Python script to generate robot model produces wrong result

Question

Platform = Ubuntu 14.04 64 bit, Blender 2.77a Here's the image:

The following simple script produces an incorrect model is from the book "Learning Robotics Using Python"; so I did not create it and I'm a complete newbie to Blender so I'm totally lost concerning fixing it.

There is no errata mentioning bug corrections on the script, so this forum is my best practical hope. The script runs, but the model generated has a bottom base plate missing and one of the wheels is a cube; the rest is okay.

The script is supposed to produce a robot model comprised of 3 vertically stacked circular plates with 4 poles in between them for support; a cube on the middle plate, two wheels with shafts and motors.

Would someone please show me what is wrong?

Thankfully,

Sam

Here is the script:

=========================

import bpy # contains all the functionalities of Blender and it can only be
       # accessed from inside the Blender application



# This function will draw base plate
def Draw_Base_Plate():


    #Added two cubes for cutting sides of base plate

    bpy.ops.mesh.primitive_cube_add(radius=0.05, location=(0.175,0,0.09))
    bpy.ops.mesh.primitive_cube_add(radius=0.05, location=(-0.175,0,0.09))

   #############################################################################################

    #Adding base plate
    bpy.ops.mesh.primitive_cylinder_add(radius=0.15,depth=0.005, location=(0,0,0.09))

    #Adding booleab difference modifier from first cube

    bpy.ops.object.modifier_add(type='BOOLEAN')
    bpy.context.object.modifiers["Boolean"].operation = 'DIFFERENCE'
    bpy.context.object.modifiers["Boolean"].object =      bpy.data.objects["Cube"]
    bpy.ops.object.modifier_apply(modifier="Boolean")


#############################################################################################

    #Adding booleab difference modifier from second cube

    bpy.ops.object.modifier_add(type='BOOLEAN')
    bpy.context.object.modifiers["Boolean"].operation = 'DIFFERENCE'
    bpy.context.object.modifiers["Boolean"].object =     bpy.data.objects["Cube.001"]
    bpy.ops.object.modifier_apply(modifier="Boolean")


#############################################################################################

    #Deselect cylinder and delete cubes
    bpy.ops.object.select_pattern(pattern="Cube")
    bpy.ops.object.select_pattern(pattern="Cube.001")
    bpy.data.objects['Cylinder'].select = False
    bpy.ops.object.delete(use_global=False)



#This function will draw motors and wheels
def Draw_Motors_Wheels():



    #Create first Wheel

    bpy.ops.mesh.primitive_cylinder_add(radius=0.045,depth=0.01, location=(0,0,0.07))
    #Rotate
    bpy.context.object.rotation_euler[1] = 1.5708
    #Transalation
    bpy.context.object.location[0] = 0.135

    #Create second wheel
    bpy.ops.mesh.primitive_cylinder_add(radius=0.045,depth=0.01, location=(0,0,0.07))
    #Rotate
    bpy.context.object.rotation_euler[1] = 1.5708
    #Transalation
    bpy.context.object.location[0] = -0.135

    #Adding motors

    bpy.ops.mesh.primitive_cylinder_add(radius=0.018,depth=0.06, location=(0.075,0,0.075))
    bpy.context.object.rotation_euler[1] = 1.5708

    bpy.ops.mesh.primitive_cylinder_add(radius=0.018,depth=0.06, location=(-0.075,0,0.075))
    bpy.context.object.rotation_euler[1] = 1.5708


    #Adding motor shaft
    bpy.ops.mesh.primitive_cylinder_add(radius=0.006,depth=0.04, location=(0.12,0,0.075))
    bpy.context.object.rotation_euler[1] = 1.5708

    bpy.ops.mesh.primitive_cylinder_add(radius=0.006,depth=0.04, location=(-0.12,0,0.075))
    bpy.context.object.rotation_euler[1] = 1.5708


#############################################################################################

    #Addubg Caster Wheel

    bpy.ops.mesh.primitive_cylinder_add(radius=0.015,depth=0.05, location=(0,0.125,0.065))
    bpy.ops.mesh.primitive_cylinder_add(radius=0.015,depth=0.05, location=(0,-0.125,0.065))

    #Adding Kinect

    bpy.ops.mesh.primitive_cube_add(radius=0.04, location=(0,0,0.26))    

#Draw middle plate
def Draw_Middle_Plate():
    bpy.ops.mesh.primitive_cylinder_add(radius=0.15,depth=0.005, location=(0,0,0.22))

#Adding top plate
def Draw_Top_Plate():
    bpy.ops.mesh.primitive_cylinder_add(radius=0.15,depth=0.005, location=(0,0,0.37))

#Adding support tubes
def Draw_Support_Tubes():
    #Cylinders
    bpy.ops.mesh.primitive_cylinder_add(radius=0.007,depth=0.30, location=(0.09,0.09,0.23))
    bpy.ops.mesh.primitive_cylinder_add(radius=0.007,depth=0.30, location=(-0.09,0.09,0.23))
    bpy.ops.mesh.primitive_cylinder_add(radius=0.007,depth=0.30, location=(-0.09,-0.09,0.23))
    bpy.ops.mesh.primitive_cylinder_add(radius=0.007,depth=0.30, location=(0.09,-0.09,0.23))

#Exporting into STL    
def Save_to_STL():
    bpy.ops.object.select_all(action='SELECT')
    #    bpy.ops.mesh.select_all(action='TOGGLE')
    bpy.ops.export_mesh.stl(check_existing=True, 
        filepath="/home/sam/Desktop/exported.stl", 
        filter_glob="*.stl", ascii=False, use_mesh_modifiers=True,   
        axis_forward='Y', axis_up='Z', global_scale=1.0)


#Main code

if __name__ == "__main__":
    Draw_Base_Plate()
    Draw_Motors_Wheels()
    Draw_Middle_Plate()
    Draw_Top_Plate()
    Draw_Support_Tubes()
    Save_to_STL()

Answer 1

The issue is in lines such as

bpy.ops.mesh.primitive_cube_add(radius=0.05, location=(0.175,0,0.09))
bpy.ops.mesh.primitive_cube_add(radius=0.05, location=(-0.175,0,0.09))

in the script assumes the two cubes created will be named "Cube" and "Cube.001", as they are later referenced for the boolean modifiers

bpy.context.object.modifiers["Boolean"].object = bpy.data.objects["Cube"]

which is assuming that you don't already have an object in your file named Cube or Cube.001 before running, in which case the newly created cubes will have unique names "Cube.nnn" where nnn is the first available unused name.

When I pasted and ran in a test file with both "Cube" and "Cube.001", already existing in another scene, ended up with a busted base plate and cubes for wheels.

My suggestion would be to a)

make a ref when creating the primitives if they are going to be used as modifier targets etc scene.objects.active gives a reference to the newly created primitive, as would context.active_object, but I've passed the scene to the method. Could pass the context I suppose, which in way is good practice as so much code Ops and Panels etc use method(self, context):

def Draw_Base_Plate(scene=bpy.context.scene):
    #Add cube for cutting sides of base plate

    bpy.ops.mesh.primitive_cube_add(radius=0.05, location=(0.175,0,0.09))
    cube1 = scene.objects.active
    # copy cube1 and link to scene (not required as example howto)
    '''
    cube2 = cube1.copy()
    cube2.location.x = -0.175
    scene.objects.link(cube2)
    '''

    #Adding base plate
    bpy.ops.mesh.primitive_cylinder_add(radius=0.15,depth=0.005, location=(0,0,0.09))
    cyl = scene.objects.active

    #Adding booleab difference modifiers
    for ob in [cube1, cube1]:
        mod = cyl.modifiers.new("FacePlateBool", type='BOOLEAN')
        mod.operation = 'DIFFERENCE'
        mod.object =  ob        
        bpy.ops.object.modifier_apply(modifier=mod.name)
        ob.location.x = -ob.location.x

    #Deselect cylinder and delete cube        
    cyl.select = False
    scene.objects.unlink(cube1)
    bpy.data.objects.remove(cube1)   

or b)

make sure you have no "Cube" or "Cube.nnn" (by renaming) in your file before running script.

Answer 2

When Blender is first invoked via the Unity launcher icon in Ubuntu linux, there is a "cube" object automatically present. (This can be verified by mouse clicking on the "Choose Screen layout" icon to the left of the text box showing "Default" at the top bar of the Blender app; then selecting "Scripting" from the drop-down menu; then typing bpy.context.selected_objects in the python console window at the bottom of the scripting layout and pressing return key on keyboard; the result is [bpy.data.objects['Cube']]).

The example code from the book mentioned above inadvertently uses the object name "Cube" when creating the first cube and then uses the object name "Cube.001" when creating the second cube. Apparently, two different objects with the name "Cube" are then in the objects list. The first "Cube" object in the object list is the automatically displayed cube upon startup of Blender, and the second "Cube" object in the list is the one we create. When deleting the cubes from the base plate cylinder to provide space for wheels, the code line bpy.ops.object.select_pattern(pattern="Cube") deletes the automatically displayed startup cube, and leaves the cube we created as part of the model, so one wheel looks like a cube.

The base plate fails to display due to the conflicts caused by having two "Cube" objects.

There are two cures to this problem:

1. Delete the startup Cube object before executing the book script (which isn't obvious that this would be necessary) by doing: bpy.ops.object.select_pattern(pattern="Cube") and bpy.ops.object.delete(use_global=False) in the Python console of the Scripting layout

or

2. Submit an errata to the book that changes the two cube names created by script to: "Cube.001" and "Cube.002" so the while hassle is avoided (unless objects with names if "Cube.001" and "Cube.002" already exist!); or perhaps an errata that explains the potential problem.