How To Get Mapping of UV and Vertex

GOAL

To get correspondence of UV coordinates and vertex id in the object in Blender scene by Blender Python. The following is output of the script.

uv: 0.625 0.5 vertex id: 0
uv: 0.875 0.5 vertex id: 4
uv: 0.875 0.75 vertex id: 6
uv: 0.625 0.75 vertex id: 2
uv: 0.375 0.75 vertex id: 3
.
.
.

Environment

Blender 2.83 (Python 3.7.4)
Windows 10

Method

Mesh Loop

Use Mesh Loop to get UV and vertex data.

In unfolded UV, the correspondence between the point of UV map and the vertex of 3D object is not one-to-one. Thus we use ‘Mesh Loop’ instead of vertex or edge to distinct point of UV map.

Mesh Loop is an element surrounding a face, which consists of edges and vertices. Each quad face consists of 4 Mesh Loops as below. For example, cube objects have 6face, 24 mesh loops.

import bpy

obj = bpy.data.objects[obj_name]
mesh_loops = obj.data.loops 
for mesh_loop in mesh_loops:
    print(mesh_loop)

Mesh UV Loop

UVs have Mesh UV Loops as a counterpart to the Mesh Loops. And the correspondence between Mesh Loop and MeshUVLoop is one-to-one.

import bpy

obj = bpy.data.objects[obj_name]
mesh_uv_loop = obj.data.uv_layers[0].data[loop_index]

Source Code

import bpy

def get_uvs(obj_name):
    obj = bpy.data.objects[obj_name]
    mesh_loops = obj.data.loops

    for i, mesh_loop in enumerate(mesh_loops):
        mesh_uv_loop = obj.data.uv_layers[0].data[i]
        print("uv:", mesh_uv_loop.uv[0], mesh_uv_loop.uv[1], "vertex id:", mesh_loop.vertex_index)

get_uvs("Cube")

Blender Addon Naming Rules

GOAL

Today’s goal is to list naming rules and conventions of Blender addon.

Reference:
AddonPreferences(bpy_struct) from Blender 2.93.2 Release Candidate Python API documentation
Script Meta Info from wiki.blender.org

Environment

Blender 2.83 (Python 3.7.4)
Windows10

directory_name and file_name.py

Addon directory and files are snake_case with only lower cases. See examples in your addon directory, C:/Program Files/Blender Foundation/Blender 2.83/2.83/scripts/addons.

Addon Name

Addon name defined in bl_info is defined with the first letter of each word capitalized.

Class Name

2.8x enforces naming conventions for class name. Refer to “Class Registration” > “Naming” in “Blender 2.80: Addon API” reference for details.

The naming convention is UPPER_CASE_{SEPARATOR}_mixed_case. And {SEPARATOR} is defined according to the type of class as below.

• Header -> _HT_
• Menu -> _MT_
• Operator -> _OT_
• Panel -> _PT_
• UIList -> _UL_

The followings are examples of panel class name.

# Header
# from "oscurart_tools" addon
class OSSELECTION_HT_OscSelection(bpy.types.Header):
    bl_label = "Selection Osc"
    bl_space_type = "VIEW_3D"

# Menu
# from "space_view3d_spacebar_menu" addon
class VIEW3D_MT_Space_Dynamic_Menu(Menu):
    bl_label = "Dynamic Context Menu"

class VIEW3D_MT_View_Menu(Menu):
    bl_label = "View"

# Operator
# from "add_camera_rigs" addon
class ADD_CAMERA_RIGS_OT_set_scene_camera(Operator):
    bl_idname = "add_camera_rigs.set_scene_camera"
    bl_label = "Make Camera Active"
    bl_description = "Makes the camera parented to this rig the active scene camera"

# Panels
# from "add_camera_rigs" addon
class ADD_CAMERA_RIGS_PT_camera_rig_ui(Panel, CameraRigMixin)
    bl_label = "Camera Rig"
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'
    bl_category = 'Item'

# UIList
# from "cycles" addon
class CYCLES_RENDER_UL_aov(bpy.types.UIList):
    def draw_item(self, ...

 bl_idname

bl_idname is specified to access the operator from python script in Blender. The bl_idname consists of snake_case words connected with dots.

# bl_idname in cycle/operators.py in cycle addon

class CYCLES_OT_use_shading_nodes(Operator):
    bl_idname = "cycles.use_shading_nodes"

class CYCLES_OT_add_aov(bpy.types.Operator):
    bl_idname="cycles.add_aov"

class CYCLES_OT_remove_aov(bpy.types.Operator):
    bl_idname="cycles.remove_aov"

class CYCLES_OT_denoise_animation(Operator):
    bl_idname = "cycles.denoise_animation"

class CYCLES_OT_merge_images(Operator):
    bl_idname = "cycles.merge_images"

 For headers, menus and panels, the bl_idname is expected to match the class name (automatic if none is specified). Refer to “Class Registration” > “Naming” in “Blender 2.80: Addon API” reference.

Other – PEP 8

Basically the naming rules of module, variable and so on is compliant with PEP 8.

variable = "hello"
variable_two = 2

def function_name(arg_name):
    pass

class ClassName():
    def __init__(self, arg_name):
        self.variable = 1
    def method_name(self):
        pass

How To Make My Addon Support “Undo” In Blender

GOAL

Today’s goal is to implement “Undo” in my Blender addon.

Environment

Blender 2.83
Windows10

Method

Add bl_options “UNDO” or “UNDO_GROUPED” into the your custom operator. It’s easy.

UNDO Undo: Push an undo event (needed for operator redo).
UNDO_GROUPED: Grouped Undo, Push a single undo event for repeated instances of this operator.

from Blender2.93.2 Python API Documentation

The following is an example.

class ADDMATRIX_add_cube(bpy.types.Operator):
    bl_idname = 'add_matrix_obj.add_cube'
    bl_label = "Add matrix cube"
    bl_options = {'REGISTER', "UNDO"} # Add "UNDO" option here

    input1: bpy.props.IntProperty()
    input2: bpy.props.IntProperty()

    def execute(self, context):
        for xi in range(self.input1):
            x = xi*1.2
            for yi in range(self.input2):
                y = yi*1.2
                bpy.ops.mesh.primitive_cube_add(size=0.5, enter_editmode=False, align='WORLD', location=(x, y, 0))
        return {'FINISHED'}

Postscript

be aware of the error where Blender crashed when undo after executing script in python console. (Reference: Fix T86293: crash undoing after executing the python console in certain)

That was fixed in Blender 2.93, the latest version. And You can fix the error by adding bl_options “UNDO_GROUPED” into the ConsoleExec operation in C:/Program Files/Blender Foundation/Blender 2.83/2.83/scripts/startup/bl_operators/console.py.

class ConsoleExec(Operator):
    """Execute the current console line as a python expression"""
    bl_idname = "console.execute"
    bl_label = "Console Execute"
    bl_options = {"UNDO_GROUPED"} # add undo here

    interactive: BoolProperty(
        options={'SKIP_SAVE'},
    )

    @classmethod
    def poll(cls, context):
        return (context.area and context.area.type == 'CONSOLE')

How To Get Icon List In Blender

GOAL

Today’s goal is to show Icon Viewer that shows the list of icon we can use in Blender and use it.

Environment

Blender2.83(LTS)
Windows10

Method

1. Activate addon “Development Icon Viewer”

2. Open Icon Viewer

Open text Editor and click “Dev” tab.

Click on any icon you like and it’s name will be copied to the clipboard. Then paste it into your editor.

Examples

1. Operator button

def draw(self, context):
    layout = self.layout
    col = layout.column(align=True)
    col.operator("mesh.primitive_monkey_add", icon="MONKEY")

2. EnumProperty

class MyProp(bpy.types.PropertyGroup):
    items = [("id1", "name1", "description 1", "HELP", 0),
             ("id2", "name2", "description 2", "GHOST_ENABLED", 1),
             ("id3", "mame3", "description 3", "FUND", 2),]
    enum: bpy.props.EnumProperty(items=items)

def draw(self, context):
    layout = self.layout
    col = layout.column(align=True)
    col.prop(context.scene.MyProp, "enum")