NateTG writes:

I'm not quite sure that I get what you're suggesting here, but it
might be related to my "riff" below.

After thinking about this a bit, I think that's it's basically what's
happening in the lasercut library.

Here's an example that pulls the def'n of clipTab (your "positive",
above) and clipInner (your "negative", above) and sticks them onto a
flat plate:

module clipTab(angle, x, y, thickness)
{
     translate([x,y,0]) rotate([0,0,angle]) union()
     {
          // make off-center
          translate([-thickness/2-1,0,0])
          {
               difference()
               {
                    translate([0,thickness/2,thickness/2]) cube([thickness+1,thickness,thickness], center=true);
                    translate([(1+thickness)/2+1,-thickness*8,0])  linear_extrude(height = thickness*3, center = true)  polygon(points=[[0,0],[0,9*thickness],[-(thickness-1), 9*thickness]]);
               }
               translate([-thickness,thickness,0])  linear_extrude(height = thickness)  polygon(points=[[0,0],[thickness+1,0],[thickness,thickness-1],[1,thickness-1]]);
          }
          translate([0.5,0,0]) cube([thickness,thickness,thickness]);
          *       cube([thickness*2+1,thickness,thickness], center=true);
     }
}

module clipInner(angle, x, y, thickness)
{
     translate([x,y,0]) rotate([0,0,angle]) union()
     {
          translate([-thickness/2-1,0,0])
          {
               translate([-(1+thickness)/2,-thickness*10,-thickness]) cube([1, thickness*11, thickness*3]);
               translate([(1+thickness)/2+1,-thickness*8,0])  linear_extrude(height = thickness*3, center = true)  polygon(points=[[0,0],[0,9*thickness],[-(thickness-1), 9*thickness]]);
               translate([(1+thickness)/2+1-thickness+1,thickness-1,-thickness]) cube([thickness/2, thickness, thickness*3]);
               translate([-thickness*3/2,0,-thickness]) cube([thickness, thickness, thickness*3]);
          }
     }
}

difference () {
     union() {
          translate([-10, -40, 0]) cube([20, 40, 3]);
          clipTab(0, 0, 0, 3);
     }
     clipInner(0, 0, 0, 3);
}

It doesn't seem like a very general approach.

As a riff on what I think you're suggesting, I guess that I could
write a pair of modules, one of which defines a bounding box for a
doohickey and one of which defines the doohickey itself.  I could
delete the bounding box (basically making a "slot") and then add the
doohickey into the resulting emptiness.

I feel like this has to be well trod ground and would appreciate
points to how other libraries/designs have approached it.

THANKS!,

g.

Hi George, how about this solution?

// A little C shaped thingy with a dingus hanging off the top.
module doohickey () {
     /*  the C shaped bit */
   if ($negative) {
          translate([-4, -3, 0]) cube([4, 6, 1]);
          translate([-3, -1, 0]) cube([3,2,1]);
   } else {
     /* the dingus */
     translate([0,3,0]) cube([2,1,1]);
  }
}

// applies positive and negative features to the first child from all
subsequent children
module apply_features() {
  union() {
    difference() {
      children(0);
      for (i = [1:$children-1]) let($negative = true) {
        children(i);
      }
    }
    for (i = [1:$children-1]) let($negative = false) {
      children(i);
    }
  }
}

apply_features() {
  cube([10, 20, 1]);
  translate([10,10,0]) doohickey();
}

On Fri, Feb 2, 2018 at 7:07 PM, George Hartzell hartzell@alerce.com wrote:

Hans L writes:

That's quite nice.  I was just playing around with deleting a bounding
cube explicitly then merging and it works.  This is a nice way to do
that programmatically.  I like the use of let and the
apply_features method.

It's too bad that I have to calculate the bounding cube "by hand", but
I've read through a couple of threads that make it sounds as if it's
not reasonably do-able in OpenSCAD at the moment.

I have a vague memory about being careful about butting two objects up
against each other, that OpenSCAD might get confused about their
edges.  I can't find it now that I'm looking at it.  Is that a
concern?

THANKS!

g.

This bounding box computation is pretty fast because minkowsky is applied
to just three cubes.

2018-02-03 1:53 GMT-02:00 NateTG nate-openscadforum@pedantic.org:

Is this supposed to actually work in OpenSCAD?  I get a compiler error
at $negative but I am not using the latest OpenSCAD.

On 2/2/2018 9:26 PM, Hans L wrote:

Hans L writes:

One other comment about this solution.

The doohickey that my original code draws in isolation looks something
like this:

             +---+
             |   |
+------------+---+
|            |
|   +--------+
|   |
|   |
|   |
|   +--------+
|            |
+------------+

My intent was that it would be glued into a bigger whole like so:

+--------------+
|              |
|              +---+
|                  |
|              +---+
|              |
|     +--------+
|     |
|     |
|     |
|     +--------+
|              |
|              |
+--------------+

In Hans' solution:

• when $negative is true it carves out negative space for each of
  the spaces it fills; and
• when $negative is false it draws all
  of its bits.

I had originally thought of simplifying this so that the negative step
carved out a bounding cube in one simple step and the positive step
filled it as it deemed appropriate.  The advantage of this is that
code for the negative case is simpler.

If I were to take this approach then I would need to modify my
original positive step to fill in the area above the "C" shape,
otherwise it would be open space in the result.

On the other hand, with that simplification there's no way to position
the top of the "C" along the top edge of a base feature and have the
little dingus hang off into space.  Hans' solution allows this, at the
cost of a slightly more complicated "negative" step.

On the gripping hand, there's the approach from the lasercut
library, where each object has a method that adds it's
positive bits and another method that carves out its negative space
and it's up to the end user to call them appropriately.

Horses for courses, I suppose.  I'll screw around with it and see if I
have a preference.  My end goal is to be able to output a
drawing for a lasercut boxes, e.g. clip-box.

g.