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key_concepts_algebra.rst

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Key Concepts (algebra mode)

Build123d's algebra mode works on objects of the classes Shape, Part, Sketch and Curve and is based on two concepts:

  1. Object arithmetic
  2. Placement arithmetic

Object arithmetic

  • Creating a box and a cylinder centered at (0, 0, 0)

    b = Box(1, 2, 3)
c = Cylinder(0.2, 5)

  • Fusing a box and a cylinder

    r = Box(1, 2, 3) + Cylinder(0.2, 5)

  • Cutting a cylinder from a box

    r = Box(1, 2, 3) - Cylinder(0.2, 5)

  • Intersecting a box and a cylinder

    r = Box(1, 2, 3) & Cylinder(0.2, 5)

Notes:

  • b, c and r are instances of class Compound and can be viewed with every viewer that can show build123d.Compound objects.
  • A discussion around performance can be found in :ref:`algebra_performance`.
  • A mathematically formal definition of the algebra can be found in :ref:`algebra_definition`.

Placement arithmetic

A Part, Sketch or Curve does not have any location or rotation parameter. The rationale is that an object defines its topology (shape, sizes and its center), but does not know where in space it will be located. Instead, it will be relocated with the * operator onto a plane and to location relative to the plane (similar moved).

The generic forms of object placement are:

  1. Placement on plane or at location relative to XY plane:

    plane * alg_compound
location * alg_compound

2. Placement on the plane and then moved relative to the plane by location (the location is relative to the local coordinate system of the plane).

plane * location * alg_compound

Details can be found in :ref:`location_arithmetics`.

Examples:

  • Box on the XY plane, centered at (0, 0, 0) (both forms are equivalent):

    Plane.XY * Box(1, 2, 3)

Box(1, 2, 3)

    Note: On the XY plane no placement is needed (mathematically Plane.XY * will not change the location of an object).

  • Box on the XY plane centered at (0, 1, 0) (all three are equivalent):

    Plane.XY * Pos(0, 1, 0) * Box(1, 2, 3)

Pos(0, 1, 0) * Box(1, 2, 3)

Pos(Y=1) * Box(1, 2, 3)

    Note: Again, Plane.XY can be omitted.

  • Box on plane Plane.XZ:

    Plane.XZ * Box(1, 2, 3)

  • Box on plane Plane.XZ with a location (X=1, Y=2, Z=3) relative to the XZ plane, i.e., using the x-, y- and z-axis of the XZ plane:

    Plane.XZ * Pos(1, 2, 3) * Box(1, 2, 3)

  • Box on plane Plane.XZ moved to (X=1, Y=2, Z=3) relative to this plane and rotated there by the angles (X=0, Y=100, Z=45) around Plane.XZ axes:

    Plane.XZ * Pos(1, 2, 3) * Rot(0, 100, 45) * Box(1, 2, 3)

Location((1, 2, 3), (0, 100, 45)) * Box(1, 2, 3)

    Note: Pos * Rot is the same as using Location directly

  • Box on plane Plane.XZ rotated on this plane by the angles (X=0, Y=100, Z=45) (using the x-, y- and z-axis of the XZ plane) and then moved to (X=1, Y=2, Z=3) relative to the XZ plane:

    Plane.XZ * Rot(0, 100, 45) * Pos(0,1,2) * Box(1, 2, 3)

Combing both concepts

Object arithmetic and Placement at locations can be combined:

b = Plane.XZ * Rot(X=30) * Box(1, 2, 3) + Plane.YZ * Pos(X=-1) * Cylinder(0.2, 5)

Note: In Python * binds stronger then +, -, &, hence brackets are not needed.