Extruding 2.5D surface

[ofmetal]

Imagine, we have a zero-thickness surface (like a slightly crumpled paper, or earth surface with hills and ravines) described as a mesh: points + triangles.
And now we want to create a solid manifold object from it by extruding in Z direction by some amount.

Current implementation of the Manifold offers Extrude() that only takes array of 2D polygons as input.

So, the simplest straightforward solution I can imagine is creating lots of triangle prisms for each mesh triangle and then applying a union operation on them. That looks ugly and probably too slow even with the ultra-fast Manifold engine.

Smarter way is to navigate through all the triangles, find border edges (half-edges with no pair), build 2.5D-contours and use them to make walls. Then somehow add tops and bottoms for each of the contours. The latter part seems not that easy for me.

Maybe I missing something and there is something hidden deep in the Manifold that might help solving this?

[elalish]

Smarter way is to navigate through all the triangles, find border edges (half-edges with no pair), build 2.5D-contours and use them to make walls. Then somehow add tops and bottoms for each of the contours.

Yes - let's imagine this surface is a 2D function on a grid: every x-y location has a corresponding z height. Now you just need to build a cube where this surface becomes the top face. You only need to add four verts: the lower corners. Then for each of the four sides of your surface you assemble an indexed side face polygon including two of the new verts. Use our TriangulateIdx() function to triangulate these faces and assemble all the triangles into a MeshGL. The bottom face is just a square. You'll know you've done it correctly if you don't need the Merge() function.