as opposed to scanning them, the results were pretty smooth. All the images were first prints - test prints if you will. Both Calamus and Photoshop will allow for further correction. But I do believe that once mastered (that's the tricky part, mastering the process) Calamus is actually easier to use in terms of correcting images.

chatobarkin@myatari.net
 

[ Top of page ]

MyAtari magazine - Feature #5, May 2002