feature request: negative hull

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feature request: negative hull

Peter Falke
Was:  [OpenSCAD] loop_extrude() to get curves continuously differentiable at points

I was wondering for a while if there is a way to do a negative_hull() like this:
(please excuse the crummy picture, Paint is all I had :-) )

Does this problem have a general (allgemeine) solution at all?

Have other CAD programs a feature like this?

What does (would)  RapCAD do?

Sincerely,

TakeItAndRun
Inline-Bild 1


2013/10/17 Ruud Vlaming <[hidden email]>
On 17-10-13 18:25, John Griessen wrote:
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-1.png"  alt="curve sketch">
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-2.png"  alt="curve sketch">

Okay i will give this a try tomorrow.
Ruud

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Re: feature request: negative hull

nophead
If you Google concave hull there are some implementations for points. Seems to be the minimum area that covers all of them. I don't know if it generalises to solids, plus it isn't the same the same thing.

The problem with Google is if you don't know what a tool is called it is hard to find it.

I think I could write one for 2D, not sure about 3D.


On 18 October 2013 23:08, Peter Falke <[hidden email]> wrote:
Was:  [OpenSCAD] loop_extrude() to get curves continuously differentiable at points

I was wondering for a while if there is a way to do a negative_hull() like this:
(please excuse the crummy picture, Paint is all I had :-) )

Does this problem have a general (allgemeine) solution at all?

Have other CAD programs a feature like this?

What does (would)  RapCAD do?

Sincerely,

TakeItAndRun
Inline-Bild 1


2013/10/17 Ruud Vlaming <[hidden email]>
On 17-10-13 18:25, John Griessen wrote:
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-1.png"  alt="curve sketch">
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-2.png"  alt="curve sketch">

Okay i will give this a try tomorrow.
Ruud

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Re: feature request: negative hull

Jeff Senn-2
Is what you want even well defined?

Imagine that red circle within the green area: what happens? (arguably nothing, 
depending on how you define it). 

Even in 2D it seems clear (to me) only when the difference area covers a line segment with no
other polygon points within the area.  I suppose the strategy is to insert a "deflection into the polygon" in 
said line -- possibly following the hull of the difference for awhile -- that can avoid the difference area
and winds up with tangents to the previous line segment end points.

This will not be possible in all cases (imagine subtracting a circle covering 
a triangle with the points sticking out).  I suppose you might limit it to a single line...

In 3D I guess it would generalize to interrupting a planar surface rather than a line.

I'm really not sure what might be intended if any line endpoint (or non-planer point or
edge in 3D) is within the difference....

On Oct 18, 2013, at 8:40 PM, Jeff Senn <[hidden email]> wrote:

Is what you want even well defined?

Imagine that red circle within the green area: what happens? (arguably nothing, 
depending on how you define it). 

Even in 2D it seems clear (to me) only when the difference area covers a line segment with no
other polygon points within the area.  I suppose the strategy is to insert a "deflection into the polygon" in 
said line -- possibly following the hull of the difference for awhile -- that can avoid the difference area
and winds up with tangents to the previous line segment end points.

This will not be possible in all cases (imagine subtracting a circle covering 
a triangle with the points sticking out).  I suppose you might limit it to a single line...

In 3D I guess it would generalize to interrupting a planar surface rather than a line.

I'm really not sure what might be intended if any line endpoint (or non-planer point or
edge in 3D) is within the difference....


On Oct 18, 2013, at 7:10 PM, nop head <[hidden email]> wrote:

If you Google concave hull there are some implementations for points. Seems to be the minimum area that covers all of them. I don't know if it generalises to solids, plus it isn't the same the same thing.

The problem with Google is if you don't know what a tool is called it is hard to find it.

I think I could write one for 2D, not sure about 3D.


On 18 October 2013 23:08, Peter Falke <[hidden email]> wrote:
Was:  [OpenSCAD] loop_extrude() to get curves continuously differentiable at points

I was wondering for a while if there is a way to do a negative_hull() like this:
(please excuse the crummy picture, Paint is all I had :-) )

Does this problem have a general (allgemeine) solution at all?

Have other CAD programs a feature like this?

What does (would)  RapCAD do?

Sincerely,

TakeItAndRun
<negative hull command.png>


2013/10/17 Ruud Vlaming <[hidden email]>
On 17-10-13 18:25, John Griessen wrote:
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-1.png"  alt="curve sketch">
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-2.png"  alt="curve sketch">

Okay i will give this a try tomorrow.
Ruud

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Re: feature request: negative hull

Carl Witty
On Fri, Oct 18, 2013 at 5:41 PM, Jeff Senn <[hidden email]> wrote:

> Is what you want even well defined?
>
> Imagine that red circle within the green area: what happens? (arguably
> nothing,
> depending on how you define it).
>
> Even in 2D it seems clear (to me) only when the difference area covers a
> line segment with no
> other polygon points within the area.  I suppose the strategy is to insert a
> "deflection into the polygon" in
> said line -- possibly following the hull of the difference for awhile --
> that can avoid the difference area
> and winds up with tangents to the previous line segment end points.
>
> This will not be possible in all cases (imagine subtracting a circle
> covering
> a triangle with the points sticking out).  I suppose you might limit it to a
> single line...
>
> In 3D I guess it would generalize to interrupting a planar surface rather
> than a line.
>
> I'm really not sure what might be intended if any line endpoint (or
> non-planer point or
> edge in 3D) is within the difference....

If this is what you want, I think it's not too hard to code yourself.

As Jeff said, we assume that the "negative" figure only interrupts a
single face (because otherwise we can't figure out what it would
mean).  Then I think (although I haven't actually tried it) that you
get what you want by taking the difference of the positive figure with
(the convex hull of (the negative figure combined with the face it
interrupts)).

Carl
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Re: feature request: negative hull

Peter Falke
In reply to this post by Jeff Senn-2
@Jeff: Your right, my picture is not well defined.
The green (truncated) circle should be a pie, so that the edges of the cutout of the pie goes through the centers of the two lime circles.
The "negative hull" should only cut into the "positive hull".
I dont understand quiet what difference you mean. Could you please explain?

Sincerely,

TakeItAndRun


2013/10/19 Jeff Senn <[hidden email]>
Is what you want even well defined?

Imagine that red circle within the green area: what happens? (arguably nothing, 
depending on how you define it). 

Even in 2D it seems clear (to me) only when the difference area covers a line segment with no
other polygon points within the area.  I suppose the strategy is to insert a "deflection into the polygon" in 
said line -- possibly following the hull of the difference for awhile -- that can avoid the difference area
and winds up with tangents to the previous line segment end points.

This will not be possible in all cases (imagine subtracting a circle covering 
a triangle with the points sticking out).  I suppose you might limit it to a single line...

In 3D I guess it would generalize to interrupting a planar surface rather than a line.

I'm really not sure what might be intended if any line endpoint (or non-planer point or
edge in 3D) is within the difference....

On Oct 18, 2013, at 8:40 PM, Jeff Senn <[hidden email]> wrote:

Is what you want even well defined?

Imagine that red circle within the green area: what happens? (arguably nothing, 
depending on how you define it). 

Even in 2D it seems clear (to me) only when the difference area covers a line segment with no
other polygon points within the area.  I suppose the strategy is to insert a "deflection into the polygon" in 
said line -- possibly following the hull of the difference for awhile -- that can avoid the difference area
and winds up with tangents to the previous line segment end points.

This will not be possible in all cases (imagine subtracting a circle covering 
a triangle with the points sticking out).  I suppose you might limit it to a single line...

In 3D I guess it would generalize to interrupting a planar surface rather than a line.

I'm really not sure what might be intended if any line endpoint (or non-planer point or
edge in 3D) is within the difference....


On Oct 18, 2013, at 7:10 PM, nop head <[hidden email]> wrote:

If you Google concave hull there are some implementations for points. Seems to be the minimum area that covers all of them. I don't know if it generalises to solids, plus it isn't the same the same thing.

The problem with Google is if you don't know what a tool is called it is hard to find it.

I think I could write one for 2D, not sure about 3D.


On 18 October 2013 23:08, Peter Falke <[hidden email]> wrote:
Was:  [OpenSCAD] loop_extrude() to get curves continuously differentiable at points

I was wondering for a while if there is a way to do a negative_hull() like this:
(please excuse the crummy picture, Paint is all I had :-) )

Does this problem have a general (allgemeine) solution at all?

Have other CAD programs a feature like this?

What does (would)  RapCAD do?

Sincerely,

TakeItAndRun
<negative hull command.png>


2013/10/17 Ruud Vlaming <[hidden email]>
On 17-10-13 18:25, John Griessen wrote:
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-1.png"  alt="curve sketch">
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-2.png"  alt="curve sketch">

Okay i will give this a try tomorrow.
Ruud

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Re: feature request: negative hull

Peter Falke
@Carl: Your way would leave a sharp edge at the beginning of the indention, and not follow the edge of the lime circle(s) enough.

Sincerely,

TakeItAndRun


2013/10/19 Peter Falke <[hidden email]>
@Jeff: Your right, my picture is not well defined.
The green (truncated) circle should be a pie, so that the edges of the cutout of the pie goes through the centers of the two lime circles.
The "negative hull" should only cut into the "positive hull".
I dont understand quiet what difference you mean. Could you please explain?

Sincerely,

TakeItAndRun


2013/10/19 Jeff Senn <[hidden email]>
Is what you want even well defined?

Imagine that red circle within the green area: what happens? (arguably nothing, 
depending on how you define it). 

Even in 2D it seems clear (to me) only when the difference area covers a line segment with no
other polygon points within the area.  I suppose the strategy is to insert a "deflection into the polygon" in 
said line -- possibly following the hull of the difference for awhile -- that can avoid the difference area
and winds up with tangents to the previous line segment end points.

This will not be possible in all cases (imagine subtracting a circle covering 
a triangle with the points sticking out).  I suppose you might limit it to a single line...

In 3D I guess it would generalize to interrupting a planar surface rather than a line.

I'm really not sure what might be intended if any line endpoint (or non-planer point or
edge in 3D) is within the difference....

On Oct 18, 2013, at 8:40 PM, Jeff Senn <[hidden email]> wrote:

Is what you want even well defined?

Imagine that red circle within the green area: what happens? (arguably nothing, 
depending on how you define it). 

Even in 2D it seems clear (to me) only when the difference area covers a line segment with no
other polygon points within the area.  I suppose the strategy is to insert a "deflection into the polygon" in 
said line -- possibly following the hull of the difference for awhile -- that can avoid the difference area
and winds up with tangents to the previous line segment end points.

This will not be possible in all cases (imagine subtracting a circle covering 
a triangle with the points sticking out).  I suppose you might limit it to a single line...

In 3D I guess it would generalize to interrupting a planar surface rather than a line.

I'm really not sure what might be intended if any line endpoint (or non-planer point or
edge in 3D) is within the difference....


On Oct 18, 2013, at 7:10 PM, nop head <[hidden email]> wrote:

If you Google concave hull there are some implementations for points. Seems to be the minimum area that covers all of them. I don't know if it generalises to solids, plus it isn't the same the same thing.

The problem with Google is if you don't know what a tool is called it is hard to find it.

I think I could write one for 2D, not sure about 3D.


On 18 October 2013 23:08, Peter Falke <[hidden email]> wrote:
Was:  [OpenSCAD] loop_extrude() to get curves continuously differentiable at points

I was wondering for a while if there is a way to do a negative_hull() like this:
(please excuse the crummy picture, Paint is all I had :-) )

Does this problem have a general (allgemeine) solution at all?

Have other CAD programs a feature like this?

What does (would)  RapCAD do?

Sincerely,

TakeItAndRun
<negative hull command.png>


2013/10/17 Ruud Vlaming <[hidden email]>
On 17-10-13 18:25, John Griessen wrote:
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-1.png"  alt="curve sketch">
> <img src="http://ecosensory.com/bitmaps/openscad-tangent-curves-sketch-simplest-2.png"  alt="curve sketch">

Okay i will give this a try tomorrow.
Ruud

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Re: feature request: negative hull

John Griessen
In reply to this post by Peter Falke
On 10/18/2013 08:13 PM, Peter Falke wrote:
> The "negative hull" should only cut into the "positive hull".
> I dont understand quiet what difference you mean.

It would have to be a difference on tangent lines/planes to be a benefit.
A sharp point as the deepest part of an indentation defeats the original purpose.
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Re: feature request: negative hull

nophead
Imagine that red circle within the green area: what happens? (arguably nothing, depending on how you define it). 

I think it should always keep the entire red circle on the outside so it would mean that result becomes more U shaped, less kidney shaped.

It should behave like a rubber band stretched around the positive objects on its inside and the negative objects on its outside.


On 19 October 2013 03:02, John Griessen <[hidden email]> wrote:
On 10/18/2013 08:13 PM, Peter Falke wrote:
> The "negative hull" should only cut into the "positive hull".
> I dont understand quiet what difference you mean.

It would have to be a difference on tangent lines/planes to be a benefit.
A sharp point as the deepest part of an indentation defeats the original purpose.
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Re: feature request: negative hull

Peter Falke
@nop head: Thats it. Very nicely formulated.




2013/10/19 nop head <[hidden email]>
Imagine that red circle within the green area: what happens? (arguably nothing, depending on how you define it). 

I think it should always keep the entire red circle on the outside so it would mean that result becomes more U shaped, less kidney shaped.

It should behave like a rubber band stretched around the positive objects on its inside and the negative objects on its outside.


On 19 October 2013 03:02, John Griessen <[hidden email]> wrote:
On 10/18/2013 08:13 PM, Peter Falke wrote:
> The "negative hull" should only cut into the "positive hull".
> I dont understand quiet what difference you mean.

It would have to be a difference on tangent lines/planes to be a benefit.
A sharp point as the deepest part of an indentation defeats the original purpose.
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Re: feature request: negative hull

nophead
I think an additional rule needs to be that only edges of the convex hull of the positive objects that would pass through a negative object are the ones that are affected by that negative object. I think that resolves some ambiguities.

So in the example above if the red circle was totally inside it would be ignored but you could extend it upwards if you wanted it to affect the top edge, or downwards if you wanted to the affect the bottom edge.

Another complication is if some of the positive vertices are inside a negative object. For example if you put the red circle on the curved side of the green shape. In that case I think the local effect should the same as difference. 


On 19 October 2013 15:22, Peter Falke <[hidden email]> wrote:
@nop head: Thats it. Very nicely formulated.




2013/10/19 nop head <[hidden email]>
Imagine that red circle within the green area: what happens? (arguably nothing, depending on how you define it). 

I think it should always keep the entire red circle on the outside so it would mean that result becomes more U shaped, less kidney shaped.

It should behave like a rubber band stretched around the positive objects on its inside and the negative objects on its outside.


On 19 October 2013 03:02, John Griessen <[hidden email]> wrote:
On 10/18/2013 08:13 PM, Peter Falke wrote:
> The "negative hull" should only cut into the "positive hull".
> I dont understand quiet what difference you mean.

It would have to be a difference on tangent lines/planes to be a benefit.
A sharp point as the deepest part of an indentation defeats the original purpose.
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