Now, how do you use all this to tune a printer? There are a number of ways to do it; this one is my personal favorite.
There's a file named `cyan-sweep.tif'. This consists of a thin bar
of cyan sweeping from white to almost pure cyan, and from pure cyan to
black. The first thing to do is to pick the appropriate
simple_dither_range_t (or create a whole new
escp2_variable_inklist_t) and comment out all but the darkest ink
(this means you'll be using the largest dots of dark ink). At 8.5"
width (the width of a letter-size piece of paper), the bar will be 1/8"
high. Printing it on wider or narrower paper will change the height
accordingly. Print it width-wise across a piece of photo quality paper
in line art mode using ordered or adaptive hybrid dither. Do not use
photographic mode; the colors in photographic mode vary non-linearly
depending upon the presence of the three color components, while in line
art mode the colors are much purer. Make sure that all the color
adjustments are set to defaults (1.0). Use the highest quality version
of the print mode you're testing to reduce banding and other artifacts.
This is much easier to do with the Gimp than with Ghostscript.
At this stage, you want to look for four things:
Repeat this stage until you have everything just right. Use the positioning entry boxes in the dialog to position each bar exactly 1/8" further down the page. Adjacent bars will be touching.
The next step is to uncomment out the second darkest dot size. If you're using variable dots, use the second largest dot size of the dark ink rather than the largest dot size of the light ink. This will give you two inks.
When you recompile the plugin, you simply need to copy the new executable into the correct place. You do not need to exit and restart the Gimp.
Print another bar adjacent to the first one. Your goal is to match the bar using a single dot size as closely as possible. You'll find that the dark region of the bar shouldn't change to any great degree, but the light half probably will. If the lighter part of the light half is too dark, you need to increase the value of the smaller dot; if it's too light, you need to decrease the value. The reasoning is that if the value is too low, the ink isn't being given enough credit for its contribution to the darkness of the ink, and vice versa. Repeat until you have a good match. Make sure you let the ink dry fully, which will take a few minutes. Wet ink will look too dark. Don't look at the paper too closely; hold it at a distance. The extra graininess of the largest dot size will probably make it look lighter than it should; if you hold it far enough away so that you can't see the dots, you'll get a more accurate picture of what's going on.
After you have what looks like a good match, print another bar using only the largest dot size (or dark ink, for single dot size 6-color printers). You want to ensure that the bars touching each other look identical, or as close as possible to it; your eye won't give you a good reading if the bars are separated from each other. You'll probably have to repeat the procedure.
The next step is to comment out all but the largest and third-largest dot size, and repeat the procedure. When they match, use all three dot sizes of dark ink. Again, the goal is to match the single dot size.
You'll probably find the match is imperfect. Now you have to figure out what region isn't right, which takes some experimentation. Even small adjustments can make a noticeable difference in what you see. At this stage, it's very important to hold the page far enough from your eye; when you use all three dot sizes, the texture will be much more even, which sometimes makes it look darker and sometimes lighter.
After this is calibrated, it's time to calibrate the light ink against the dark ink. To do this, comment out all but the large dot version of the two inks, and repeat the procedure. This is trickier, because the hues of the inks might not be quite identical. Look at the dark half of the bar as well as the light half to see that the hue really doesn't change as you sweep from cyan to black. Sometimes it's easier to judge that way. You may find that it looks blotchy, in which case you should switch from ordered dither to adaptive hybrid.
After you have the light and dark inks calibrated against each other, it's time to add everything back in. Usually you don't want to use the largest dot size of light ink. These dots will be much larger than the small dots of dark ink, but they'll still be lighter. This will cause problems when printing mixed colors, since you'll be depositing more ink on lighter regions of the page, and you'll probably get strange color casts that you can't get rid of in neutral tones. I normally use only the smallest one or two dot sizes of light ink.
After you've tweaked everything, print the color bar with saturation set to zero. This will print neutral tones using color inks. Your goal here is to look for neutral tonality. If you're using a 6-color printer and get a yellow cast, it means that the values for your light inks are too high (remember, that means they're getting too much credit, so you're not depositing enough cyan and magenta ink, and the yellow dominates). If you get a bluish or bluish-purple cast, your light inks are too low (you're not giving them enough credit, so too much cyan and magenta is deposited, which overwhelms the yellow). Make sure you do this on very white, very high grade inkjet paper that's designed for 1440x720 dpi or higher; otherwise the ink will spread on contact and you'll get values that aren't really true for high grade paper. You can, of course, calibrate for low grade paper if that's what you're going to use, but that shouldn't be put into the distribution.
You can also fully desaturate this bar inside the Gimp and print it as monochrome (don't print the cyan as monochrome; the driver does funny things with luminance), for comparison. You'll find it very hard to get rid of all color casts.
There are other ways of tuning printers, but this one works pretty well for me.
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