attachments fixes/tweaks

tutorials/Attachments.md

@@ -681,6 +681,55 @@ prismoid(150,60,100)
     show_anchors(s=45);
 ```
 
+Is is also possible to attach to edges and corners of the parent
+object.  The anchors for edges spin the child so its BACK direction is
+aligned with the edge.  If the edge belongs to a top or bottom
+horizontal face, then the BACK directions will point clockwise around
+the face, as seen from outside the shape.  (This is the same direction
+required for construction of valid faces in OpenSCAD.)  Otherwise, the
+BACK direction will point upwards.
+
+Examine the red flags below, where only edge anchors appear on a
+prismoid.  The top face shows the red flags pointing clockwise.
+The sloped side edges point along the edges, generally upward, and
+the bottom ones appear to point counter-clockwise, but if we viewed
+the shape from the bottom they would also appear clockwise.  
+
+```openscad-3D;Big
+include <BOSL2/std.scad>
+prismoid([100,175],[55,88], h=55)
+  for(i=[-1:1], j=[-1:1], k=[-1:1])
+    let(anchor=[i,j,k])
+       if (sum(v_abs(anchor))==2)
+         attach(anchor,BOT)anchor_arrow(40);
+```
+
+In this example cylinders sink half-way into the top edges of the
+prismoid:
+
+```openscad-3D;Big
+include <BOSL2/std.scad>
+$fn=16;
+r=6;
+prismoid([100,175],[55,88], h=55){
+   attach([TOP+RIGHT,TOP+LEFT],LEFT,overlap=r/2) cyl(r=r,l=88+2*r,rounding=r);
+   attach([TOP+FWD,TOP+BACK],LEFT,overlap=r/2) cyl(r=r,l=55+2*r, rounding=r);   
+}
+```
+
+This type of edge attachment is useful for attaching 3d edge masks to
+edges:
+
+```openscad-3D;Big
+include <BOSL2/std.scad>
+$fn=32;
+diff()
+cuboid(75)
+   attach([FRONT+LEFT, FRONT+RIGHT, BACK+LEFT, BACK+RIGHT],
+          FWD+LEFT,inside=true)
+     rounding_edge_mask(l=76, r1=8,r2=28);
+```
+
 ## Parent-Child Anchor Attachment (Double Argument Attachment)
 
 The `attach()` module has two different modes of operation,