PPA Printer calibration for pnm2ppa

The pnm2ppa project team ppa-rpms@users.sourceforge.net

Overview of Printer Calibration.

There are three tasks you may need to carry out to fine-tune your PPA printer:

• Adjust the x-offset and y-offset to correctly center the printed page on the paper
• Adjust the relative offsets between black and colored ink printing - this should be done each time you replace an ink cartridge.
• Match the colors of printed images to some other output device - this is color correction and is described in COLOR.html (or COLOR.txt).

These instructions assume that you are already able to print postscript files with your PPA printer. If your print filters are properly configured, something like "lpr test.ps" will successfully invoke ghostscript and pnm2ppa to print the postscript file test.ps. Alternatively, you may be using shell scripts like "lprbw", "lprcolor" and "lpreco" supplied with the pnm2ppa source code, and modified by you for your defaults; you may also have defined special printing devices in /etc/printcap, e.g. a "coloreco" device so you print with lpr -P coloreco test.ps. These details vary with the operating system and distribution you use.

(1) Centering the printed page on the paper.

This is done by adjusting the x-offset and y-offset parameters xoff and yoff in the pnm2ppa configuration file (usually /etc/pnm2ppa.conf). The parameters are given in units of 1/600 inch; increasing xoff moves the image to the right, and increasing yoff moves it downwards. These offsets now have default values defined for each printer model, and you will probably not need to adjust them. Uncomment and make small adjustments to the default values for these in the configuration file, until the printout is satifactory. (You can also edit the defaults.h file and recompile pnm2ppa, or use the '-x' and '-y' command-line parameters.)

There are two ways to check the offsets.

• The easy way: printing a suitable test page.
• An older way: using the "calibrate" program.

On Red Hat or Mandrake Linux distributions, a suitable test page is /usr/lib/rhs/rhs-printfilters/testpage.ps (US Letter paper) or /usr/lib/rhs/rhs-printfilters/testpage-a4.ps (A4 paper). (If these are not available, similar test pages are supplied in the tarball distribution pnm2ppa-1.0.tar.gz). These pages have margins drawn at standard distances from the edges of the paper, and can easily be used to check the centering.

The "calibrate" program is a stand-alone program distributed with pnm2ppa which creates a calibrated grid pattern in the raw pixmap format accepted by pnm2ppa. (The three paper sizes, US Letter, US Legal, and A4, are supported). If your paper size is A4, and your printer filter accepts the "-l" (direct output) option to lpr, the command to run is: calibrate -a4 | pnm2ppa -i - -o - | lpr -l. (Substitute "letter" or "legal" for "a4" to change the papersize; if no paper size is specified, it is assumed to be letter.) If "lpr -l" is not valid, you need to know which port your printer is attached to. Assuming it is /dev/lp0, the above command becomes calibrate | pnm2ppa -i - -o /dev/lp0. The numerical values marking grid intersections are pixel coordinates. Unfortunately, these coordinates are probably cut off before the edge of the paper. You'll have to use a ruler to estimate the pixel coordinate of the left and top edges of the actual sheet of paper (should be within +/- 300, may be negative, there are 600 pixels per inch). When properly calibrated, the center mark should be in the center of the paper.

(2) Calibrating the X and Y Color Offsets.

The color ink offsets now have to be calibrated with respect to the black ink. Because the relative position of the two ink cartridges can move slightly, this must be done whenever an ink cartridge is removed and replaced.

The calibration can be done by printing the test page test.ps (found in the tarball pnm2ppa-1.0.tar.gz; or installed with the pnm2ppa documentation. (This is often in /usr/doc/pnm2ppa*/.) The settings to change in the configuration file are ColOffsX and ColOffsY, again in units of 1/600 inch.

On the printout you see several images:

• top left, you see a square with both black and colored items. These should align correctly, or you have to change your settings.
• You see a large vertical bar, and also a horizontal bar. On the sides of the vertical bar there are a blue and a green line. These should align with the 0 mark.
• You also see a small vertical, and horizontal, bar, with a red bar on its side.

If these items do NOT line up, you should take the following steps:

• (1) take for the large bar the number of the tickmark just smaller than the place where the green and blue lines point to. The minor tick marks are 25 points.
• (2) To this number, add the number of the tick mark at which a red tick mark exactly lines up. This might not work if the offsets are way off; in that case, apply the correction of step 1, and print again.
• (3) Add the horizontal offset to the value of ColOffsX, and the vertical offset to the value of ColOffsY in the configuration file.

Now you can print test.ps again to see whether this procedure worked.

Other adjustments

Many of these are documented in the sample configuration file.

If bidirectional printing causes "shearing" of vertical lines (horizontal offsets of those parts of the line printed by left-to-right print-head sweeps relative to those printed on right-to-left sweeps) you can make small adjustments (in 1/600 inch) units using the blackshear and colorshear parameters in the configuration file. You can also suppress bidirectional printing with the unimode=1 parameter.

The parameter blackness adjusts the number of black ink drops printed per pixel. It takes values 1, 2, 3, or 4 (2 is the default). Other values, like 0, suppress black ink.