Cross section of 3d shape

Fridolin

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

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

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

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

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

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

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.