Empty group counts as child.
This doesn't seem correct to me
module test() {
echo($i, $children);
children();
}
for($i = [0,1])
test()
if($i)
sphere();
ECHO: 0, 1
ECHO: 1, 1
When $i is 0 test() has no child but $children is still one. The CSG tree
look like this:
group() {
group() {
group();
group() {
group();
}
}
group() {
group();
group() {
group() {
sphere($fn = 0, $fa = 12, $fs = 2, r = 1);
}
}
}
}
So it looks like a false if generates an empty group, why? And that an
empty group counts as a child.
Both look like bugs to me. Why so many groups? All the empty ones and the
ones with one entry look like they can be recursively pruned without any
semantic change. Applying that here would just give.
sphere($fn = 0, $fa = 12, $fs = 2, r = 1);
Which is exactly what is on the screen.
- Yes this looks like a flaw indeed. $children seems to have wrong default
value, which might indeed lead to wrong semantics e.g. in code like this:
module test() {
if($children)
echo("children exist");
}
for(i = [0,1])
test()
if(i)
sphere();
- You are also right that empty structures could be extinguished. That
would demand some postprocessing with respect to the recursive nature of
such a structure. On the other hand, empty groups are for sure not very too
time consuming. So why not just live with them?
--
Sent from: http://forum.openscad.org/
2017-11-25 11:59 GMT-02:00 Parkinbot rudolf@parkinbot.com:
- You are also right that empty structures could be extinguished. That
would demand some postprocessing with respect to the recursive nature of
such a structure.
- On the other hand, empty groups are for sure not very too time
consuming.* So why not just live with them?
That is not exactly true. It may require a lot of time and memory. See:
http://forum.openscad.org/Recursion-issue-td20882.html
So why not just live with them?
They totally bloat out the CSG view and make $children report the wrong
number. But why create them in the first place? Why does an if that is
false need to create anything?
On 25 November 2017 at 15:38, Ronaldo Persiano rcmpersiano@gmail.com
wrote:
2017-11-25 11:59 GMT-02:00 Parkinbot rudolf@parkinbot.com:
- You are also right that empty structures could be extinguished. That
would demand some postprocessing with respect to the recursive nature of
such a structure.
- On the other hand, empty groups are for sure not very too time
consuming.* So why not just live with them?
That is not exactly true. It may require a lot of time and memory. See:
http://forum.openscad.org/Recursion-issue-td20882.html
OpenSCAD mailing list
Discuss@lists.openscad.org
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
2017-11-25 13:58 GMT-02:00 nop head nop.head@gmail.com:
So why not just live with them?
They totally bloat out the CSG view and make $children report the wrong
number. But why create them in the first place? Why does an if that is
false need to create anything?
And not only conditionals cause wrong answers.
module test() {
if($children)
echo($children,"children exist");
}
module empty() {}
test() { translate(); }
test() { empty(); }
Tree dump:
group() {
group() {
group() ;
}
}
group() {
group() {
group() ;
}
}
On Nov 25, 2017, at 10:58 AM, nop head nop.head@gmail.com wrote:
They totally bloat out the CSG view and make $children report the wrong number. But why create them in the first place? Why does an if that is false need to create anything?
The underlying reason for this is that “if” is not a language construct, it’s a built-in module. Since it’s a module, it always returns a node, even when it has no children, similar to an empty union.
Changing the contract of modules to optionally return nothing is possible. Pruning empty sub trees is also a possible post-processing step.
Another option would be to see this in connection with the old "implicit union” story, whose work-in-progress suggests allowing modules to return a (possibly empty) list of nodes, where emptyness is something we can manage in a better way: https://github.com/openscad/openscad/issues/350
-Marius
Hmm, I don't want a lazy union that outputs self intersecting objects
because the slicer I use does not union them it produces the symmetric
difference. I.e. each skin causes a transition from inside to outside.
My idea of a lazy union would only union things that actually do overlap
and take a short when they are disjoint. It would produce the same results
as now but a lot faster where ever you have multiple objects, for example
when you make an array of holes. Currently the only practical way to do
that is to do it in 2D and then extrude it to difference from a 3D object.
Getting rid of empty and single item groups seems like a separate issue as
children and for loops don't really work properly at the moment.
On 25 November 2017 at 17:11, Marius Kintel marius@kintel.net wrote:
On Nov 25, 2017, at 10:58 AM, nop head nop.head@gmail.com wrote:
They totally bloat out the CSG view and make $children report the wrong
number. But why create them in the first place? Why does an if that is
false need to create anything?
The underlying reason for this is that “if” is not a language construct,
it’s a built-in module. Since it’s a module, it always returns a node, even
when it has no children, similar to an empty union.
Changing the contract of modules to optionally return nothing is possible.
Pruning empty sub trees is also a possible post-processing step.
Another option would be to see this in connection with the old "implicit
union” story, whose work-in-progress suggests allowing modules to return a
(possibly empty) list of nodes, where emptyness is something we can manage
in a better way: https://github.com/openscad/openscad/issues/350
-Marius
OpenSCAD mailing list
Discuss@lists.openscad.org
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
2017-11-25 16:49 GMT-02:00 nop head nop.head@gmail.com:
My idea of a lazy union would only union things that actually do overlap
and take a short when they are disjoint.
But to check overlaps wouldn't be almost so time consuming as a regular
union?
No in most case you can just check the bounding boxes. You don't get any
false negatives and false positives are no worse than the current situation.
On 25 November 2017 at 20:14, Ronaldo Persiano rcmpersiano@gmail.com
wrote:
2017-11-25 16:49 GMT-02:00 nop head nop.head@gmail.com:
My idea of a lazy union would only union things that actually do overlap
and take a short when they are disjoint.
But to check overlaps wouldn't be almost so time consuming as a regular
union?
OpenSCAD mailing list
Discuss@lists.openscad.org
http://lists.openscad.org/mailman/listinfo/discuss_lists.openscad.org
My thought was that the CSG parser will do away with them anyway and will not
take more time than a decicated postprocessor step. But you are right, since
$children is a node count, proper semantics demands getting rid of empty
tails.
nophead wrote
So why not just live with them?
They totally bloat out the CSG view and make $children report the wrong
number. But why create them in the first place? Why does an if that is
false need to create anything?
--
Sent from: http://forum.openscad.org/