Index: trunk/doc/user/07_io/3_1_export_openscad/index.html
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--- trunk/doc/user/07_io/3_1_export_openscad/index.html	(revision 37249)
+++ trunk/doc/user/07_io/3_1_export_openscad/index.html	(revision 37250)
@@ -29,6 +29,7 @@
 	<li> subcircuit (3d) models (optional)
 </ul>
 
+<h4> 7.2.1.1 Subcircuit 3d models </h4>
 <p>
 Subcircuit models are loaded from external files referenced from the subcircuit
 in the pcb-rnd board file. Model files are searched recursively under the
@@ -61,3 +62,43 @@
 module name in the same line, { in the following line. One instance of
 each model is copied into the output file so that the resulting openscad
 script has no external reference and is self-contained.
+
+<h4> 7.2.1.2 no export objects </h4>
+<p>
+If an object shall not be exported to openscad, add the attribute
+<i>noexport:openscad</i> to it with value <i>1</i>. This will omit
+the object only from the openscad export. Any drawing object can be omitted.
+
+<h4> 7.2.1.3 namespace </h4>
+<p>
+By default all exported module names are prefixed by pcb_. When the openscad
+scripts of multiple boards are included in a complex project this would cause
+conflicts because of matching module names. This can be fixed by specifying
+a unique prefix on export using the prefix export option for each board.
+
+<h4> 7.2.1.4 board thickness </h4>
+<p>
+The openscad exporter respects standard layer group thickness attributes when
+calculating board body thickness: it sums all insulating layers between
+top and bottom copper. Copper/silk/mask layer thickness is not respected
+because they would normally be too thin for reasonable openscad rendering.
+<p>
+If board thickness can not be determined, a fallback default value of 1.6mm
+is used.
+
+<h4> 7.2.1.5 board origin </h4>
+<p>
+When exporing a board that would be part of a bigger 3d model it is important
+to be able to position the board properly in openscad. Normally the origin
+(0;0 coordinate of the openscad board model) will be the bottom left corner of
+the board's bounding box (when the board is looked down at from the top).
+<p>
+In many situations the board could be better positioned by an object, typically
+by a padstack (hole). This can be done by taking a drawing object and
+adding the <i>openscad-origin</i> attribute to it with the value of <i>1</i>.
+When such an object is present on the board, the center point of the object
+is used as the openscad origin and the board model is translated so that
+the center point of the object is at 0;0.
+<p>
+If there are multiple openscad-origin objects present on the board, one is
+picked randomly.