Gradient¶

class odl.discr.diff_ops.Gradient(*args, **kwargs)[source]¶

Bases: odl.operator.tensor_ops.PointwiseTensorFieldOperator

Spatial gradient operator for DiscretizedSpace spaces.

Calls helper function finite_diff to calculate each component of the resulting product space element. For the adjoint of the Gradient operator, zero padding is assumed to match the negative Divergence operator

Attributes

adjoint: Adjoint of this operator.
base_space: Base space X of this operator’s domain and range.
domain: Set of objects on which this operator can be evaluated.
inverse: Return the operator inverse.
is_functional: True if this operator’s range is a Field.
is_linear: True if this operator is linear.
range: Set in which the result of an evaluation of this operator lies.

Methods

`_call`(self, x[, out])	Calculate the spatial gradient of `x`.
`derivative`(self[, point])	Return the derivative operator.
`norm`(self[, estimate])	Return the operator norm of this operator.

__init__(self, domain=None, range=None, method='forward', pad_mode='constant', pad_const=0)[source]¶

Initialize a new instance.

Zero padding is assumed for the adjoint of the Gradient operator to match negative Divergence operator.

Parameters

domainDiscretizedSpace, optional

Space of elements which the operator acts on. This is required if range is not given.

rangepower space of DiscretizedSpace, optional

Space of elements to which the operator maps. This is required if domain is not given.

method{‘forward’, ‘backward’, ‘central’}, optional

Finite difference method to be used.

pad_modestring, optional

The padding mode to use outside the domain.

'constant': Fill with pad_const.

'symmetric': Reflect at the boundaries, not doubling the outmost values.

'periodic': Fill in values from the other side, keeping the order.

'order0': Extend constantly with the outmost values (ensures continuity).

'order1': Extend with constant slope (ensures continuity of the first derivative). This requires at least 2 values along each axis where padding is applied.

'order2': Extend with second order accuracy (ensures continuity of the second derivative). This requires at least 3 values along each axis.

pad_constfloat, optional

For pad_mode == 'constant', f assumes pad_const for indices outside the domain of f

Examples

Creating a Gradient operator:

>>> dom = odl.uniform_discr([0, 0], [1, 1], (10, 20))
>>> ran = odl.ProductSpace(dom, dom.ndim)  # 2-dimensional
>>> grad_op = Gradient(dom)
>>> grad_op.range == ran
True
>>> grad_op2 = Gradient(range=ran)
>>> grad_op2.domain == dom
True
>>> grad_op3 = Gradient(domain=dom, range=ran)
>>> grad_op3.domain == dom
True
>>> grad_op3.range == ran
True

Calling the operator:

>>> data = np.array([[ 0., 1., 2., 3., 4.],
...                  [ 0., 2., 4., 6., 8.]])
>>> discr = odl.uniform_discr([0, 0], [2, 5], data.shape)
>>> f = discr.element(data)
>>> grad = Gradient(discr)
>>> grad_f = grad(f)
>>> grad_f[0]
uniform_discr([ 0.,  0.], [ 2.,  5.], (2, 5)).element(
    [[ 0.,  1.,  2.,  3.,  4.],
     [ 0., -2., -4., -6., -8.]]
)
>>> grad_f[1]
uniform_discr([ 0.,  0.], [ 2.,  5.], (2, 5)).element(
    [[ 1.,  1.,  1.,  1., -4.],
     [ 2.,  2.,  2.,  2., -8.]]
)

Verify adjoint:

>>> g = grad.range.element((data, data ** 2))
>>> adj_g = grad.adjoint(g)
>>> adj_g
uniform_discr([ 0.,  0.], [ 2.,  5.], (2, 5)).element(
    [[  0.,  -2.,  -5.,  -8., -11.],
     [  0.,  -5., -14., -23., -32.]]
)
>>> g.inner(grad_f) / f.inner(adj_g)
1.0