That’s what I was planning on doing :-)  But now imagine that you have another perpendicular surface that penetrates it…

In OpenSCAD I’d like to “roll the curved surface” - see where the next part intersects it… then compute the difference of the rolled surface (i.e. with a slot where the other part cuts it)… then “roll” the rolled part “flat” so that I know exactly what to laser cut, so that when I physically roll it back into shape the slot will be in just the right spot for the other part?

Does that make more sense?

Difficult for me to describe elegantly!

Well, I guess that’s fine in principle, but it kind of negates the coolest feature of OpenSCAD somehow.

If the user’s eye strength shifts, all I have to do is shift the plate holding the lens moves along the Z axis. The “difference” calculations with the tabs on that plate and the surrounding case automatically (at the moment) produce all the tab holes in the right place.

If I go down that route, then I am mixing two totally different approaches towards doing things - where I’m implicitly forming holes based on simulating construction, and another where I’m explicitly defining holes based on replicating a bunch of calculations about where the tab should be (which may require replicating a lot of steps as different groups of assemblies are moved into position).

I hope there’s another way?

okobloko wrote

Congratulation, this is a quite ambitious project - especially for somebody
who started with OpenSCAD only a week ago.

Despite all its merits, OpenSCAD also has its drawbacks that pop up in the
moment you want to do really fancy stuff. One is, that it doesn't allow you
to query an object. Therefore you don't get hands on the vertex coordinates
of an object. Being able to access these coordinates you could think about
defining a reverse transformation function that unrolls the object ...
Another one is that you can't define and apply non-affine transformations on
objects. Rolling and unrolling would be such a transformation.
rotate_extrude() is an operator that maps from 2D into 3D. You are looking
for an operator that maps from 3D to 2D. The only one OpenSCAD offers is
again affine: projection()

However, there is a technique that rolls a 3D object along a plane, cuts
slices by means of intersection() and unions the result. But this is an
exhausting and slow process with an unsatisfactory result.

http://forum.openscad.org/file/t887/CWCCW.png

See the following code:

r=40;  // radius
a=4;  // height
b=50;  // width
resolution = 8;

i=180*$t;  // to animate select view/Animate and set  FPS=10, Steps=40
animate();
unroll(i);  //

module animate()
{
rad = i/180PIr;
rotate([0,90,0])
translate([-r-5,rad,0])
rotate([0,0,-i])
piece();

translate([0,rad,0]) window(resolution);

}

module unroll(j)
{
for(i=[0:resolution:j])
{
rad = i/180PIr;
intersection()
{
rotate([0,90,0])
translate([-r-a,rad,0])
rotate([0,0,-i])
piece();

translate([0,rad,0]) window(resolution); 

}
}
}

module window(width)
{
translate([0, -width/2, 0])cube([b, width, 2*a]);
}

module piece()
{
difference()
{
rotate_extrude(angle=180) translate([r, 0]) square([a, b]);
rotate([-60, 0, -45])cylinder(r=5, h=100);
}
}

--
Sent from: http://forum.openscad.org/

:-)  I actually love this approach.  Unfortunately it won’t produce results that will work well for a laser cutter I suspect… but it’s still great :-)

Do you think non-affine transformations might be on the cards for OpenSCAD at some point?

I didn’t do anything clever (ten hours work from first opening OpenSCAD, and it shows :-/ )  but to give a clearer idea of the situation if you’re interested: here’s the file I was working on without rounded edges https://cloud.eyeskills.org/s/CbcqJH4eisoqRfa

It's a lot of work to implement properly.

First, non-affine transformations can introduce self-intersections, and make the mesh non-manifold. So we would need automatic mesh repair to fix a transformed mesh before a boolean operation could be applied to it.

Second, consider twisting or bending a cube or a cuboid. You would need to subdivide the existing faces into smaller faces, add more vertices, before applying the transformation. So we'd need a mesh subdivision operation.

Third, non-affine transformations like bend can cause vertices to get farther apart in some regions, and closer together in other regions. If you have first subdivided the mesh finely enough to handle the worst case regions of the shape, then after the transformation, you may need to simplify the mesh in the areas where there are more vertices than necessary. So automatic mesh simplification is another operation that would be useful to have.

okobloko wrote

As I wrote, it is not fast, but it will work with a laser cutter if you
project it down and and export it as DXF or SVG. Of course the rounded face
at least needs a form that can be mathematically explicitly expressed and
unrolled. Use my code and try

projection() unroll(180);

okobloko wrote

We've had discussions about this years ago, when I was starting with
OpenSCAD. Meanwhile I've lost hope to see solutions for both of the
mentioned drawbacks. If you have another language, you can use it to parse
an STL, apply any non-affine transformation you like, write it out and
reimport it to OpenSCAD. However, as Doug already mentioned, the task is not
trivial as it can involve a lot of additional considerations and work and it
requires you to know exactly what you do.

--
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