Triangulation and Convex Polygons

cotty

Have a look at split donut (works with your example, but maybe not in all cases?)

mrapsouthern

Hi,

I tried and it works with the simple example I gave.

BUT for the following real case pictured, it does not.
Does it work for you?

Ultimately the problem is that the 3rd party software does not see a hole. The bounding rectangle fills the hole in. Hence why I need to split it up before export.

Here is the SKP file:
http://dl.dropbox.com/u/22511393/example.skp

thomthom

In the first example, that's not really triangulation though. There are triangulation plugins out there - that expose the underlying triangulation of SketchUp's faces. But I gather you just want to split up faces to avoid holes?

What exporter do you use? (Since you mention you experience problems when transferring the model to other applications.)

mrapsouthern

Hi,

I export using http://www.rahe-kraft.de/cms/su2catt/index.htm

Just to be clear, I want the hole to be present.

I could use a triangulation plugin, this does solve the problem in theory (and practice sometimes).

However, for the acoustic software it is better to reduce the number of polygons and also to avoid skinny/narrow/spikey triangles.

Thanks
Alex

pilou

quadrilateralizer by tig ?
but normally your surface must be automatically triangulated

thomthom

@unknownuser said:

quadrilateralizer by tig ?
but normally your surface must be automatically triangulated

That actually creates more faces than just normal triangulations.

thomthom

@mrapsouthern said:

Just to be clear, I want the hole to be present.

Is there some consistency to the mesh? Writing a generic tool would be pretty much impossible as there are so many mutations. But if the holes are always square - then the task is more predictable.

thomthom

Oh, right - you posted that curvy thingy...

pilou

I have drawn a flat form like your. No Made any split !
Export in OBJ format by Tig OBJ Exporter

No Problem ! I have hole and only triangles in Wings3D for example!
I don't understand your little problem

mrapsouthern

@Pilou Basically the exporter and 3rd party software I use cause the hole to be filled in. It looks like the OBJ exporter you used exported the underlying triangles defined by Sketchup.

I can achieve the same by using a triangulation plugin prior to export.

That is as far as I have got. As you have also shown, it works, but the issue is that the triangles are very skinny/narrow/spikey (however you want to describe them).

I think this is as good as it can be, but the skinny triangles are not desirable. The only way to avoid them that I can see is to approximate the hole with fewr polygons, at it is the curves that cause the thin triangles.

Thanks for your help. Further suggestions are welcome though.

Cheers
Alex

thomthom

what if... for each face with a hole in it one edge is drawn from the inner loop to the outer loop? (Even here there are cases where it could be difficult to do by script - for instance if there are multiple holes in the face.)

mrapsouthern

Got a solution in theory...

What I need is a plugin that will allow me to select a face with one (or multiple) holes in it and then divide that face in some way so that there would be multiple faces without holes.

i.e. for each hole in a face you only need to add two lines to split the face with 1 hole into 2 faces without holes.

Similarly,
a face with 2 holes requires 4 dividing lines and results in 3 faces with no holes.
a face with 3 holes requires 6 dividing lines and results in 4 faces with no holes.
a face with 4 holes requires 8 dividing lines and results in 5 faces with no holes.

This solution would mean you can represent a complex geometry with holes without splitting the whole model into triangles.

Is there anything already out there?

Thanks
Alex

MattC

Thomthom's idea should work great in case there is only one hole in a face, and if You make more vertices on the outside loop. The problem with 'spiky' triangles is that You have only 4 vertices to which the edges are drawn.

Assume that You have two sets of points:
[inner_vertices]
[outer_vertices] - for example 25 points per rectangle side

For every point from [inner_vertices] find closest point from [outer_vertices] and draw an edge between those two.

(Possible problem) when two outer points have the same distance to inner point. Which point to choose ?

Cheers
Matt

pilou

Can you post an image with result wished ?

thomthom

I was thinking one edge from inner to outer was enough - in order to reduce the number of faces.

But thinking of it more, ultimately everything breaks down to triangles, so if you have a polygonal face in SU - internally it'll be made up of many triangles. The same thing goes for any other software. So converting a face into it's internal triangles doesn't really make the model heavier - in terms of total number of triangles the graphic card needs to draw.