(gimpprint.info.gz) Tuning Epson printers

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 Tuning the printer
 ==================
 
    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:
 
   1. The black near the center of the line is solid, but not more so
      than that.
 
   2. The cyan immediately to the left of the black is _almost_ solid.
 
   3. The dark cyan at the far right of the page is solid, but not more
      so.  You can try tuning the density so that it isn't quite solid,
      then nudging up the density until it is.
 
   4. Both sweeps sweep smoothly from light to dark.  In particular, the
      dark half of the bar shouldn't visibly change color; it should go
      smoothly from cyan to black.
 
    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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