Select all Concave edges on a Mesh

Hello all,

I am trying to recreate what would be the path of water across this triangulated terrain.

For this I need to find the line segments that represent the ditches/canals.

These would be where two faces of opposite direction meet:

Thank you.

Attachments:

Ditch Water.gh, 19 KB

Replies to This Discussion

Permalink Reply by Michael Pryor on September 7, 2017 at 3:17pm

IT would be easier to use a loop for that. Check out anemone looping plug in for GH. I am pretty sure it has a water drainage example file.

Permalink Reply by Michael Pryor on September 7, 2017 at 3:21pm

Attached is the drainage example file in case you cannot find it.

Attachments:

04-Anmn_drainage.gh, 75 KB

Permalink Reply by Ivan Fischer on September 7, 2017 at 4:27pm

Hi Michael, thank you. I have anemone and could run the example file. But I am afraid it does not work ok with meshes. Tried modifying the file but no luck.

Still the curves might not be exactly what I am after, I specifically need those ditch segments. Just need a way of recognizing which lines are concave or convex.

Permalink Reply by Daniel González Abalde on September 7, 2017 at 8:08pm

Try this, the idea is to move the point in the direction of the slope in x and y.

Attachments:

DitchWater.gh, 27 KB

Permalink Reply by Daniel González Abalde on September 7, 2017 at 8:19pm

If you replace de [Amplitude] by [Multiplication] for example, you can make it advance faster where there is more slope.

Permalink Reply by Ivan Fischer on September 8, 2017 at 11:03am

Hey Daniel, thank you, but in the end this gives me a visual recognition of the ditch places right? How could I grab these "ditches" axis?

Permalink Reply by Daniel González Abalde on September 8, 2017 at 11:15am

Oh sorry, I forgot to mention that. On hoopsnake double click, you will get a window, there you have a loop button. I have used Hoopsnake here precisely because it has that button, because it is a slow process due to the need to take small steps for when it reaches the edges.
But you can replace it with anemone if you prefer.

Permalink Reply by Ivan Fischer on September 8, 2017 at 12:13pm

I been doing some research on how to test whether an edge is concave or convex and found this:

https://www.gamedev.net/forums/topic/508442-determining-whether-an-...

If n_a and n_b are the normals of the both adjacent faces, and p_a and p_b vertices of the both faces that are not connected to the edge in question, wherein n_a and p_a belongs to the face A, and n_b and p_b to the face B, then

( p_b - p_a ) . n_a <= 0 => ridge edge

as well as

( p_b - p_a ) . n_a > 0 => valley edge

Seems to work, (trying with two faces) but now I need to translate it to the whole mesh.

Attachments:

Mesh Concave or Convex Edges.gh, 19 KB

Permalink Reply by Ivan Fischer on September 9, 2017 at 12:00pm

Any ideas?

Permalink Reply by Daniel Piker on September 9, 2017 at 1:16pm

Assuming the mesh is always triangulated - the 'HingePoints' component in Kangaroo extracts the 2 points at the end of each non-boundary edge, and the points at the tips of the 2 adjacent triangles. From this you can get the normals of the 2 triangles, and then whether the cross product of these is in the same or opposite direction as the edge tells you if it is concave or convex.

Attachments: