blender-syntax-mesh

name	blender-syntax-mesh
description	Covers Blender mesh data access including vertices, edges, faces, loops, BMesh creation and editing, from_pydata, UV layers, vertex colors/attributes, normals, and foreach_get/set for bulk operations. Activates when creating or editing mesh geometry, working with BMesh, or manipulating vertex data programmatically.
license	MIT
compatibility	Designed for Claude Code. Requires Blender 3.x/4.x/5.x with Python.
metadata	{"author":"OpenAEC-Foundation","version":"1.0"}

blender-syntax-mesh

Quick Reference

Critical Warnings

NEVER access mesh.vertices, mesh.edges, mesh.polygons, or mesh.loops while in Edit Mode — data is not synchronized. Use bmesh.from_edit_mesh() instead.

NEVER forget mesh.update() after mesh.from_pydata() — normals and derived data will be incorrect.

NEVER forget bm.free() after BMesh operations in Object Mode — causes memory leaks. Exception: edit-mode BMesh from bmesh.from_edit_mesh() must NOT be freed.

NEVER access BMesh elements by index without calling ensure_lookup_table() first — raises IndexError or undefined behavior.

NEVER use mesh.calc_normals() in Blender 4.0+ — removed, normals are computed automatically.

NEVER use mesh.use_auto_smooth or mesh.auto_smooth_angle in Blender 4.1+ — removed, auto-smooth is automatic when custom or sharp normals exist.

NEVER use MeshEdge.bevel_weight, MeshEdge.crease, or MeshVertex.bevel_weight in Blender 4.0+ — replaced by mesh.attributes["bevel_weight_edge"], mesh.attributes["crease_edge"], etc.

ALWAYS call mesh.update() after modifying mesh data via foreach_set or direct property writes.

ALWAYS link created objects to a collection — bpy.context.collection.objects.link(obj). Objects without collection links are invisible.

---

Version Matrix: Mesh API Breaking Changes

Feature	Blender 3.x	Blender 4.0	Blender 4.1	Blender 4.3
Bevel weight	edge.bevel_weight	mesh.attributes["bevel_weight_edge"]	Same as 4.0	Same as 4.0
Edge crease	edge.crease	mesh.attributes["crease_edge"]	Same as 4.0	Same as 4.0
Vertex colors	mesh.vertex_colors	mesh.color_attributes	Same as 4.0	Same as 4.0
calc_normals()	Available	Removed (automatic)	Removed	Removed
use_auto_smooth	Available	Available	Removed (automatic)	Removed
calc_normals_split()	Available	Available	Removed	Removed
MeshLoop.normal	Read/Write	Read/Write	Read-only	Read-only
Custom normals	create/calc/free_normals_split()	Same as 3.x	mesh.corner_normals (auto)	Same as 4.1
AttributeGroup	N/A	Single type	Single type	Split into AttributeGroupMesh etc.
foreach_set types	Silent coercion	Silent coercion	Strict TypeError	Strict + auto-updates

---

Decision Tree: Which Mesh Approach?

Need to create mesh from vertex/face data?
├─ Simple mesh (< 10K verts) → mesh.from_pydata()
├─ Large mesh (> 10K verts) → Pre-allocate + foreach_set
└─ Procedural/algorithmic → BMesh (verts.new, faces.new)

Need to read mesh data?
├─ Small mesh (< 1K elements) → Direct iteration (for v in mesh.vertices)
├─ Large mesh (> 1K elements) → foreach_get + numpy
└─ In Edit Mode → bmesh.from_edit_mesh()

Need to modify topology (add/remove verts/faces)?
├─ In Object Mode → BMesh (from_mesh → edit → to_mesh → free)
└─ In Edit Mode → bmesh.from_edit_mesh() → update_edit_mesh()

Need to modify vertex positions only?
├─ Few vertices → Direct: vertex.co = (x, y, z)
├─ Many vertices → foreach_set("co", flat_array)
└─ In Edit Mode → BMesh vert.co = Vector(...)

Need UV data?
├─ Read existing → mesh.uv_layers.active.data[loop_idx].uv
├─ Create new layer → mesh.uv_layers.new(name="UVMap")
├─ Bulk read/write → foreach_get/set on uv_layer.data
└─ In BMesh → bm.loops.layers.uv.active; loop[uv_lay].uv

Need custom attributes?
├─ Create → mesh.attributes.new(name, type, domain)
├─ Read → mesh.attributes["name"].data[i].value
├─ Bulk read/write → attr.data.foreach_get/set("value", array)
└─ Built-in names: "bevel_weight_edge", "crease_edge", "sharp_edge", "sharp_face"

---

Essential Patterns

Pattern 1: Create Mesh with from_pydata (All Versions)

import bpy

mesh = bpy.data.meshes.new("MyMesh")
verts = [(0, 0, 0), (1, 0, 0), (1, 1, 0), (0, 1, 0)]
edges = []  # Empty = auto-generate from faces
faces = [(0, 1, 2, 3)]
mesh.from_pydata(verts, edges, faces)
mesh.update()  # REQUIRED after from_pydata

obj = bpy.data.objects.new("MyObject", mesh)
bpy.context.collection.objects.link(obj)  # REQUIRED to make visible

Pattern 2: High-Performance Mesh Creation with foreach_set (All Versions)

import bpy
import numpy as np

n_verts = 10000
n_tris = 9998

mesh = bpy.data.meshes.new("FastMesh")
mesh.vertices.add(n_verts)
mesh.loops.add(n_tris * 3)
mesh.polygons.add(n_tris)

# Vertex coordinates — flat array, length = n_verts * 3
coords = np.random.rand(n_verts * 3).astype(np.float32)
mesh.vertices.foreach_set("co", coords)

# Loop vertex indices — flat array, length = n_tris * 3
loop_verts = np.arange(n_tris * 3, dtype=np.int32)
mesh.loops.foreach_set("vertex_index", loop_verts)

# Polygon definitions
loop_starts = np.arange(0, n_tris * 3, 3, dtype=np.int32)
loop_totals = np.full(n_tris, 3, dtype=np.int32)
mesh.polygons.foreach_set("loop_start", loop_starts)
mesh.polygons.foreach_set("loop_total", loop_totals)

mesh.update(calc_edges=True)
mesh.validate()

Pattern 3: BMesh in Object Mode (All Versions)

import bpy, bmesh

obj = bpy.context.active_object
bm = bmesh.new()
bm.from_mesh(obj.data)

bm.verts.ensure_lookup_table()  # REQUIRED before index access
bm.edges.ensure_lookup_table()
bm.faces.ensure_lookup_table()

# Create geometry
v1 = bm.verts.new((0, 0, 0))
v2 = bm.verts.new((1, 0, 0))
v3 = bm.verts.new((1, 1, 0))
face = bm.faces.new((v1, v2, v3))

# Write back and clean up
bm.to_mesh(obj.data)
obj.data.update()
bm.free()  # ALWAYS free in Object Mode

Pattern 4: BMesh in Edit Mode (All Versions)

import bpy, bmesh

obj = bpy.context.edit_object  # Must be in Edit Mode
bm = bmesh.from_edit_mesh(obj.data)

# Modify geometry
v = bm.verts.new((2, 0, 0))
bm.verts.ensure_lookup_table()

# Sync changes — do NOT call bm.free()
bmesh.update_edit_mesh(obj.data, loop_triangles=True, destructive=True)

Pattern 5: Bulk Data Read with foreach_get (All Versions)

import bpy
import numpy as np

mesh = bpy.context.active_object.data

# Read all vertex positions
n = len(mesh.vertices)
coords = np.empty(n * 3, dtype=np.float32)
mesh.vertices.foreach_get("co", coords)
coords = coords.reshape(-1, 3)  # Shape: (n, 3)

# Read all face normals
n_polys = len(mesh.polygons)
normals = np.empty(n_polys * 3, dtype=np.float32)
mesh.polygons.foreach_get("normal", normals)
normals = normals.reshape(-1, 3)

Pattern 6: UV Layer Access (All Versions)

import bpy

mesh = bpy.context.active_object.data

# Create UV layer if none exists
if not mesh.uv_layers:
    mesh.uv_layers.new(name="UVMap")

uv_layer = mesh.uv_layers.active

# Per-loop UV access
for poly in mesh.polygons:
    for loop_idx in poly.loop_indices:
        uv = uv_layer.data[loop_idx].uv  # mathutils.Vector (2D)
        print(f"Loop {loop_idx}: UV ({uv.x:.3f}, {uv.y:.3f})")

Pattern 7: Generic Attributes (Blender 4.0+)

import bpy
import numpy as np

mesh = bpy.context.active_object.data

# Create a per-vertex float attribute
attr = mesh.attributes.new("my_weight", type='FLOAT', domain='POINT')

# Write values
values = np.linspace(0.0, 1.0, len(mesh.vertices), dtype=np.float32)
attr.data.foreach_set("value", values)

# Read values
result = np.empty(len(mesh.vertices), dtype=np.float32)
attr.data.foreach_get("value", result)

# Access built-in attributes (4.0+ replacements)
bevel = mesh.attributes.get("bevel_weight_edge")
if bevel is None:
    bevel = mesh.attributes.new("bevel_weight_edge", 'FLOAT', 'EDGE')

Pattern 8: Custom Normals (Version-Dependent)

import bpy

mesh = bpy.context.active_object.data

# Blender 3.x: enable auto-smooth first
if bpy.app.version < (4, 1, 0):
    mesh.use_auto_smooth = True

# Set custom per-loop normals (all versions that support it)
# normals: list of (x, y, z) tuples, one per loop
custom_normals = [(0, 0, 1)] * len(mesh.loops)
mesh.normals_split_custom_set(custom_normals)
mesh.update()

# Read normals (4.1+): use corner_normals (auto-computed)
if bpy.app.version >= (4, 1, 0):
    for cn in mesh.corner_normals:
        print(cn.vector)

---

Common Operations

Read Mesh Data

mesh = obj.data  # bpy.types.Mesh

# Vertices
for v in mesh.vertices:
    v.co          # mathutils.Vector — position
    v.normal      # mathutils.Vector — vertex normal (read-only)
    v.index       # int — index
    v.select      # bool — selection state
    v.groups      # VertexGroupElement collection

# Edges
for e in mesh.edges:
    e.vertices    # int[2] — vertex indices
    e.index       # int
    e.use_seam    # bool — UV seam
    e.use_edge_sharp  # bool — sharp edge

# Polygons (faces)
for p in mesh.polygons:
    p.vertices[:]    # list[int] — vertex indices
    p.normal         # mathutils.Vector — face normal
    p.center         # mathutils.Vector — face center
    p.area           # float — surface area
    p.loop_start     # int — first loop index
    p.loop_total     # int — loop count
    p.material_index # int — material slot
    p.use_smooth     # bool — smooth shading

# Loops (face corners)
for l in mesh.loops:
    l.vertex_index   # int — associated vertex
    l.edge_index     # int — associated edge
    l.normal         # float[3] — loop normal (read-only in 4.1+)

Transform Mesh

import mathutils

mesh = obj.data
mat = mathutils.Matrix.Scale(2.0, 4)
mesh.transform(mat)
mesh.update()

Evaluated Mesh (Post-Modifiers)

depsgraph = bpy.context.evaluated_depsgraph_get()
obj_eval = obj.evaluated_get(depsgraph)
mesh_eval = obj_eval.to_mesh()

for v in mesh_eval.vertices:
    print(v.co)

obj_eval.to_mesh_clear()  # ALWAYS clean up

---

Performance Guide

Operation	Method	Relative Speed
Read < 1K verts	for v in mesh.vertices	1x (baseline)
Read > 1K verts	foreach_get("co", array)	10-100x faster
Write positions	foreach_set("co", array)	10-100x faster
Create simple mesh	mesh.from_pydata()	Good
Create large mesh	Pre-allocate + foreach_set	Best
Topology changes	BMesh	Required
Interactive edits	bmesh.from_edit_mesh()	Best for edit mode

foreach_get/set rules:

• Array MUST be flat 1D — for co (3 floats per vertex): length = len(vertices) * 3
• Use np.float32 for float data, np.int32 for integer data
• Blender 4.1+: strict type checking — mismatched types raise TypeError
• Blender 4.3+: foreach_set on attribute data triggers property updates automatically

---

Attribute Type Reference (Blender 4.0+)

Data Types

Type	Description	foreach_get/set key
'FLOAT'	Single float	"value"
'INT'	Integer	"value"
'FLOAT_VECTOR'	3D vector	"vector"
'FLOAT_COLOR'	RGBA linear float	"color"
'BYTE_COLOR'	RGBA sRGB byte	"color"
'BOOLEAN'	Boolean	"value"
'FLOAT2'	2D vector (UV-like)	"vector"
'QUATERNION'	Quaternion	"value"

Domains

Domain	Applies to	Size equals
'POINT'	Vertices	len(mesh.vertices)
'EDGE'	Edges	len(mesh.edges)
'FACE'	Polygons	len(mesh.polygons)
'CORNER'	Loops	len(mesh.loops)

Built-in Named Attributes (Blender 4.0+)

Name	Type	Domain	Replaces
"bevel_weight_vert"	FLOAT	POINT	MeshVertex.bevel_weight
"bevel_weight_edge"	FLOAT	EDGE	MeshEdge.bevel_weight
"crease_vert"	FLOAT	POINT	Vertex crease
"crease_edge"	FLOAT	EDGE	MeshEdge.crease
"sharp_edge"	BOOLEAN	EDGE	Edge sharp flag
"sharp_face"	BOOLEAN	FACE	Inverse of use_smooth
".sculpt_mask"	FLOAT	POINT	vertex_paint_mask (4.1+)

---

Reference Links

• references/methods.md — Complete API signatures for Mesh, BMesh, MeshVertex, MeshPolygon, MeshLoop, UV layers, Attributes
• references/examples.md — Working code examples: from_pydata, foreach_set, BMesh workflows, UV, attributes, normals
• references/anti-patterns.md — What NOT to do: 15 mesh-specific anti-patterns with version context

Official Sources

• https://docs.blender.org/api/current/bpy.types.Mesh.html
• https://docs.blender.org/api/current/bmesh.html
• https://docs.blender.org/api/current/bmesh.types.html
• https://docs.blender.org/api/current/bmesh.ops.html
• https://docs.blender.org/api/current/bpy.types.MeshVertex.html
• https://docs.blender.org/api/current/bpy.types.MeshPolygon.html
• https://docs.blender.org/api/current/bpy.types.MeshLoop.html
• https://docs.blender.org/api/current/bpy.types.AttributeGroup.html
• https://developer.blender.org/docs/release_notes/4.0/python_api/
• https://developer.blender.org/docs/release_notes/4.1/python_api/
• https://developer.blender.org/docs/release_notes/4.3/python_api/