How can I unsubdivide regions of flat faces with Python?

Question

First: I have a very detailed mesh (16M tris) but there are many regions where lots of faces have the same height and angle.

An optimization approach (real time rendering is the goal) is to select a region's face, go to select linked flat faces, and unsubdivide a bunch.

That's all fine but there are hundreds of regions like this and I'd really love to lose about 5M++ tris WITHOUT doing it by hand (mind you this won't be the only mesh).

So... I really not new to programming (but to Python, which shouldn't be a problem.........) and I came up with a VERY simple, brute force algorithm to do it for me, which's pseudo-code looks like this(sorry if I use C-like syntax):

for(int i = 0; i < mesh.faces.count; i++)
{
    List<face> linkedFaces = mesh.faces[i].SelectLinkedFaces(0.02f);
    linkedFaces.UnSubdivide(10);
}

Well... OBVIOUSLY I don't know anything about the blender-python API. I just want to select every single face, execute the "Select linked faces" option with an angle of 0.02 degrees and then un-subdivide all faces selected that way ten times. Can anyone help me out, even if it's just a guide? Thank you!

Answer 1

Edit Mode bmesh script.

Tested on subdivided and poked cube.

• Tags all edges that have a face angle > EPS
• Gets all triangular faces made up of only edges not tagged and joins them. (To Quad)
• Feeds all verts that have no linked edges tagged into bmesh unsubdivide operator.

Can edit properties of both to quads, eg max face angle, and iterations for unsubdivide

Test script, run in edit mode. Needs no selection.

import bmesh
import bpy
from math import radians

EPS = 1e-5 # edge angle
ITER = 10 # unsubdiv iters
AST = radians(40)
AFT = radians(40)        

me = bpy.context.object.data
bm = bmesh.from_edit_mesh(me)

for e in bm.edges:
    if len(e.link_faces) < 2:
        e.tag = True
    else:
        e.tag = e.calc_face_angle() > EPS


ret = bmesh.ops.join_triangles(
        bm,
        faces=[f for f in bm.faces if all(not e.tag for e in f.edges)],
        angle_face_threshold=AFT, 
        angle_shape_threshold=AST,
        )

bmesh.ops.unsubdivide(bm,
        verts=[v for v in bm.verts if not any(e.tag for e in v.link_edges)],
        iterations=ITER,
        )

bmesh.update_edit_mesh(me) 

Alternatively Dissolve edges with 0 face angle.

In edge select mode select an edge with no face angle, then select similar by face angle

Hit X

and dissolve edges.

A script

A quick edit mode bmesh script to do this.

import bmesh
import bpy

EPS = 1e-5


me = bpy.context.object.data
bm = bmesh.from_edit_mesh(me)


bmesh.ops.dissolve_edges(bm, 
        edges = [e for e in bm.edges if e.calc_face_angle() < EPS],
        )

bmesh.update_edit_mesh(me)        

Not sure how it will go on 5M ++ faces. Could change slightly to work on selection.

        edges = [e for e in bm.edges if e.calc_face_angle() < EPS and e.select]

This will ofcourse create a number of ngons, that can be grid filled, and fed into the unsubdivided.

Answer 2

Thanks to a hint from @batFINGER, turns out the best solution is simply to select an edge (Edge Select mode) with no face angles, go to Select > Select Similar > Face Angles. Then go to Face Select mode and un-subdivide.

No scripting needed. For simple shapes, Dissolve Edges will work way better. But beware of face-groups that are selected and connected to other face-groups at an angle with more complex models.