Hierarchical assemblies

Displaying a complex or animated scene often involves many related interactive objects moving relative to one another according to particular rules.

ZenCad provides the zencad.assemble library and its main tool, zencad.assemble.unit, to make this easier.

Assembly unit (zencad.assemble.unit)

An assembly unit has its own local coordinate system, relative to which its interactive objects and child units are placed. Units can form a tree, with each unit positioned relative to its parent (unit.parent). A unit without a parent is positioned relative to the global coordinate system.

A unit holds two transforms: location and global_location.

• location sets placement relative to the parent. It can be changed directly or with relocate.
• global_location is the placement in global coordinates, used for display. It is derived from the tree of unit.location values and can be updated with location_update, relocate and other operations.

Adding an object

Creates an interactive object from the geometric object obj and attaches it to the unit.

If an interactive object is passed, the unit takes control of its placement.

Signature:

u.add(obj, color=None)

Example:

m = box(10)
i = box(10).right(20)
u.add(m)
u.add(i)

Adding a child unit

Makes u the parent of child. The placement of objects in child and its descendants will then take u's placement into account.

Signature:

u.link(child)

Example:

from zencad.assemble import unit

u = unit()
child = unit()
u.link(child)

Updating global placement

Updates the object's global placement from its local placement and its parent's global placement. view redraws a displayed object at its new placement. deep applies the update recursively to descendants.

Signature:

u.location_update(deep=True, view=True)

Changing local placement

Sets the local placement to location and calls location_update with the deep and view options.

Signature:

u.relocate(location, deep=False, view=True)

Displaying in a scene

Signature:

u.bind_to_scene(scene)

Adds the unit and its descendants to scene. In an ordinary script, display(u) handles the scene binding. Set colors when adding geometry with u.add(obj, color=...).

Assembly example

Both parts are in the coordinate system of base. Moving the parent moves them together while preserving their relative placement:

from zencad import *
from zencad.assemble import unit

base = unit()
base.add(box(20, 10, 3), color=blue)
post = unit(parent=base, location=translate(10, 5, 3))
post.add(cylinder(2, 12), color=yellow)
base.relocate(translate(30, 0, 0), deep=True)

assert tuple(post.global_location.translation()) == (40, 5, 3)
display(base)
show()