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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.