Index: trunk/doc/developer/hid_render_comp.html
===================================================================
--- trunk/doc/developer/hid_render_comp.html	(revision 11046)
+++ trunk/doc/developer/hid_render_comp.html	(revision 11047)
@@ -79,5 +79,18 @@
 A GC can be initialized only after <i>render_burst</i> / <i>start</i>
 and all GCs will be uninitialized before <i>render_burst</i> / <i>end</i>.
 
+<H2> colors </H2>
+<p>
+In negative drawing mode, drawing an object always clear on the current canvas,
+regardless of the GC color.
+<p>
+In positive drawing mode, objects are drawn with whatever color the
+current GC has. The special color "drill" is used to draw holes that go thru
+all layers of the board. It is possible that a drill is drawn over existing
+objects and new objects are drawn on top of a drilled hole.
+<p>
+(Note: there's no "erase" color anymore.)
+
+
 </body>
 </html>