topic Re: Clipping a Terrain Mesh to Leave a Road in Modeling

Clipping a Terrain Mesh to Leave a Road

Anonymous — Wed, 24 May 2023 18:00:37 GMT

I don't know how to clip a mesh with a polygon leaving the polygon and cutting away the mesh outside the polygon.

In other words, I can make a hole in a mesh. But how do I keep the hole and discard the donut?

The reason I need this is that I wish to create a road from a complicated site mesh and I have made a copy of the mesh and was hoping to cut the road from it by using the road polygon...

Oh, and I don't want to use SEO for this.

Thanks,
Greg

Re: Clipping a Terrain Mesh to Leave a Road

Anonymous — Thu, 05 Nov 2009 21:15:04 GMT

Greg,
I wrote this quite a while ago but I think it will still work.

• After the topo mesh is complete with all the ridges at there correct
elevations, rename it "topo proposed" in the ID field
and draw the outline of the roadway with the polyline tool in the correct
position on the mesh.

• Edit the contours so they are correct for a road. This could involve
considerable editing to make the the transverse cross-section of the road
level and the grade of the roadway even and at the correct slope.

• Select the mesh and the polyline and drag a copy a known distance away
from the original. This new mesh is going to be the roadway mesh with
every thing else removed.

• With the mesh tool selected, click on the edge of the mesh, the pet
palette comes up, select the MINUS button and trace a closed polygon
around all areas of the mesh that are not part of the roadway.
Where the roadway ends at the edge of the mesh continue the selection
line outside the mesh around to the point where the roadway meets the
edge of the mesh again,when the selection line comes back to where it
started that portion of the mesh that is within the selection polygon will
be removed.

• When all that remains of the mesh is the roadway select the mesh and
assign it an appropriate material and pen color and rename it "road
proposed" to distinguish it from the "topo proposed" mesh.

• Drag the roadway poly line back to it's original position on the
"topo proposed" mesh. Using the same MINUS button method trace
completely around the roadway, when the selection line comes back to
it's starting point the roadway area is removed. Delete the polyline and
drag the "road proposed" mesh into position in the "topo proposed"
mesh.

Peter Devlin

Re: Clipping a Terrain Mesh to Leave a Road

Anonymous — Thu, 05 Nov 2009 21:49:35 GMT

Peter, thanks so much for the detailed response.

That is more or less what I ended up doing. I was hoping that there was a way to pick whether the large area is subtracted away and use the magic wand tool. I had to go around the edge of the road polygon and click a hundred or so points because the magic wand always subtracts the smaller polygon...

As I was writing this response I figured out a work around that simplified things a bit... My reference to the smaller polygon got me thinking.
I made a masking polygon that was the inverse of the road shape and much larger than the entire terrain mesh.

I made a road polygon. I then made a polygon larger than the size of the entire terrain mesh. I subtracted the road polygon from the larger polygon with the magic wand to create a road mask.

I then placed the road mask over the terrain mesh. I then cut the road mask polygon in half down the road more or less. Then I subtracted one half of the road mask from the terrain mesh with the magic wand. Repeat with second half of road mask.

Left a perfect shaped mesh with all topo info intact with much less than the hundred clicks I did originally (about 6 magic wand clicks)...

Ahhh, much better on my carpally inflamed wrists.

Thanks

Re: Clipping a Terrain Mesh to Leave a Road

Erika Epstein — Fri, 06 Nov 2009 05:11:47 GMT

g.h.
Sweet