Loop at intersection of two objects (script)

Question

I'm looking for a Script or Add-on for Blender 2.79 that creates verts and a lines where two objects intersect.

Practical example:

I need loops where the mountains intersect with the grass and where the grass intersects with the water for many purposes:

• Hitboxes around everything preventing player from leaving level (Extrude loops upward)
• AI boundaries where Ranged AI should not walk (Water)
• AI boundaries where Melee AI should not walk into / see player
• Boundaries for generating Vegetation using the Particle System

Image below is a part of the scene.

Some people recommended boolean and knife tools. It doesn't work out of the box, but I figured out a way and put it in a seperate answer below.

Answer 1

Script of Witold Jaworski generates intersection of two selected meshes.

• Select two objects
• Press W (in 2.79), for 2.8x search Intersection
• Press P (Separate) > Selected

• Version for 2.79
• Version for 2.8x

For more detailed instructions visit his site.

For future broken links – blend with script for version 2.79

For complex mesh like terrain is better select roughly vertices in intersected area to get script calcutlat faster, and I did quick cleaning of result (not perfect):

• in Edit mode select one of the longer edge than ussual
• search Similar > Lenght > Greater, Delete X
• than Select all and Search Remove Doubles (in 2.8x Merge by Distance)

(In tool properties you can increase minimum distance limit, it depence on how precise final intersection has to be. Higher value clean it better but in cost of preciseness (detail). Three gaps I had to connect manually. Pink line is the intersection object, converted into curve just to be visible in the image better.)

Answer 2

Bmesh bisect plane.

Here is a script that bisects one mesh with a plane and creates a new mesh object with the intersecting edges.

In test example

• Grid is a 100 x 100 grid displaced with a cloud texture via a modifier (not applied).
• Plane is a single face default plane, named "Plane".

• Move plane to desired location and run script.

• New object "Bisect" has the edges of the bisection only.

Still of result, since I hit stop on gif to quickly

2.8 version

import bpy
import bmesh

context = bpy.context
dg = context.evaluated_depsgraph_get()
scene = context.scene 
ob = context.object
plane = scene.objects.get("Plane")

if plane and ob:
    pmw = plane.matrix_world
    face = plane.data.polygons[0]
    plane_co = pmw @ face.center
    plane_no = pmw @ (face.center + face.normal) - plane_co 
    bm = bmesh.new()
    bm.from_object(ob, dg)
    bmesh.ops.transform(bm,
            verts=bm.verts,
            matrix=ob.matrix_world)

    x = bmesh.ops.bisect_plane(bm,
            geom=bm.faces[:] + bm.edges[:] + bm.verts[:],
            clear_inner=True,
            clear_outer=True,
            plane_co=plane_co,
            plane_no=plane_no,
            )
    # new object
    me = bpy.data.meshes.new("Bisect")
    bm.to_mesh(me)
    ob = bpy.data.objects.new("Bisect", me)
    context.collection.objects.link(ob)

2.7x version

Result of running in 2.79 on modified grid, using a one face plane object named "Plane"

import bpy
import bmesh

context = bpy.context

scene = context.scene 
ob = context.object
plane = scene.objects.get("Plane")

if plane and ob:
    pmw = plane.matrix_world
    plane_co = pmw.translation
    face = plane.data.polygons[0]
    plane_no = pmw * (face.center + face.normal) - pmw * face.center 
    bm = bmesh.new()
    bm.from_object(ob, scene)
    bmesh.ops.transform(bm, verts=bm.verts, matrix=ob.matrix_world)
    x = bmesh.ops.bisect_plane(bm,
            geom=bm.faces[:] + bm.edges[:] + bm.verts[:],
            clear_inner=True,
            clear_outer=True,
            plane_co=plane_co,
            plane_no=plane_no,
            )
    # new object
    me = bpy.data.meshes.new("Bisect")
    bm.to_mesh(me)
    ob = bpy.data.objects.new("Bisect", me)
    scene.objects.link(ob)

Answer 3

Just a reminder of Bisect tool in Blender 2.8x (as native tool) that can do similar kind of job (cut by plane).

• Duplicate your object
• Search Bisect
• Enable Clear Inner and Clear Outer in tool properties in left-down corner
• Adjust cut position (can be set numerically in properties too)

Done

For horizontal cut set Plane Normal values to 0/0/1 and move only Z axis of Plane Point parametr.