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scikit-image/skimage/transform/_geometric.py
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Stefan van der Walt 2566171929 Space code more readably in transform.
2013-06-29 10:55:34 -05:00

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import math
import numpy as np
from scipy import ndimage, spatial
from skimage.util import img_as_float
from ._warps_cy import _warp_fast
from skimage._shared.utils import get_bound_method_class
from skimage._shared import six
class GeometricTransform(object):
"""Perform geometric transformations on a set of coordinates.
"""
def __call__(self, coords):
"""Apply forward transformation.
Parameters
----------
coords : (N, 2) array
Source coordinates.
Returns
-------
coords : (N, 2) array
Transformed coordinates.
"""
raise NotImplementedError()
def inverse(self, coords):
"""Apply inverse transformation.
Parameters
----------
coords : (N, 2) array
Source coordinates.
Returns
-------
coords : (N, 2) array
Transformed coordinates.
"""
raise NotImplementedError()
def residuals(self, src, dst):
"""Determine residuals of transformed destination coordinates.
For each transformed source coordinate the euclidean distance to the
respective destination coordinate is determined.
Parameters
----------
src : (N, 2) array
Source coordinates.
dst : (N, 2) array
Destination coordinates.
Returns
-------
residuals : (N, ) array
Residual for coordinate.
"""
return np.sqrt(np.sum((self(src) - dst)**2, axis=1))
def __add__(self, other):
"""Combine this transformation with another.
"""
raise NotImplementedError()
class ProjectiveTransform(GeometricTransform):
"""Matrix transformation.
Apply a projective transformation (homography) on coordinates.
For each homogeneous coordinate :math:`\mathbf{x} = [x, y, 1]^T`, its
target position is calculated by multiplying with the given matrix,
:math:`H`, to give :math:`H \mathbf{x}`::
[[a0 a1 a2]
[b0 b1 b2]
[c0 c1 1 ]].
E.g., to rotate by theta degrees clockwise, the matrix should be::
[[cos(theta) -sin(theta) 0]
[sin(theta) cos(theta) 0]
[0 0 1]]
or, to translate x by 10 and y by 20::
[[1 0 10]
[0 1 20]
[0 0 1 ]].
Parameters
----------
matrix : (3, 3) array, optional
Homogeneous transformation matrix.
"""
_coeffs = range(8)
def __init__(self, matrix=None):
if matrix is None:
# default to an identity transform
matrix = np.eye(3)
if matrix.shape != (3, 3):
raise ValueError("invalid shape of transformation matrix")
self._matrix = matrix
@property
def _inv_matrix(self):
return np.linalg.inv(self._matrix)
def _apply_mat(self, coords, matrix):
coords = np.array(coords, copy=False, ndmin=2)
x, y = np.transpose(coords)
src = np.vstack((x, y, np.ones_like(x)))
dst = np.dot(src.transpose(), matrix.transpose())
# rescale to homogeneous coordinates
dst[:, 0] /= dst[:, 2]
dst[:, 1] /= dst[:, 2]
return dst[:, :2]
def __call__(self, coords):
return self._apply_mat(coords, self._matrix)
def inverse(self, coords):
"""Apply inverse transformation.
Parameters
----------
coords : (N, 2) array
Source coordinates.
Returns
-------
coords : (N, 2) array
Transformed coordinates.
"""
return self._apply_mat(coords, self._inv_matrix)
def estimate(self, src, dst):
"""Set the transformation matrix with the explicit transformation
parameters.
You can determine the over-, well- and under-determined parameters
with the total least-squares method.
Number of source and destination coordinates must match.
The transformation is defined as::
X = (a0*x + a1*y + a2) / (c0*x + c1*y + 1)
Y = (b0*x + b1*y + b2) / (c0*x + c1*y + 1)
These equations can be transformed to the following form::
0 = a0*x + a1*y + a2 - c0*x*X - c1*y*X - X
0 = b0*x + b1*y + b2 - c0*x*Y - c1*y*Y - Y
which exist for each set of corresponding points, so we have a set of
N * 2 equations. The coefficients appear linearly so we can write
A x = 0, where::
A = [[x y 1 0 0 0 -x*X -y*X -X]
[0 0 0 x y 1 -x*Y -y*Y -Y]
...
...
]
x.T = [a0 a1 a2 b0 b1 b2 c0 c1 c3]
In case of total least-squares the solution of this homogeneous system
of equations is the right singular vector of A which corresponds to the
smallest singular value normed by the coefficient c3.
In case of the affine transformation the coefficients c0 and c1 are 0.
Thus the system of equations is::
A = [[x y 1 0 0 0 -X]
[0 0 0 x y 1 -Y]
...
...
]
x.T = [a0 a1 a2 b0 b1 b2 c3]
Parameters
----------
src : (N, 2) array
Source coordinates.
dst : (N, 2) array
Destination coordinates.
"""
xs = src[:, 0]
ys = src[:, 1]
xd = dst[:, 0]
yd = dst[:, 1]
rows = src.shape[0]
# params: a0, a1, a2, b0, b1, b2, c0, c1
A = np.zeros((rows * 2, 9))
A[:rows, 0] = xs
A[:rows, 1] = ys
A[:rows, 2] = 1
A[:rows, 6] = - xd * xs
A[:rows, 7] = - xd * ys
A[rows:, 3] = xs
A[rows:, 4] = ys
A[rows:, 5] = 1
A[rows:, 6] = - yd * xs
A[rows:, 7] = - yd * ys
A[:rows, 8] = xd
A[rows:, 8] = yd
# Select relevant columns, depending on params
A = A[:, list(self._coeffs) + [8]]
_, _, V = np.linalg.svd(A)
H = np.zeros((3, 3))
# solution is right singular vector that corresponds to smallest
# singular value
H.flat[list(self._coeffs) + [8]] = - V[-1, :-1] / V[-1, -1]
H[2, 2] = 1
self._matrix = H
def __add__(self, other):
"""Combine this transformation with another.
"""
if isinstance(other, ProjectiveTransform):
# combination of the same types result in a transformation of this
# type again, otherwise use general projective transformation
if type(self) == type(other):
tform = self.__class__
else:
tform = ProjectiveTransform
return tform(other._matrix.dot(self._matrix))
else:
raise TypeError("Cannot combine transformations of differing "
"types.")
class AffineTransform(ProjectiveTransform):
"""2D affine transformation of the form::
X = a0*x + a1*y + a2 =
= sx*x*cos(rotation) - sy*y*sin(rotation + shear) + a2
Y = b0*x + b1*y + b2 =
= sx*x*sin(rotation) + sy*y*cos(rotation + shear) + b2
where ``sx`` and ``sy`` are zoom factors in the x and y directions,
and the homogeneous transformation matrix is::
[[a0 a1 a2]
[b0 b1 b2]
[0 0 1]]
Parameters
----------
matrix : (3, 3) array, optional
Homogeneous transformation matrix.
scale : (sx, sy) as array, list or tuple, optional
Scale factors.
rotation : float, optional
Rotation angle in counter-clockwise direction as radians.
shear : float, optional
Shear angle in counter-clockwise direction as radians.
translation : (tx, ty) as array, list or tuple, optional
Translation parameters.
"""
_coeffs = range(6)
def __init__(self, matrix=None, scale=None, rotation=None, shear=None,
translation=None):
params = any(param is not None
for param in (scale, rotation, shear, translation))
if params and matrix is not None:
raise ValueError("You cannot specify the transformation matrix and"
" the implicit parameters at the same time.")
elif matrix is not None:
if matrix.shape != (3, 3):
raise ValueError("Invalid shape of transformation matrix.")
self._matrix = matrix
elif params:
if scale is None:
scale = (1, 1)
if rotation is None:
rotation = 0
if shear is None:
shear = 0
if translation is None:
translation = (0, 0)
sx, sy = scale
self._matrix = np.array([
[sx * math.cos(rotation), -sy * math.sin(rotation + shear), 0],
[sx * math.sin(rotation), sy * math.cos(rotation + shear), 0],
[ 0, 0, 1]
])
self._matrix[0:2, 2] = translation
else:
# default to an identity transform
self._matrix = np.eye(3)
@property
def scale(self):
sx = math.sqrt(self._matrix[0, 0] ** 2 + self._matrix[1, 0] ** 2)
sy = math.sqrt(self._matrix[0, 1] ** 2 + self._matrix[1, 1] ** 2)
return sx, sy
@property
def rotation(self):
return math.atan2(self._matrix[1, 0], self._matrix[0, 0])
@property
def shear(self):
beta = math.atan2(- self._matrix[0, 1], self._matrix[1, 1])
return beta - self.rotation
@property
def translation(self):
return self._matrix[0:2, 2]
class PiecewiseAffineTransform(ProjectiveTransform):
"""2D piecewise affine transformation.
Control points are used to define the mapping. The transform is based on
a Delaunay triangulation of the points to form a mesh. Each triangle is
used to find a local affine transform.
"""
def __init__(self):
self._tesselation = None
self._inverse_tesselation = None
self.affines = []
self.inverse_affines = []
def estimate(self, src, dst):
"""Set the control points with which to perform the piecewise mapping.
Number of source and destination coordinates must match.
Parameters
----------
src : (N, 2) array
Source coordinates.
dst : (N, 2) array
Destination coordinates.
"""
# forward piecewise affine
# triangulate input positions into mesh
self._tesselation = spatial.Delaunay(src)
# find affine mapping from source positions to destination
self.affines = []
for tri in self._tesselation.vertices:
affine = AffineTransform()
affine.estimate(src[tri, :], dst[tri, :])
self.affines.append(affine)
# inverse piecewise affine
# triangulate input positions into mesh
self._inverse_tesselation = spatial.Delaunay(dst)
# find affine mapping from source positions to destination
self.inverse_affines = []
for tri in self._inverse_tesselation.vertices:
affine = AffineTransform()
affine.estimate(dst[tri, :], src[tri, :])
self.inverse_affines.append(affine)
def __call__(self, coords):
"""Apply forward transformation.
Coordinates outside of the mesh will be set to `- 1`.
Parameters
----------
coords : (N, 2) array
Source coordinates.
Returns
-------
coords : (N, 2) array
Transformed coordinates.
"""
out = np.empty_like(coords, np.double)
# determine triangle index for each coordinate
simplex = self._tesselation.find_simplex(coords)
# coordinates outside of mesh
out[simplex == -1, :] = -1
for index in range(len(self._tesselation.vertices)):
# affine transform for triangle
affine = self.affines[index]
# all coordinates within triangle
index_mask = simplex == index
out[index_mask, :] = affine(coords[index_mask, :])
return out
def inverse(self, coords):
"""Apply inverse transformation.
Coordinates outside of the mesh will be set to `- 1`.
Parameters
----------
coords : (N, 2) array
Source coordinates.
Returns
-------
coords : (N, 2) array
Transformed coordinates.
"""
out = np.empty_like(coords, np.double)
# determine triangle index for each coordinate
simplex = self._inverse_tesselation.find_simplex(coords)
# coordinates outside of mesh
out[simplex == -1, :] = -1
for index in range(len(self._inverse_tesselation.vertices)):
# affine transform for triangle
affine = self.inverse_affines[index]
# all coordinates within triangle
index_mask = simplex == index
out[index_mask, :] = affine(coords[index_mask, :])
return out
class SimilarityTransform(ProjectiveTransform):
"""2D similarity transformation of the form::
X = a0*x - b0*y + a1 =
= m*x*cos(rotation) + m*y*sin(rotation) + a1
Y = b0*x + a0*y + b1 =
= m*x*sin(rotation) + m*y*cos(rotation) + b1
where ``m`` is a zoom factor and the homogeneous transformation matrix is::
[[a0 b0 a1]
[b0 a0 b1]
[0 0 1]]
Parameters
----------
matrix : (3, 3) array, optional
Homogeneous transformation matrix.
scale : float, optional
Scale factor.
rotation : float, optional
Rotation angle in counter-clockwise direction as radians.
translation : (tx, ty) as array, list or tuple, optional
x, y translation parameters.
"""
def __init__(self, matrix=None, scale=None, rotation=None,
translation=None):
params = any(param is not None
for param in (scale, rotation, translation))
if params and matrix is not None:
raise ValueError("You cannot specify the transformation matrix and"
" the implicit parameters at the same time.")
elif matrix is not None:
if matrix.shape != (3, 3):
raise ValueError("Invalid shape of transformation matrix.")
self._matrix = matrix
elif params:
if scale is None:
scale = 1
if rotation is None:
rotation = 0
if translation is None:
translation = (0, 0)
self._matrix = np.array([
[math.cos(rotation), - math.sin(rotation), 0],
[math.sin(rotation), math.cos(rotation), 0],
[ 0, 0, 1]
])
self._matrix *= scale
self._matrix[0:2, 2] = translation
else:
# default to an identity transform
self._matrix = np.eye(3)
def estimate(self, src, dst):
"""Set the transformation matrix with the explicit parameters.
You can determine the over-, well- and under-determined parameters
with the total least-squares method.
Number of source and destination coordinates must match.
The transformation is defined as::
X = a0*x - b0*y + a1
Y = b0*x + a0*y + b1
These equations can be transformed to the following form::
0 = a0*x - b0*y + a1 - X
0 = b0*x + a0*y + b1 - Y
which exist for each set of corresponding points, so we have a set of
N * 2 equations. The coefficients appear linearly so we can write
A x = 0, where::
A = [[x 1 -y 0 -X]
[y 0 x 1 -Y]
...
...
]
x.T = [a0 a1 b0 b1 c3]
In case of total least-squares the solution of this homogeneous system
of equations is the right singular vector of A which corresponds to the
smallest singular value normed by the coefficient c3.
Parameters
----------
src : (N, 2) array
Source coordinates.
dst : (N, 2) array
Destination coordinates.
"""
xs = src[:, 0]
ys = src[:, 1]
xd = dst[:, 0]
yd = dst[:, 1]
rows = src.shape[0]
# params: a0, a1, b0, b1
A = np.zeros((rows * 2, 5))
A[:rows, 0] = xs
A[:rows, 2] = - ys
A[:rows, 1] = 1
A[rows:, 2] = xs
A[rows:, 0] = ys
A[rows:, 3] = 1
A[:rows, 4] = xd
A[rows:, 4] = yd
_, _, V = np.linalg.svd(A)
# solution is right singular vector that corresponds to smallest
# singular value
a0, a1, b0, b1 = - V[-1, :-1] / V[-1, -1]
self._matrix = np.array([[a0, -b0, a1],
[b0, a0, b1],
[ 0, 0, 1]])
@property
def scale(self):
if math.cos(self.rotation) == 0:
# sin(self.rotation) == 1
scale = self._matrix[0, 1]
else:
scale = self._matrix[0, 0] / math.cos(self.rotation)
return scale
@property
def rotation(self):
return math.atan2(self._matrix[1, 0], self._matrix[1, 1])
@property
def translation(self):
return self._matrix[0:2, 2]
class PolynomialTransform(GeometricTransform):
"""2D transformation of the form::
X = sum[j=0:order]( sum[i=0:j]( a_ji * x**(j - i) * y**i ))
Y = sum[j=0:order]( sum[i=0:j]( b_ji * x**(j - i) * y**i ))
Parameters
----------
params : (2, N) array, optional
Polynomial coefficients where `N * 2 = (order + 1) * (order + 2)`. So,
a_ji is defined in `params[0, :]` and b_ji in `params[1, :]`.
"""
def __init__(self, params=None):
if params is None:
# default to transformation which preserves original coordinates
params = np.array([[0, 1, 0], [0, 0, 1]])
if params.shape[0] != 2:
raise ValueError("invalid shape of transformation parameters")
self._params = params
def estimate(self, src, dst, order=2):
"""Set the transformation matrix with the explicit transformation
parameters.
You can determine the over-, well- and under-determined parameters
with the total least-squares method.
Number of source and destination coordinates must match.
The transformation is defined as::
X = sum[j=0:order]( sum[i=0:j]( a_ji * x**(j - i) * y**i ))
Y = sum[j=0:order]( sum[i=0:j]( b_ji * x**(j - i) * y**i ))
These equations can be transformed to the following form::
0 = sum[j=0:order]( sum[i=0:j]( a_ji * x**(j - i) * y**i )) - X
0 = sum[j=0:order]( sum[i=0:j]( b_ji * x**(j - i) * y**i )) - Y
which exist for each set of corresponding points, so we have a set of
N * 2 equations. The coefficients appear linearly so we can write
A x = 0, where::
A = [[1 x y x**2 x*y y**2 ... 0 ... 0 -X]
[0 ... 0 1 x y x**2 x*y y**2 -Y]
...
...
]
x.T = [a00 a10 a11 a20 a21 a22 ... ann
b00 b10 b11 b20 b21 b22 ... bnn c3]
In case of total least-squares the solution of this homogeneous system
of equations is the right singular vector of A which corresponds to the
smallest singular value normed by the coefficient c3.
Parameters
----------
src : (N, 2) array
Source coordinates.
dst : (N, 2) array
Destination coordinates.
order : int, optional
Polynomial order (number of coefficients is order + 1).
"""
xs = src[:, 0]
ys = src[:, 1]
xd = dst[:, 0]
yd = dst[:, 1]
rows = src.shape[0]
# number of unknown polynomial coefficients
u = (order + 1) * (order + 2)
A = np.zeros((rows * 2, u + 1))
pidx = 0
for j in range(order + 1):
for i in range(j + 1):
A[:rows, pidx] = xs ** (j - i) * ys ** i
A[rows:, pidx + u / 2] = xs ** (j - i) * ys ** i
pidx += 1
A[:rows, -1] = xd
A[rows:, -1] = yd
_, _, V = np.linalg.svd(A)
# solution is right singular vector that corresponds to smallest
# singular value
params = - V[-1, :-1] / V[-1, -1]
self._params = params.reshape((2, u / 2))
def __call__(self, coords):
"""Apply forward transformation.
Parameters
----------
coords : (N, 2) array
source coordinates
Returns
-------
coords : (N, 2) array
Transformed coordinates.
"""
x = coords[:, 0]
y = coords[:, 1]
u = len(self._params.ravel())
# number of coefficients -> u = (order + 1) * (order + 2)
order = int((- 3 + math.sqrt(9 - 4 * (2 - u))) / 2)
dst = np.zeros(coords.shape)
pidx = 0
for j in range(order + 1):
for i in range(j + 1):
dst[:, 0] += self._params[0, pidx] * x ** (j - i) * y ** i
dst[:, 1] += self._params[1, pidx] * x ** (j - i) * y ** i
pidx += 1
return dst
def inverse(self, coords):
raise Exception(
'There is no explicit way to do the inverse polynomial '
'transformation. Instead, estimate the inverse transformation '
'parameters by exchanging source and destination coordinates,'
'then apply the forward transformation.')
TRANSFORMS = {
'similarity': SimilarityTransform,
'affine': AffineTransform,
'piecewise-affine': PiecewiseAffineTransform,
'projective': ProjectiveTransform,
'polynomial': PolynomialTransform,
}
HOMOGRAPHY_TRANSFORMS = (
SimilarityTransform,
AffineTransform,
ProjectiveTransform
)
def estimate_transform(ttype, src, dst, **kwargs):
"""Estimate 2D geometric transformation parameters.
You can determine the over-, well- and under-determined parameters
with the total least-squares method.
Number of source and destination coordinates must match.
Parameters
----------
ttype : {'similarity', 'affine', 'piecewise-affine', 'projective', \
'polynomial'}
Type of transform.
kwargs : array or int
Function parameters (src, dst, n, angle)::
NAME / TTYPE FUNCTION PARAMETERS
'similarity' `src, `dst`
'affine' `src, `dst`
'piecewise-affine' `src, `dst`
'projective' `src, `dst`
'polynomial' `src, `dst`, `order` (polynomial order,
default order is 2)
Also see examples below.
Returns
-------
tform : :class:`GeometricTransform`
Transform object containing the transformation parameters and providing
access to forward and inverse transformation functions.
Examples
--------
>>> import numpy as np
>>> from skimage import transform as tf
>>> # estimate transformation parameters
>>> src = np.array([0, 0, 10, 10]).reshape((2, 2))
>>> dst = np.array([12, 14, 1, -20]).reshape((2, 2))
>>> tform = tf.estimate_transform('similarity', src, dst)
>>> tform.inverse(tform(src)) # == src
>>> # warp image using the estimated transformation
>>> from skimage import data
>>> image = data.camera()
>>> warp(image, inverse_map=tform.inverse)
>>> # create transformation with explicit parameters
>>> tform2 = tf.SimilarityTransform(scale=1.1, rotation=1,
... translation=(10, 20))
>>> # unite transformations, applied in order from left to right
>>> tform3 = tform + tform2
>>> tform3(src) # == tform2(tform(src))
"""
ttype = ttype.lower()
if ttype not in TRANSFORMS:
raise ValueError('the transformation type \'%s\' is not'
'implemented' % ttype)
tform = TRANSFORMS[ttype]()
tform.estimate(src, dst, **kwargs)
return tform
def matrix_transform(coords, matrix):
"""Apply 2D matrix transform.
Parameters
----------
coords : (N, 2) array
x, y coordinates to transform
matrix : (3, 3) array
Homogeneous transformation matrix.
Returns
-------
coords : (N, 2) array
Transformed coordinates.
"""
return ProjectiveTransform(matrix)(coords)
def _stackcopy(a, b):
"""Copy b into each color layer of a, such that::
a[:,:,0] = a[:,:,1] = ... = b
Parameters
----------
a : (M, N) or (M, N, P) ndarray
Target array.
b : (M, N)
Source array.
Notes
-----
Color images are stored as an ``(M, N, 3)`` or ``(M, N, 4)`` arrays.
"""
if a.ndim == 3:
a[:] = b[:, :, np.newaxis]
else:
a[:] = b
def warp_coords(coord_map, shape, dtype=np.float64):
"""Build the source coordinates for the output pixels of an image warp.
Parameters
----------
coord_map : callable like GeometricTransform.inverse
Return input coordinates for given output coordinates.
Coordinates are in the shape (P, 2), where P is the number
of coordinates and each element is a ``(x, y)`` pair.
shape : tuple
Shape of output image ``(rows, cols[, bands])``.
dtype : np.dtype or string
dtype for return value (sane choices: float32 or float64).
Returns
-------
coords : (ndim, rows, cols[, bands]) array of dtype `dtype`
Coordinates for `scipy.ndimage.map_coordinates`, that will yield
an image of shape (orows, ocols, bands) by drawing from source
points according to the `coord_transform_fn`.
Notes
-----
This is a lower-level routine that produces the source coordinates used by
`warp()`.
It is provided separately from `warp` to give additional flexibility to
users who would like, for example, to re-use a particular coordinate
mapping, to use specific dtypes at various points along the the
image-warping process, or to implement different post-processing logic
than `warp` performs after the call to `ndimage.map_coordinates`.
Examples
--------
Produce a coordinate map that Shifts an image up and to the right:
>>> from skimage import data
>>> from scipy.ndimage import map_coordinates
>>>
>>> def shift_up10_left20(xy):
... return xy - np.array([-20, 10])[None, :]
>>>
>>> image = data.lena().astype(np.float32)
>>> coords = warp_coords(shift_up10_left20, image.shape)
>>> warped_image = map_coordinates(image, coords)
"""
rows, cols = shape[0], shape[1]
coords_shape = [len(shape), rows, cols]
if len(shape) == 3:
coords_shape.append(shape[2])
coords = np.empty(coords_shape, dtype=dtype)
# Reshape grid coordinates into a (P, 2) array of (x, y) pairs
tf_coords = np.indices((cols, rows), dtype=dtype).reshape(2, -1).T
# Map each (x, y) pair to the source image according to
# the user-provided mapping
tf_coords = coord_map(tf_coords)
# Reshape back to a (2, M, N) coordinate grid
tf_coords = tf_coords.T.reshape((-1, cols, rows)).swapaxes(1, 2)
# Place the y-coordinate mapping
_stackcopy(coords[1, ...], tf_coords[0, ...])
# Place the x-coordinate mapping
_stackcopy(coords[0, ...], tf_coords[1, ...])
if len(shape) == 3:
coords[2, ...] = range(shape[2])
return coords
def warp(image, inverse_map=None, map_args={}, output_shape=None, order=1,
mode='constant', cval=0., reverse_map=None):
"""Warp an image according to a given coordinate transformation.
Parameters
----------
image : 2-D or 3-D array
Input image.
inverse_map : transformation object, callable ``xy = f(xy, **kwargs)``
Inverse coordinate map. A function that transforms a (N, 2) array of
``(x, y)`` coordinates in the *output image* into their corresponding
coordinates in the *source image* (e.g. a transformation object or its
inverse).
map_args : dict, optional
Keyword arguments passed to `inverse_map`.
output_shape : tuple (rows, cols), optional
Shape of the output image generated. By default the shape of the input
image is preserved.
order : int, optional
The order of the spline interpolation, default is 3. The order has to
be in the range 0-5.
mode : string, optional
Points outside the boundaries of the input are filled according
to the given mode ('constant', 'nearest', 'reflect' or 'wrap').
cval : float, optional
Used in conjunction with mode 'constant', the value outside
the image boundaries.
Examples
--------
Shift an image to the right:
>>> from skimage.transform import warp
>>> from skimage import data
>>> image = data.camera()
>>>
>>> def shift_right(xy):
... xy[:, 0] -= 10
... return xy
>>>
>>> warp(image, shift_right)
Use a geometric transform to warp an image:
>>> from skimage.transform import SimilarityTransform
>>> tform = SimilarityTransform(scale=0.1, rotation=0.1)
>>> warp(image, tform)
"""
# Backward API compatibility
if reverse_map is not None:
inverse_map = reverse_map
if image.ndim < 2:
raise ValueError("Input must have more than 1 dimension.")
orig_ndim = image.ndim
image = np.atleast_3d(img_as_float(image))
ishape = np.array(image.shape)
bands = ishape[2]
out = None
# use fast Cython version for specific interpolation orders
if order in range(4) and not map_args:
matrix = None
if inverse_map in HOMOGRAPHY_TRANSFORMS:
matrix = inverse_map._matrix
elif hasattr(inverse_map, '__name__') \
and inverse_map.__name__ == 'inverse' \
and get_bound_method_class(inverse_map) in HOMOGRAPHY_TRANSFORMS:
matrix = np.linalg.inv(six.get_method_self(inverse_map)._matrix)
if matrix is not None:
# transform all bands
dims = []
for dim in range(image.shape[2]):
dims.append(_warp_fast(image[..., dim], matrix,
output_shape=output_shape,
order=order, mode=mode, cval=cval))
out = np.dstack(dims)
if orig_ndim == 2:
out = out[..., 0]
if out is None: # use ndimage.map_coordinates
if output_shape is None:
output_shape = ishape
rows, cols = output_shape[:2]
def coord_map(*args):
return inverse_map(*args, **map_args)
coords = warp_coords(coord_map, (rows, cols, bands))
# Prefilter not necessary for order 1 interpolation
prefilter = order > 1
out = ndimage.map_coordinates(image, coords, prefilter=prefilter,
mode=mode, order=order, cval=cval)
# The spline filters sometimes return results outside [0, 1],
# so clip to ensure valid data
clipped = np.clip(out, 0, 1)
if mode == 'constant' and not (0 <= cval <= 1):
clipped[out == cval] = cval
if clipped.ndim == 3 and orig_ndim == 2:
# remove singleton dim introduced by atleast_3d
return clipped[..., 0]
else:
return clipped
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