Wish: explicitly negative volume components
OpenSCAD already has a feature to remove a part of a solid using the
difference() operator.
However, the (very intuitive) inherently additive nature of OpenSCAD
modelling does not allow
one to perform subtractive modelling using modules. That is, one cannot
return from a module
a volume to be implicitly subtracted from the already existing objects. To
achieve this currently
one has to split the model into two parts: the first one adds volume, the
other subtracts it.
This is very tedious and doesn't allow for good modularization of the
design. In reality the
module-defined comonents have both positive volume parts (they add solid
pieces to the
universe) and the negative ones (they indicate holes to be drilled,
trenches milled, texts
embossed etc.). To support this, I propose the following extension:
negative() { contents }
This acts like a union within the braces and then indicates that the
combined result
should be permanently subtractive. Then it could be seamlessly integrated
with the
default additive semantics as follows:
module ball_hole() {
negative() sphere(r=1, center=true);
}
cube(1, center=true);
ball_hole();
The real power comes from the ability to combine both positive and negative
volumes within a single module.
module ball_hole() {
union { negative() sphere(r=1, center=true);
cube(0.1, center=true);
}
}
To be well defined, it is necessary to know what results from composing the
operator negative() with other operators besides union() like:
intersection() {A() ; negative() B() ;}
difference() {negative() A() ; B() ; negative() C() ;}
and what would be rendered for each:
negative() cube() ;
negative() difference() {cube() ; cube()}
Em 15 de out de 2017 06:59, "Piotr Wyderski" piotr.wyderski@gmail.com
escreveu:
OpenSCAD already has a feature to remove a part of a solid using the
difference() operator.
However, the (very intuitive) inherently additive nature of OpenSCAD
modelling does not allow
one to perform subtractive modelling using modules. That is, one cannot
return from a module
a volume to be implicitly subtracted from the already existing objects. To
achieve this currently
one has to split the model into two parts: the first one adds volume, the
other subtracts it.
This is very tedious and doesn't allow for good modularization of the
design. In reality the
module-defined comonents have both positive volume parts (they add solid
pieces to the
universe) and the negative ones (they indicate holes to be drilled,
trenches milled, texts
embossed etc.). To support this, I propose the following extension:
negative() { contents }
This acts like a union within the braces and then indicates that the
combined result
should be permanently subtractive. Then it could be seamlessly integrated
with the
default additive semantics as follows:
module ball_hole() {
negative() sphere(r=1, center=true);
}
cube(1, center=true);
ball_hole();
The real power comes from the ability to combine both positive and negative
volumes within a single module.
module ball_hole() {
union { negative() sphere(r=1, center=true);
cube(0.1, center=true);
}
}
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You can make a module that operates on it's childeren
module makePin()
{
union()
{
difference()
{
// here the magic happens:
union(){ children(); }
cube([5, 3, 10], center=true);
cube([3, 5, 10], center=true);
}
cube([5, 1, 1], center=true);
cube([1, 5, 1], center=true);
}
}
// subtract a cross from the cube and add soething into it
//
makePin() cube([10, 10, 1], center=true);
--
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The Relativity library already has support for negative volumes.
https://www.thingiverse.com/thing:349943
You would do this by tagging negative volumes with the class name
"negative" (or any name you want), then subtract the negative shapes from
the model at the top level.
Issue #1608 discusses a proposal for adding a similar feature to OpenSCAD
in a general way (the terminology used here is "part names" rather than
"class names".
https://github.com/openscad/openscad/issues/1608
On 15 October 2017 at 04:58, Piotr Wyderski piotr.wyderski@gmail.com
wrote:
OpenSCAD already has a feature to remove a part of a solid using the
difference() operator.
However, the (very intuitive) inherently additive nature of OpenSCAD
modelling does not allow
one to perform subtractive modelling using modules. That is, one cannot
return from a module
a volume to be implicitly subtracted from the already existing objects. To
achieve this currently
one has to split the model into two parts: the first one adds volume, the
other subtracts it.
This is very tedious and doesn't allow for good modularization of the
design. In reality the
module-defined comonents have both positive volume parts (they add solid
pieces to the
universe) and the negative ones (they indicate holes to be drilled,
trenches milled, texts
embossed etc.). To support this, I propose the following extension:
negative() { contents }
This acts like a union within the braces and then indicates that the
combined result
should be permanently subtractive. Then it could be seamlessly integrated
with the
default additive semantics as follows:
module ball_hole() {
negative() sphere(r=1, center=true);
}
cube(1, center=true);
ball_hole();
The real power comes from the ability to combine both positive and negative
volumes within a single module.
module ball_hole() {
union { negative() sphere(r=1, center=true);
cube(0.1, center=true);
}
}
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On 2017-10-15 09:58, Piotr Wyderski wrote:
That is, one cannot return from a module
a volume to be implicitly subtracted from the already existing objects.
I have done this by having a negative flag that's set false by default,
so the module creates its positive objects, and the module is then
called again from difference () with flag set to let it subtract its
negative spaces.
OK, so a bit messy, but it does give the localisation of positive and
negative into the same module and lots of transformations can be shared.
I just factor the positioning transformations so that they can be shared.
I do complex assemblies that join and make mounting holes, etc without any
desire for negative volumes or named attachments.
On 16 October 2017 at 12:01, Gadgetmind lists@foxhill.co.uk wrote:
On 2017-10-15 09:58, Piotr Wyderski wrote:
That is, one cannot return from a module
a volume to be implicitly subtracted from the already existing objects.
I have done this by having a negative flag that's set false by default, so
the module creates its positive objects, and the module is then called
again from difference () with flag set to let it subtract its negative
spaces.
OK, so a bit messy, but it does give the localisation of positive and
negative into the same module and lots of transformations can be shared.
OpenSCAD mailing list
Discuss@lists.openscad.org
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
Agreed, you can do all that transformations manually, but you just
shouldn't: it is the task of a machine, not a human. A human should
be provided with a set of convenient tools to create models efficiently
and correctly. After several days of using OpenSCAD I was able to
identify two of them, hence the wish list posts. I have modelled the
parts correctly and then suddenly discovered that there are no tools
to compose them into a higher-order entity.
Best regards, Piotr
2017-10-16 13:53 GMT+02:00 nop head nop.head@gmail.com:
I just factor the positioning transformations so that they can be shared.
I do complex assemblies that join and make mounting holes, etc without any
desire for negative volumes or named attachments.
On 16 October 2017 at 12:01, Gadgetmind lists@foxhill.co.uk wrote:
On 2017-10-15 09:58, Piotr Wyderski wrote:
That is, one cannot return from a module
a volume to be implicitly subtracted from the already existing objects.
I have done this by having a negative flag that's set false by default,
so the module creates its positive objects, and the module is then called
again from difference () with flag set to let it subtract its negative
spaces.
OK, so a bit messy, but it does give the localisation of positive and
negative into the same module and lots of transformations can be shared.
OpenSCAD mailing list
Discuss@lists.openscad.org
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
This just gave me an idea. When doing odd shapes, hull is good for doing
convex corners, but not as good for concave corners.
With negative objects, this could be solved. Where a positive object
"stretches the surface" outside it, the negative object would instead
stretch it so that the object is outside the resulting object.
So, for example, a boomerang could be modelled (and I'm simplifying things
here, the actual viability of the boomerang is not the point) as the hull of
three short, wide cylinders in a triangle, and a negative cylinder in the
"V".
Of course, for this to work, the order of the objects needs to be taken into
account.
Would be very useful in making softer, more organic shapes.
Still, has some problems which needs to be sorted out, of course. I'm just
planting ideas...
--
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Yes I proposed a concave_hull() operator a while ago. It would be like
difference where the first object is positive and the rest are negative.
On 7 December 2017 at 09:50, Troberg troberg.anders@gmail.com wrote:
This just gave me an idea. When doing odd shapes, hull is good for doing
convex corners, but not as good for concave corners.
With negative objects, this could be solved. Where a positive object
"stretches the surface" outside it, the negative object would instead
stretch it so that the object is outside the resulting object.
So, for example, a boomerang could be modelled (and I'm simplifying things
here, the actual viability of the boomerang is not the point) as the hull
of
three short, wide cylinders in a triangle, and a negative cylinder in the
"V".
Of course, for this to work, the order of the objects needs to be taken
into
account.
Would be very useful in making softer, more organic shapes.
Still, has some problems which needs to be sorted out, of course. I'm just
planting ideas...
--
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Piotr Wyderski wrote
The real power comes from the ability to combine both positive and
negative
volumes within a single module.
module ball_hole() {
union { negative() sphere(r=1, center=true);
cube(0.1, center=true);
}
}
I would say, the real power comes from using the appropriate Boolean
operation:
module ball_hole()
difference { cube(0.1, center=true); sphere(r=1, center=true); }
A negative object will exend to infinite. It can't be rendered and will
obviously create a bag full of problems if used as operand in the Boolean
operations difference() and union(), while just intersection is save.
Where is the problem in using a difference() operation, which was indeed
introduced to get along without negation?
--
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