How to get an outline of a 2D object?
On 18.11.2020 12:53, Parkinbot wrote:
A boundingbox function would probably return me the coordinates of the
diagonal and I could calculate the center and a translation, just like you
do it.
The center of a boundingbox is not guaranteed to be located within its
2d shape, although it will in most cases.
A general solution would be to find the different edge loops and discard
all but the one covering the largest area.
Carsten Arnholm
For convex outlines, hull() would be enough to get the outline.
It is not difficult to get an approximation of the bounding box center as a
very small square. But the hard issue is that the bounding box center will
not be a "visual center" in general (non-convex-cases). In some cases, the
outline has no visual center. Consider the following polygon for instance:
module polyg()
difference() {
square(20,center=true);
for(x=7*[-1,1], y=7*[-1,1])
translate([x,y])
circle(5);
circle(3);
}
projection()
linear_extrude(height = 1, scale =0)
polyg();
We can relax the "visual center" condition. If P is a list of points such
that all vertices of the outline are visible by at least one point in P,
then fill_holes(P) will compute the correct outline:
module fill_holes2(P)
intersection_for(p=P)
translate(p) {
projection()
linear_extrude(1, scale=0.0001)
translate( -p) children();
scale(.01) hull() children();
}
For example:
P = [ [10,0], [-10,0], [0,10], [0,-10], [0,0] ];
fill_holes2(P) polyg();
However, I can't see a simple way to find a list P like that.
Btw, linear_extrude with scale=0 will produce a non-manifold when the
polygon has holes.
Em qua., 18 de nov. de 2020 às 11:54, Parkinbot rudolf@digitaldocument.de
escreveu:
A boundingbox function would probably return me the coordinates of the
diagonal and I could calculate the center and a translation, just like you
do it.
Of course a boundingbox function can also be implemented to just return the
[dx, dy, dz] extents according to the three axes. But this would be the
worst way to implement it. Usually it is implemented to return a pair of
3D-points or a box object with an origin and the [dx, dy, dz] extents.
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Ronaldo wrote
Nice designs,
I guess that if you could collect all arc and circle center points in a
list P, fill_holes2(P) would give you the outline you want.
Thanks!
Well, I do the arcs by using offset() to get the rounded shapes, and I do it
in layers (small radius for outside corners, large for inside corners, then
I add some elements (such as the hinges) and put a small radius on the new
inside corners. All this on parametric designs.
While I suppose I could track all this, it's not a task I would enjoy...
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cacb wrote
The center of a boundingbox is not guaranteed to be located within its
2d shape, although it will in most cases.
Ups, yeah, of course you are right. Non-convexity is a mess.
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Another example where this would be nice, and where the inside geometry isn't
defined by me, so I can't "reverse engineer" my code to remove the holes. A
very typical laser cutter example:
Say that you are making a sign, with some text, and a baseline to hold the
characters together. So, basically, a union of a square and some text. Very
common laser cutting task.
You want to engrave the letters, but cut the outline of the object (with a
slight offset), but you don't want any holes in it.
I can't reverse engineer the font, a hull() would just make a mess of
everything, using offset to remove holes would destroy the outline as well.
The only way I can figure out to make it work is if there were some kind of
built-in outline() module.
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Can't you just use Ronaldo's fill_holes2(P) solution? All you have to do
is nominate some positions on the object that together can see all the
vertices of the perimeter. For the item you describe, that would seem not
to be many and they don't need great precision.
On Mon, 23 Nov 2020 at 11:13, Troberg troberg.anders@gmail.com wrote:
Another example where this would be nice, and where the inside geometry
isn't
defined by me, so I can't "reverse engineer" my code to remove the holes. A
very typical laser cutter example:
Say that you are making a sign, with some text, and a baseline to hold the
characters together. So, basically, a union of a square and some text. Very
common laser cutting task.
You want to engrave the letters, but cut the outline of the object (with a
slight offset), but you don't want any holes in it.
I can't reverse engineer the font, a hull() would just make a mess of
everything, using offset to remove holes would destroy the outline as well.
The only way I can figure out to make it work is if there were some kind of
built-in outline() module.
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Unhappily that process fails miserably in some cases. The char "@" for
instance cannot be correctly processed by fill_holes2() with any set of
points. It is hard to distinguish channels from holes in the polygon. I
don't see any general solution with the tools at hand.
A segunda, 23/11/2020, 08:26, nop head nop.head@gmail.com escreveu:
Can't you just use Ronaldo's fill_holes2(P) solution? All you have to do
is nominate some positions on the object that together can see all the
vertices of the perimeter. For the item you describe, that would seem not
to be many and they don't need great precision.
On Mon, 23 Nov 2020 at 11:13, Troberg troberg.anders@gmail.com wrote:
Another example where this would be nice, and where the inside geometry
isn't
defined by me, so I can't "reverse engineer" my code to remove the holes.
A
very typical laser cutter example:
Say that you are making a sign, with some text, and a baseline to hold the
characters together. So, basically, a union of a square and some text.
Very
common laser cutting task.
You want to engrave the letters, but cut the outline of the object (with a
slight offset), but you don't want any holes in it.
I can't reverse engineer the font, a hull() would just make a mess of
everything, using offset to remove holes would destroy the outline as
well.
The only way I can figure out to make it work is if there were some kind
of
built-in outline() module.
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On 23.11.2020 12:12, Troberg wrote:
Say that you are making a sign, with some text, and a baseline to hold the
characters together. So, basically, a union of a square and some text. Very
common laser cutting task.
You want to engrave the letters, but cut the outline of the object (with a
slight offset), but you don't want any holes in it.
I don't think you can solve it 100% in OpenSCAD, but let me show you a
work-around that starts and ends with OpenSCAD. Imagine this is your
code for a sign:
eclipse.scad
$fa=0.01;
difference() {
circle(r=100);
translate([0,-40])
circle(r=70);
translate([0,55])
scale(3)
text("eclipse"
,halign="center"
,valign="center");
}
It is a concave outline with some text engraved, and you want to remove
the "eclipse" text but keep the rest.
If this was your code you could just remove the text, but let us assume
it was someone else's code, or maybe a DXF or SVG file you got from
someone and imported, and you still wanted to remove the "eclipse" text
holes. Also assume it is too complex to fix manually, or that you have
many such cases and want an automatic process. In any case, you can fix
it using a couple of command line steps:
First, run OpenSCAD and create a DXF file from the model. The DXF will
be used for extracting the final outline:
$ openscad eclipse.scad --o=eclipse_outline.dxf
This DXF contains everything, including the holes.
Second, create a DXF with only the outline
Run AngelCADs "dxfread" and create a function from the DXF.
(the installation directory is here assumed to be in system PATH)
$ dxfread eclipse_outline.dxf -asfunc
This creates "eclipse_outline.as" containing a function called
"eclipse_outline_dxf()", returning a 2d shape with the complete DXF file
geometry, including the holes. Open the file in AngelCAD and add the
following somewhere in the file (outside of the function body):
void main()
{
shape2d@ outline = fill2d(eclipse_outline_dxf());
outline.write_xcsg(GetInputFullPath());
}
Notice the use of "fill2d", it is key. It removes the holes, but keeps
the outline. Now build the file (F6) (it is also possible from command
line). If dxf is selected as an output format, you get a new DXF file in
a subfolder:
xcsg/eclipse_outline.dxf
This contains the outline only.
Third, view the outline DXF in OpenSCAD
import("xcsg/eclipse_outline.dxf");
Carsten Arnholm
I'm thinking more like this:
module sign(){
text("Boobies!",100);
square([520,5]);
}
//engrave
color([0,0,0])
translate([0,0,1])
sign();
//cut, but without the holes
offset(10)
sign();
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On 23.11.2020 14:47, Troberg wrote:
I'm thinking more like this:
module sign(){
text("Boobies!",100);
square([520,5]);
}
//engrave
color([0,0,0])
translate([0,0,1])
sign();
//cut, but without the holes
offset(10)
sign();
Sure, no problem.
Carsten Arnholm