Hi,

I'm having problems to “scale” a concave polygon. Let's say my polygon is L-shaped (or even worse). I need to reduce its size a little bit, but I need the resulting polygon to be INSIDE the original shape, and if I do a regular “scale” in a polyextrude the new polygon is smaller, but it gets out of the boundaries of the original shape. For a convex polygon a regular scale is ok, but not for a concave one.
I though I could use a combination of “ray” and “point” nodes to generate the normals along which to do the extrusion, but I can't get it working right.
Any ideas on how to do that scaling?

Thank you VERY much!!

cheers

dagush.-

Hi dagush. There's a VOP that does exactly what you're trying to do - Displace Along Normal. If you set the displacement amount to a negative number, you'll erode in your shape.

dagush
Hi,

I'm having problems to “scale” a concave polygon. Let's say my polygon is L-shaped (or even worse). I need to reduce its size a little bit, but I need the resulting polygon to be INSIDE the original shape, …
Any ideas on how to do that scaling?

try using “inset” instead of scale

if you have normals you can use the peak-sop.

Seb

Hi,

Thank you VERY much for your answers. I've been trying all solutions, the solution using Displace Along Normal, the “inset” and the one using peak, but they do not work for my perticular case. Ayway, I think the solution is the Displace Along Normal, but, basically, the problem now comes from the fact I (still) don't have normals I'm working with 2D polygons I generated with some Python code, which defines 2D blocks from a vector-based city map representation. At the end, I plan to plug this code into the rest of our urbanEngine project. To generate the block geometry, I used the explanations (and the code) in

http//www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=11472&highlight=python+geometry [http]

So, now, the problem is how to add the exterior normal for every vertex, so that the displacement will work properly… I should keep thinking on this…

thank you all for your answers!!! You are a GREAT help!!!

cheers

dagush.-

Hi Dagush,

If I understand correctly you need some outward facing normals.

The dead simplest way to this, but not accurate at all, is to subtract the Point position from the centroid of the object.


A more accurate trick is to polyextrude your face up, then take the normals from the extruded surface, and apply those back to your mesh.

Hope this helps!


cheers,
-j

inset should do what you want, either in polyextrude or starburst SOP
if not, then you can at least use it to generate normals for your method

Hi,

Here you have an example of what I'm trying to do. Node HOM_Geo is the result of some new algorithm we're testing, and is supposed to be a set of city blocks. But I have to reduce the blocks a little bit to leave some room for the street itself. I tried a direct polyextrude scale, but you can see that we have some VERY concave polygons that generated an unusable result. Then I tried what you suggested, and I got to this

1- I added a delete operator to eliminate an undesired polygon (I'll check the code to see why it is there)
2- I used a polyextrude to generate the normals for the block edges
3- I plugged the VopSop as shown by brianBurke
4- I added a point op to flatten the result. I don't know why, but the resulting polys were slightly bended upwards…
5- I deleted the side faces from the polyextrude above…

I think I'm doing too many operations here, but, at least, I have a working solution. Probably, I'll try to generate the normals directly along with the geometry, inside the HOM_Geo node, and avoid a few steps…

THANK YOU ALL FOR YOUR INVALUABLE HELP!!!!

cheers

dagush.-

here is the inset method if anyone is interested (or if you want streets of constant width)

or here is the method of using inset to compute normals
if you want uneven width streets

That's it!!
It is simple PERFECT!
When I tried the inset, I testet too large values! Now, it works perfectly!

thanks!!!

cheers

dagush.-

Haven't checked the files sent by others, but as a general thing I find the PolyFrame SOP very useful for generating various useful vectors which can eventually be copied into N with an Attribute SOP and then used for Displace along normal, Peak, PolyExtrudes etc.

Dragos