Problem using difference() with linear_extrude()

I'm having a problem using difference() with an object I created with
linear_extrude(). The extruded shape is different and wrong when it is used
in difference() than when it isn't used in difference(). The example code
extrudes a rectangle with twist. When it is extruded by itself, the shape is
correct. When the extruded shape is subtracted from a cube using
difference(), the shape is incorrect. The sides are jagged and the top is
too narrow in the middle. I'm using OpenSCAD 2019.05 on Windows 10.
Example images:  top_view.JPG
<http://forum.openscad.org/file/t1635/top_view.JPG>    side_view.JPG
<http://forum.openscad.org/file/t1635/side_view.JPG> 
Example code:

/*
    Problem: the extruded shape is different when it is used
    in difference() than when it isn't used in difference()
*/

difference() {
    translate([-10, -10, 0])
        cube([20, 20, 20]);   
    extrude_it();
}
translate([21, 0, 0])
    extrude_it();


module extrude_it() {
    linear_extrude(21, twist=45, slices=10, convexity=3)
        square([1, 30], center=true);
}



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

_______________________________________________
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org

I tried increasing slices to 100 and 200. There are no inside out bits, but
the edges are still jagged which they shouldn't be. You can see it when you
zoom in.



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

_______________________________________________
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org

When I extrude the shape outside of difference(), the shape is correct. When
I extrude the shape inside of difference() the shape is different and
incorrect. It has zig-zag edges, unlike the shape outside of difference().



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

_______________________________________________
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org

When I extrude the shape outside of difference(), the shape is correct. When
I extrude the shape inside of difference() the shape is different and
incorrect. It has zig-zag edges, unlike the shape outside of difference().



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

_______________________________________________
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org

Ronaldo wrote
> The issue is in the linear_extrude with twist.
> Try this:
> ...

That is a big improvement. Thank you for the helpful response!



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

_______________________________________________
OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org

On Tue, Aug 18, 2020 at 05:04:15PM +0100, nop head wrote:
Well everything is jagged in OpenSCAD, there are no curves. The twisted
shape has facets made of two triangles at an angle, that give it a rippled
surface. If you zoom you can see the ripples and that gives the jagged edge
when subtracted.

WELL...... 


For example this: 

difference () {
  linear_extrude (height=10, twist=360, convexity = 10,$fn=20)
    square ([10,10]);
  translate ([0,0,-1]) cube (14.2);
}

clearly shows a jagged edge near the Z-axis. (those elswhere: I
haven't investigated)

Now, normally with say "cylinder" or "sphere" you can increase $fn
and get the model to match more closely to the expected mathematical
shape. Here you can increase $fn too and.... That doesn't help. 

The issue is that the inside edge of the square (the edge that starts
out on the X axis.) is multiplied into two triangles at each
slice. Both of these triangles are "skinny". One has a base (smallest
edge) near the Z axis of about 10mm/$fn and the other is a triangle
with base 2*pi*10mm. 

The first triangle is completely vertical. The other is at an angle of
about atan (1/(2*pi)) . So these meet at an angle close to 90 degrees.

It doesn't matter how fine you make the slices: These triangles keep
meeting at that angle. 

When you approximate a sphere with more and more triangular patches,
the angle between the patches approaches: "flat". So the curved nature
of the object is better approximated the more triangles you use. But
here increasing the number of triangles does not help.

IMHO, an extra $fa or something should be added to
linear_extrude. When the angle between such a pair of triangles
exceeds $fa, then the triangles should be split.until the angles
become less than $fa. 

So in my example, the first, half-way split would give us FOUR
triangles, One of them still vertical on the Z axis, but now
the other triangle touching the Z axis with its tip is already
at atan (1/pi) a much bigger angle, closer to vertical...

If in my example, you replace the square with this almost-square: 

    polygon ([[0,0], [10,0],[10,10], [0,10], [0.001,5],[0.0011,2.5],[0.001,1.25], [0.0011,0.625]   ]);

the desired mathematical shape is already approximated a lot more
closely. it would be useful and "complete" for the openscad language
if the language would support something that allows a user to specify
how accurate to approximate the mathematical object. (Note that i've
fixed one side, not the other, so you can compare the results in one
object.).

When you move the square a bit to the -x direction, you'll get much
more pronounced spikes that the TS is also seeing.

These artefacts make some objects difficult to 3D print. They also end
up having a lousy fit... 


	Roger. 



There is nothing wrong with difference().

On Tue, 18 Aug 2020 at 16:35, jamcultur <[hidden email]> wrote:

 I tried increasing slices to 100 and 200. There are no inside out bits, but
 the edges are still jagged which they shouldn't be. You can see it when you
 zoom in.



 --
 Sent from: http://forum.openscad.org/
 OpenSCAD mailing list
 [hidden email]
 http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org


OpenSCAD mailing list
[hidden email]
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org