Cross section of 3d shape

Fridolin · July 2015

I'm trying to get a cross section of a 3d shape from any given Z position. My current sketch only shows the top and bottom plane.

void setup() {
  size(400, 400, P3D);
  ortho();
}

void draw() {
  background(0);
  translate(200, 200, 145);
  box(200);
  ortho(0, width, 0, height, mouseY, mouseY + 1);
}

What would be the correct way to do this?

Fridolin · July 2015

The problem with this approach seems to be that the box isn't filled with anything and so only the top and bottom planes become visible. I don't know if it's possible to fill it. Maybe someone knows a better way to get a cross section.

TfGuy44 · July 2015

As you have discovered, the box is empty. There is no easy way to do this.

But why are you trying to do this? Why bother getting a cross-section of an empty box?

Now if you had a 3D data set... Now that would be more interesting...

Fridolin · July 2015

Now if you had a 3D data set...

I chose the box to keep my example simple and easy to try out. The goal is to be able to load a .obj file and get a cross-section of that.

TfGuy44 · July 2015

Again, there is no easy way to do this. The trouble lies in being able to tell if a point is "inside" or "outside" an object. This is incredibly hard to determine.

For a simple geometric shape, e.g. cubes, cones, spheres, it might be possible to do mathematically. But for a generic object this will be a lot harder (if not impossible).

koogs · July 2015

the front clipping plane might be useful for this - move the plane so it slices through the object (or move the object so it straddles the plane).

z-near here: https://processing.org/reference/perspective_.html

(i think raw opengl lets you specify general clipping planes, not just front and back)

Fridolin · July 2015

the front clipping plane might be useful for this

The example I posted in the OP actually uses the clipping planes of ortho()

The trouble lies in being able to tell if a point is "inside" or "outside" an object.

I looked into that a little. There seems to be a method with counting intersections of a ray starting at the point I want to check. I don't know how to count these intersections though.

It doesn't need to be verry fast by the way.

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Cross section of 3d shape

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