bugGNU Octave - Bugs: bug #49774, convhull doesn't accept option...


bug #49774: convhull doesn't accept option simplify

Submitter:  Juan Pablo Carbajal <juanpi>
Submitted:  Tue 06 Dec 2016 01:09:40 AM UTC
Category:  Octave Function Severity:  3 - Normal
Priority:  5 - Normal Item Group:  Matlab Compatibility
Status:  Confirmed Assigned to:  None
Originator Name:  jpi Open/Closed:  * Open
Release:  * dev Operating System:  * Any
Fixed Release:  None Planned Release:  None
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Tue 13 Nov 2018 02:26:53 AM UTC, comment #4: 

I've only been looking at this today, but that said, I'm not sure if simplify is not extraneous.  Small experimentation with qhull gets me a set of random points in a square with

rbox 50 D2 | qconvex G > a

In this case the interior points are not displayed in geomview.  To display the interior and as I understand it simplified points we need to run qconvex Ga instead of G.  This shows all the points in the interior of the convex hull and the hull itself.  My question is if the default behaviour in qhull is to make the convex hull and ignore the interior points, what is simplified by simplify?

That said, I'm sure mathworks doesn't put things like simplify in their code without reason.

Chris Gorman <chrisgorman>
Sun 11 Dec 2016 09:57:42 PM UTC, comment #3: 

In the script from comment #1, ks=k.

You can see the behavior of "simplify"= true in the script

xx = -3:1:3;
yy = -3:1:3;
[x,y] = meshgrid(xx,yy);
k = convhull(x,y);
ks = convhull(x,y, 'simplify', true);
save chull1.mat

chull1.mat attached,

Note that 'simplify'= true omits all the points that if we drop them, the convex hull will not change.
In the 2D case it's simple:
if a few points are on a straight segment, the inner points are
omitted, and the end points are left.

(file #39198)

Avinoam Kalma <avinoam>
Group Member
Fri 09 Dec 2016 03:08:05 PM UTC, comment #2: 

@Juan: Can you try the example in comment #1 and report back.  I can't implement the feature until I know what it's supposed to do.

Rik <rik5>
Group administrator
Tue 06 Dec 2016 05:02:28 PM UTC, comment #1: 

Could you run the sample script in Matlab and upload the results so we can see what the "simplify" option does?

xx = -1:.05:1;
yy = abs(sqrt(xx));
[x,y] = pol2cart(xx,yy);
k = convhull(x,y);
ks = convhull(x,y, 'simplify', true);
save chull.mat

Rik <rik5>
Group administrator
Tue 06 Dec 2016 01:09:40 AM UTC, original submission:  

Option 'simplify' is not accepted. Couldn't find the equivalent for qhull


Juan Pablo Carbajal <juanpi>
Group Member


(Note: upload size limit is set to 16384 kB, after insertion of the required escape characters.)

Attach Files:

Attached Files
file #39198:  chull1.mat added by avinoam (488B - application/octet-stream)


Depends on the following items: None found

Items that depend on this one: None found


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  • -email is unavailable- added by juanpi (Submitted the item)

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    Follow 3 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2016-12-13 rik5 StatusNeed Info Confirmed
    2016-12-11 avinoam Attached File- Added chull1.mat, #39198
    2016-12-06 rik5 StatusNone Need Info

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