set_mesh_origin_to_bottom.py

#
# Object > Set Mesh Origin to Bottom
# A python script compatible with Blender 2.90 API.
#
# Takes the currently selected mesh-type objects,
# and moves the object origin location to the "bottom" of the mesh.
# The bottom is calculated as the average X and Y and the minimum Z.
#
# This is useful in 3D Printing, for example, when importing published
# STL geometry files.  Typically the bottom of the shape is flat and
# meant to align to the build plate.  The center of many published
# files is offset to random position that was convenient to the
# designer, in whatever software they used to export STL data.
#

bl_info = {
    "name": "Set Mesh Origin to Bottom",
    "category": "Object",
    "blender": (2, 90, 0),
}

import bpy
import mathutils
from mathutils import Vector

class ObjectSetMeshOriginToBottom(bpy.types.Operator):
    """Realigns Object's Origin to Mesh's Average X, Y and Minimum Z.""" # tooltip
    bl_idname = "object.set_mesh_origin_to_bottom"                       # unique identifier
    bl_label = "Set Mesh Origin to Bottom"                               # display name
    bl_options = {'REGISTER', 'UNDO'}                                    # enable undo

    def execute(self, context):

        Z = bpy.context.scene.cursor.location

        for ob in context.selected_objects:
            if ob.type == "MESH":
                self.execute_per_mesh(context, ob)

        bpy.context.scene.cursor.location = Z
        
        return {'FINISHED'} # success

    def execute_per_mesh(self, context, ob):

        V = ob.data.vertices
        if not len(V):
            return

        Px = sum(v.co.x for v in V) / len(V)
        Py = sum(v.co.y for v in V) / len(V)
        Pz = min(v.co.z for v in V)
        P = ob.matrix_world @ Vector((Px, Py, Pz))

        context.scene.cursor.location = P
        M = ob.mode
        if M == "EDIT":
            bpy.ops.object.editmode_toggle()
        bpy.ops.object.origin_set(type='ORIGIN_CURSOR', center='MEDIAN')
        if M == "EDIT":
            bpy.ops.object.editmode_toggle()

def menu_func(self, context):
    # blender 2.90+ will hide registered operators unless they are added to a menu
    self.layout.operator(ObjectSetMeshOriginToBottom.bl_idname)

def register():
    bpy.utils.register_class(ObjectSetMeshOriginToBottom)
    bpy.types.VIEW3D_MT_object.append(menu_func)

def unregister():
    bpy.utils.unregister_class(ObjectSetMeshOriginToBottom)

# run the script directly from blenders text editor
# to test the addon without having to install it
#
if __name__ == "__main__":
    register()