add position $align

attachments.scad

@@ -477,22 +477,57 @@ _ANCHOR_TYPES = ["intersect","hull"];
 //   `$attach_anchor` for each `from=` anchor given, this is set to the `[ANCHOR, POSITION, ORIENT, SPIN]` information for that anchor.
 //   `$attach_to` is set to `undef`.
 //   `$attach_norot` is set to `true`.
+//   `$align` set to the anchor that will position a child flush to the edge of the parent at specified position.  
 // Example:
 //   spheroid(d=20) {
 //       position(TOP) cyl(l=10, d1=10, d2=5, anchor=BOTTOM);
 //       position(RIGHT) cyl(l=10, d1=10, d2=5, anchor=BOTTOM);
 //       position(FRONT) cyl(l=10, d1=10, d2=5, anchor=BOTTOM);
 //   }
+// Example: Child would require anchor of RIGHT+FRONT+BOT if given explicitly.  
+//   cuboid([50,40,15])
+//     position(RIGHT+FRONT+TOP)
+//       color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+// Example: Child requires a different anchor for each position, so explicit specification of the anchor is impossible in this case.
+//   cuboid([50,40,15])
+//     position([RIGHT+TOP,LEFT+TOP])
+//       color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+// Example: If you try to spin your child, the spin happens after the position anchor, so the child will not be flush:
+//   cuboid([50,40,15])
+//     position([RIGHT+TOP])
+//       color("lightblue")prismoid([10,5],[7,4],height=4,
+//                                  anchor=$align, spin=90);
+// Example: You can instead spin the attached children using {{orient()}}.  In this example, the required anchor is BOT+FWD, which is less obvious.
+//   cuboid([50,40,15])
+//     position(RIGHT+TOP)
+//       orient(TOP, spin=90)
+//         color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+// Example: Of course, {{orient()}} can also place children on different sides of the parent.  In this case you don't have to figure out that the required anchor is BOT+BACK.  
+//   cuboid([50,40,15])
+//     position(RIGHT+TOP)
+//       orient(RIGHT)
+//         color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+// Example: You can combine this with {{diff()}} to remove the child.  Note that it's more intuitive to shift the child after positioning, relative to the global coordinate system:
+//   diff()
+//     cuboid([50,40,15])
+//       right(.1)up(.1)
+//         position(RIGHT+TOP)
+//           orient(LEFT)
+//             tag("remove")cuboid([10,5,4], anchor=$align);
 module position(from)
 {
     req_children($children);
     assert($parent_geom != undef, "No object to attach to!");
     anchors = (is_vector(from)||is_string(from))? [from] : from;
+    two_d = _attach_geom_2d($parent_geom);
     for (anchr = anchors) {
         anch = _find_anchor(anchr, $parent_geom);
         $attach_to = undef;
         $attach_anchor = anch;
         $attach_norot = true;
+        $align=two_d && anchr.y!=0 ? [anchr.x,-anchr.y]
+              :!two_d && anchr.z!=0 ? [anchr.x, anchr.y, -anchr.z]
+              : -anchr;
         translate(anch[1]) children();
     }
 }
@@ -506,6 +541,8 @@ module position(from)
 //   PARENT() orient(anchor, [spin]) CHILDREN;
 // Description:
 //   Orients children such that their top is tilted in the direction of the specified parent anchor point.
+//   The `$align` variable can help you anchor the child, aligned with the edges of the parent, based
+//   on the position specified by a parent {{position()}} module.  See {{position()}} for examples using `$align`. 
 //   For a step-by-step explanation of attachments, see the [Attachments Tutorial](Tutorial-Attachments).
 // Arguments:
 //   anchor = The anchor on the parent which you want to match the orientation of.
@@ -514,6 +551,7 @@ module position(from)
 //   `$attach_anchor` is set to the `[ANCHOR, POSITION, ORIENT, SPIN]` information for the `anchor=`, if given.
 //   `$attach_to` is set to `undef`.
 //   `$attach_norot` is set to `true`.
+//   `$align` is set to the anchor that will position the child flush on the parent at a designated {{position()}}
 //
 // Example: When orienting to an anchor, the spin of the anchor may cause confusion:
 //   prismoid([50,50],[30,30],h=40) {
@@ -541,16 +579,27 @@ module orient(anchor, spin) {
     anch = _find_anchor(anchor, $parent_geom);
     two_d = _attach_geom_2d($parent_geom);
     fromvec = two_d? BACK : UP;
+    spin = default(spin, anch[3]);
+    assert(is_finite(spin));
+
+    $align = is_undef($attach_anchor) ? undef
+           : two_d ? let(newalign=rot(from=anch[2], to=fromvec, p=zrot(-spin,$attach_anchor[0])))
+                     [sign(newalign.x), -1]
+           : let(newalign=rot(spin, from=fromvec, to=anch[2], reverse=true, p=$attach_anchor[0]))
+             [sign(newalign.x), sign(newalign.y), -1];
     $attach_to = undef;
     $attach_anchor = anch;
     $attach_norot = true;
-    spin = default(spin, anch[3]);
+    if (two_d)
-    assert(is_finite(spin));
+        rot(spin)rot(from=fromvec, to=anch[2]) children();
-    rot(spin, from=fromvec, to=anch[2]) children();
+    else
+        rot(spin, from=fromvec, to=anch[2]) children();
 }
 
 
 
+
+
 // Module: attach()
 // Synopsis: Attaches children to a parent object at an anchor point and orientation.
 // Topics: Attachments
@@ -2011,6 +2060,7 @@ module attachable(
         $parent_geom   = geom;
         $parent_size   = _attach_geom_size(geom);
         $attach_to   = undef;
+        $align=undef;
         if (_is_shown())
             _color($color) children(0);
         if (is_def($save_color)) {

tutorials/Attachments.md

@@ -375,6 +375,31 @@ cube([50,50,20],center=true)
     position(TOP+RIGHT) left(5) cube([4,50,10], anchor=RIGHT+BOT);
 ```
 
+When you position a child at an edge or corner of a parent object, you very likely
+need to change the anchor for the child in a matching way to align the child with the surface and edge
+of the parent.  You can automate this process by making use of the `$align` special variable, which
+is set by `position()`.  To align a child with a corner you can do:
+
+```openscad-3D
+include<BOSL2/std.scad>
+cuboid([50,40,15])
+    position(RIGHT+FRONT+TOP)
+        color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+```
+
+In this example, the `$align` variable replaces the explicit anchor of `RIGHT+FRONT+BOT` that
+would be required for this case. The `position()` module can also accept a list of positions,
+and it will place a copy of the child at each location.  However, if you want to align multiple
+children to the parent in different locations, each child needs a different anchor.  This can
+be achieved using the `$align` variable:
+
+```openscad-3D
+cuboid([50,40,15])
+    position([RIGHT+TOP,LEFT+TOP])
+        color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+```
+
+
 
 Positioning objects works the same way in 2D.
 
@@ -463,6 +488,28 @@ prismoid([50,50],[30,30],h=40)
         anchor_arrow(40);
 ```
 
+The potential confusion that spin creates for the proper anchoring of children 
+can be eliminated using the `$align` variable which takes into account the spin
+that the `orient()` module applies.
+
+```openscad-3D
+cuboid([50,40,15])
+    position(RIGHT+TOP)
+        orient(RIGHT)
+            color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+```
+
+In this case, the correct anchor is chosen to place the child on the right side
+with the appropriate anchor taking the spin into account.  You can use this method
+for children placed on the top by using orient with a TOP direction, but nonzero spin.
+
+```openscad-3D
+cuboid([50,40,15])
+    position(RIGHT+TOP)
+        orient(TOP, spin=90)
+            color("lightblue")prismoid([10,5],[7,4],height=4, anchor=$align);
+```
+
 
 ## Attachment overview