Vectors

A vector in Taichi can have two forms:

  • as a temporary local variable. An n component vector consists of n scalar values.
  • as an element of a global tensor. In this case, the tensor is an N-dimensional array of n component vectors.

In fact, Vector is simply an alias of Matrix, just with m = 1. See Matrices and Tensors and matrices for more details.

Declaration

As global tensors of vectors

ti.Vector(n, dt, shape = None, offset = None)
Parameters:
  • n – (scalar) the number of components in the vector
  • dt – (DataType) data type of the components
  • shape – (optional, scalar or tuple) shape the tensor of vectors, see Tensors and matrices
  • offset – (optional, scalar or tuple) see Coordinate offsets

For example, this creates a 5x4 tensor of 3 component vectors:

# Python-scope
a = ti.Vector(3, dt=ti.f32, shape=(5, 4))

Note

In Python-scope, ti.var declares Tensors of scalars, while ti.Vector declares tensors of vectors.

As a temporary local variable

ti.Vector([x, y, ...])
Parameters:
  • x – (scalar) the first component of the vector
  • y – (scalar) the second component of the vector

For example, this creates a 3D vector with components (2, 3, 4):

# Taichi-scope
a = ti.Vector([2, 3, 4])

Accessing components

As global tensors of vectors

a[p, q, ...][i]
Parameters:
  • a – (Vector) the vector
  • p – (scalar) index of the first tensor dimension
  • q – (scalar) index of the second tensor dimension
  • i – (scalar) index of the vector component

This extracts the first component of vector a[6, 3]:

x = a[6, 3][0]

# or
vec = a[6, 3]
x = vec[0]

Note

Always use two pairs of square brackets to access scalar elements from tensors of vectors.

  • The indices in the first pair of brackets locate the vector inside the tensor of vectors;
  • The indices in the second pair of brackets locate the scalar element inside the vector.

For 0-D tensors of vectors, indices in the first pair of brackets should be [None].

As a temporary local variable

a[i]
Parameters:
  • a – (Vector) the vector
  • i – (scalar) index of the component

For example, this extracts the first component of vector a:

x = a[0]

This sets the second component of a to 4:

a[1] = 4

TODO: add descriptions about a(i, j)

Methods

a.norm(eps = 0)
Parameters:
  • a – (Vector)
  • eps – (optional, scalar) a safe-guard value for sqrt, usually 0. See the note below.
Returns:

(scalar) the magnitude / length / norm of vector

For example,

a = ti.Vector([3, 4])
a.norm() # sqrt(3*3 + 4*4 + 0) = 5

a.norm(eps) is equivalent to ti.sqrt(a.dot(a) + eps)

Note

Set eps = 1e-5 for example, to safe guard the operator’s gradient on zero vectors during differentiable programming.

a.dot(b)
Parameters:
  • a – (Vector)
  • b – (Vector)
Returns:

(scalar) the dot (inner) product of a and b

E.g.,

a = ti.Vector([1, 3])
b = ti.Vector([2, 4])
a.dot(b) # 1*2 + 3*4 = 14
a.cross(b)
Parameters:
  • a – (Vector, 2 or 3 components)
  • b – (Vector of the same size as a)
Returns:

(scalar (for 2D inputs), or 3D Vector (for 3D inputs)) the cross product of a and b

We use a right-handed coordinate system. E.g.,

a = ti.Vector([1, 2, 3])
b = ti.Vector([4, 5, 6])
c = ti.cross(a, b)
# c = [2*6 - 5*3, 4*3 - 1*6, 1*5 - 4*2] = [-3, 6, -3]

p = ti.Vector([1, 2])
q = ti.Vector([4, 5])
r = ti.cross(a, b)
# r = 1*5 - 4*2 = -3
a.outer_product(b)
Parameters:
  • a – (Vector)
  • b – (Vector)
Returns:

(Matrix) the outer product of a and b

E.g.,

a = ti.Vector([1, 2])
b = ti.Vector([4, 5, 6])
c = ti.outer_product(a, b) # NOTE: c[i, j] = a[i] * b[j]
# c = [[1*4, 1*5, 1*6], [2*4, 2*5, 2*6]]

Note

This have no common with ti.cross. a and b do not have to be 3 or 2 component vectors.

a.cast(dt)
Parameters:
  • a – (Vector)
  • dt – (DataType)
Returns:

(Vector) vector with all components of a casted into type dt

E.g.,

# Taichi-scope
a = ti.Vector([1.6, 2.3])
a.cast(ti.i32) # [2, 3]

Note

Vectors are special matrices with only 1 column. In fact, ti.Vector is just an alias of ti.Matrix.

TODO: add element wise operations docs