Printer calibration

The printer calibration dialog was broken for long. In fact it was broken for longer than pcb-rnd even existed: the version of gEDA/PCB it was forked from already had a partly defunct calibration. (The bug is that after the calibration the print dialog does not remember any of the calibrated values).

The whole idea of printer calibration embedded in pcb-rnd is probably not that good. The lpr plugin is hard-wired to the ps export. The ps file format does have units, so the only reason anyone would need a calibration is working around printer bugs or paper shrink problems. But those problems are not related to pcb-rnd, thus the calibration should probably be done only once, independently of the software used.

Thus the calibration dialog box and code got removed from pcb-rnd. A new, optional config subtree is added for export_lpr where the user can save calibration values so that the lpr exporter remembers them. These preconfigured calibration values are used upon any print from pcb-rnd, if command line arguments or GUI export dialogs don't override them.

The recommended manual process for a printer calibration is:

• 1. draw a rectangle in pcb-rnd that is large enough for your printer and paper size; preferably use round sizes, e.g. on an A/4 paper, assuming portrait printing: 150*250 mm (this leaves enough margin); use thin lines
• 2. print it with pcb-rnd
• 3. measure width and height and calculate the calibration value per x (horizontal) and y (vertical) direction; the calibration value is: measured_size / drawn_size; the result should be close to 1.0.
• 4. final check: print again, but this time enter the calculated calibration values in the print dialog box xcalib and ycalib fields; the resulting printout should be very close to the exact size
• 5. save the calibration values in user config, either using the preferences window tree tab and looking for plugins/export_lpr or by writing a config file with a text editor (see below)

Hand written config file: create a file with a text editor, call it ~/.pcb-rnd/lpr_printer_calib.conf and copy the following content in it:

li:pcb-rnd-conf-v1 {
	ha:overwrite {
		ha:plugins {
			ha:export_lpr {
				default_xcalib = 1.0
				default_ycalib = 1.0
			}
		}
	}
}

Modify the values of default_xcalib and default_ycalib to the ones calculated during the above calibration process.