drop_to_ground.py

# ##### BEGIN GPL LICENSE BLOCK #####
#
#  This program is free software; you can redistribute it and/or
#  modify it under the terms of the GNU General Public License
#  as published by the Free Software Foundation; either version 2
#  of the License, or (at your option) any later version.
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#  but WITHOUT ANY WARRANTY; without even the implied warranty of
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#  Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
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# ##### END GPL LICENSE BLOCK #####

bl_info = {
    'name': 'Drop to Ground',
    'author': 'Unnikrishnan(kodemax), Florian Meyer(testscreenings)',
    'version': (1, 2),
    "blender": (2, 83, 0),
    'location': '3D View > Toolshelf > Tools Tab',
    'description': 'Drop selected objects on active object',
    'warning': '',
    'wiki_url': 'http://wiki.blender.org/index.php/Extensions:2.6/Py/'
        'Scripts/Object/Drop_to_ground',
    "tracker_url": "https://developer.blender.org/maniphest/task/edit/form/2/",
    'category': 'Object'}


import bpy, bmesh
from mathutils import *
from bpy.types import Operator
from bpy.props import *

def get_align_matrix(location, normal):
    up = Vector((0,0,1))
    angle = normal.angle(up)
    axis = up.cross(normal)
    mat_rot = Matrix.Rotation(angle, 4, axis)
    mat_loc = Matrix.Translation(location)
    mat_align = mat_rot @ mat_loc
    return mat_align

def transform_ground_to_world(layer, ground):
    depsgraph = bpy.context.evaluated_depsgraph_get()
    object_eval = ground.evaluated_get(depsgraph)
    tmpMesh = bpy.data.meshes.new_from_object(object_eval)        
    tmpMesh.transform(ground.matrix_world)
    tmp_ground = bpy.data.objects.new(name='tmpGround', object_data=tmpMesh)
    layer.objects.link(tmp_ground)
    layer.objects.update()
    return tmp_ground

def get_lowest_world_co_from_mesh(ob, mat_parent=None):
    bme = bmesh.new()
    bme.from_mesh(ob.data)
    mat_to_world = ob.matrix_world.copy()
    if mat_parent:
        mat_to_world = mat_parent @ mat_to_world
    lowest=None
    #bme.verts.index_update() #probably not needed
    for v in bme.verts:
        if not lowest:
            lowest = v
        if (mat_to_world @ v.co).z < (mat_to_world @ lowest.co).z:
            lowest = v
    lowest_co = mat_to_world @ lowest.co
    bme.free()
    return lowest_co

def get_lowest_world_co(context, ob, mat_parent=None):
    if ob.type == 'MESH':
        return get_lowest_world_co_from_mesh(ob)

    elif ob.type == 'EMPTY' and ob.dupli_type == 'GROUP':
        if not ob.dupli_group:
            return None

        else:
            lowest_co = None
            for ob_l in ob.dupli_group.objects:
                if ob_l.type == 'MESH':
                    lowest_ob_l = get_lowest_world_co_from_mesh(ob_l, ob.matrix_world)
                    if not lowest_co:
                        lowest_co = lowest_ob_l
                    if lowest_ob_l.z < lowest_co.z:
                        lowest_co = lowest_ob_l

            return lowest_co
def drop_objectsall(self, context):
    ground =  bpy.context.active_object 
    name = ground.name
    
    for obs in bpy.context.scene.objects:
        obs.select_set(state=True)
        if obs.name == name:
            
            obs.select_set(state=False)
    obs2 = context.selected_objects
        
    #obs.remove(ground)
    tmp_ground = transform_ground_to_world(context.scene.collection, ground)
    down = Vector((0, 0, -10000))

    for ob in obs2:
        if self.use_origin:
            lowest_world_co = ob.location
        else:
            lowest_world_co = get_lowest_world_co(context, ob)
        if not lowest_world_co:
            print(ob.type, 'is not supported. Failed to drop', ob.name)
            continue
        is_hit, hit_location, hit_normal, hit_index = tmp_ground.ray_cast(lowest_world_co, down)
        if not is_hit:
            print(ob.name, 'didn\'t hit the ground')
            continue

        # simple drop down
        to_ground_vec =  hit_location - lowest_world_co
        ob.location += to_ground_vec

        # drop with align to hit normal
        if self.align:
            to_center_vec = ob.location - hit_location #vec: hit_loc to origin
            # rotate object to align with face normal
            mat_normal = get_align_matrix(hit_location, hit_normal)
            rot_euler = mat_normal.to_euler()
            mat_ob_tmp = ob.matrix_world.copy().to_3x3()
            mat_ob_tmp.rotate(rot_euler)
            mat_ob_tmp = mat_ob_tmp.to_4x4()
            ob.matrix_world = mat_ob_tmp
            # move_object to hit_location
            ob.location = hit_location
            # move object above surface again
            to_center_vec.rotate(rot_euler)
            ob.location += to_center_vec


    #cleanup
    bpy.ops.object.select_all(action='DESELECT')
    tmp_ground.select_set(state=True)
    bpy.ops.object.delete('EXEC_DEFAULT')
    for ob in obs2:
        ob.select_set(state=True)
 #############################################################################   
    
    
def drop_objects(self, context):
    ground = context.object
    obs = context.selected_objects
    obs.remove(ground)
    tmp_ground = transform_ground_to_world(context.scene.collection, ground)
    down = Vector((0, 0, -10000))

    for ob in obs:
        if self.use_origin:
            lowest_world_co = ob.location
        else:
            lowest_world_co = get_lowest_world_co(context, ob)
        if not lowest_world_co:
            print(ob.type, 'is not supported. Failed to drop', ob.name)
            continue
        is_hit, hit_location, hit_normal, hit_index = tmp_ground.ray_cast(lowest_world_co, down)
        if not is_hit:
            print(ob.name, 'didn\'t hit the ground')
            continue

        # simple drop down
        to_ground_vec =  hit_location - lowest_world_co
        ob.location += to_ground_vec

        # drop with align to hit normal
        if self.align:
            to_center_vec = ob.location - hit_location #vec: hit_loc to origin
            # rotate object to align with face normal
            mat_normal = get_align_matrix(hit_location, hit_normal)
            rot_euler = mat_normal.to_euler()
            mat_ob_tmp = ob.matrix_world.copy().to_3x3()
            mat_ob_tmp.rotate(rot_euler)
            mat_ob_tmp = mat_ob_tmp.to_4x4()
            ob.matrix_world = mat_ob_tmp
            # move_object to hit_location
            ob.location = hit_location
            # move object above surface again
            to_center_vec.rotate(rot_euler)
            ob.location += to_center_vec


    #cleanup
    bpy.ops.object.select_all(action='DESELECT')
    bpy.context.active_object.select_set(state=True)
    bpy.ops.object.delete('EXEC_DEFAULT')
    for ob in obs:
        ob.select_set(state=True)

#################################################################
class OBJECT_OT_drop_to_ground(Operator):
    """Drop selected objects on active object"""
    bl_idname = "object.drop_on_active"
    bl_label = "Drop to Ground"
    bl_options = {'REGISTER', 'UNDO'}
    bl_description = "Drop selected objects on active object"

    align = BoolProperty(
            name="Align to ground",
            description="Aligns the object to the ground",
            default=True)
    use_origin = BoolProperty(
            name="Use Center",
            description="Drop to objects origins",
            default=False)

    ##### POLL #####
    @classmethod
    def poll(cls, context):
        return len(context.selected_objects) >= 2

    ##### EXECUTE #####
    def execute(self, context):
        print('\nDropping Objects')
        drop_objects(self, context)
        return {'FINISHED'}

#################################################################
#################################################################
class OBJECT_OT_drop_all_ground(Operator):
    """Drop selected objects on active object"""
    bl_idname = "object.drop_all_active"
    bl_label = "Drop to Ground"
    bl_options = {'REGISTER', 'UNDO'}
    bl_description = "Drop all objects in the scene to active object"

    align = BoolProperty(
            name="Align to ground",
            description="Aligns the object to the ground",
            default=True)
    use_origin = BoolProperty(
            name="Use Center",
            description="Drop to objects origins",
            default=False)

    ##### POLL #####
   

    ##### EXECUTE #####
    def execute(self, context):
        print('\nDropping Objects')
        drop_objectsall(self, context)
        return {'FINISHED'}

#################################################################        
class PANEL_PT_Operator_Panel(bpy.types.Panel):
    bl_label = "Drop To Ground"
    bl_region_type = 'UI'
    bl_space_type = "VIEW_3D"
    bl_options = {'DEFAULT_CLOSED'}
    bl_context = "objectmode"
    bl_category = "Tools"

    def draw(self,context):
        sce = context.scene
        layout = self.layout
        row = layout.row()
        row = layout.split()
        row.operator(OBJECT_OT_drop_to_ground.bl_idname,
                         text="Drop Selected")
        row = layout.row()
        row.operator(OBJECT_OT_drop_all_ground.bl_idname,
                         text="Drop All")


# register the class
def register():
    bpy.utils.register_class(PANEL_PT_Operator_Panel)
    bpy.utils.register_class(OBJECT_OT_drop_all_ground)
    bpy.utils.register_class(OBJECT_OT_drop_to_ground)

def unregister():
    bpy.utils.unregister_class(PANEL_PT_Operator_Panel)
    bpy.utils.unregister_class(OBJECT_OT_drop_all_ground)
    bpy.utils.unregister_class(OBJECT_OT_drop_to_ground)

if __name__ == "__main__":
    register()

Updated to 2.83c