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image.py
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"""
The :mod:`sklearn.feature_extraction.image` submodule gathers utilities to
extract features from images.
"""
# Authors: Emmanuelle Gouillart <emmanuelle.gouillart@normalesup.org>
# Gael Varoquaux <gael.varoquaux@normalesup.org>
# Olivier Grisel
# Vlad Niculae
# License: BSD 3 clause
from itertools import product
import numbers
import numpy as np
from scipy import sparse
from numpy.lib.stride_tricks import as_strided
from ..utils import check_array, check_random_state
from ..base import BaseEstimator
__all__ = ['PatchExtractor',
'extract_patches_2d',
'grid_to_graph',
'img_to_graph',
'reconstruct_from_patches_2d']
###############################################################################
# From an image to a graph
def _make_edges_3d(n_x, n_y, n_z=1):
"""Returns a list of edges for a 3D image.
Parameters
===========
n_x : integer
The size of the grid in the x direction.
n_y : integer
The size of the grid in the y direction.
n_z : integer, optional
The size of the grid in the z direction, defaults to 1
"""
vertices = np.arange(n_x * n_y * n_z).reshape((n_x, n_y, n_z))
edges_deep = np.vstack((vertices[:, :, :-1].ravel(),
vertices[:, :, 1:].ravel()))
edges_right = np.vstack((vertices[:, :-1].ravel(),
vertices[:, 1:].ravel()))
edges_down = np.vstack((vertices[:-1].ravel(), vertices[1:].ravel()))
edges = np.hstack((edges_deep, edges_right, edges_down))
return edges
def _compute_gradient_3d(edges, img):
n_x, n_y, n_z = img.shape
gradient = np.abs(img[edges[0] // (n_y * n_z),
(edges[0] % (n_y * n_z)) // n_z,
(edges[0] % (n_y * n_z)) % n_z] -
img[edges[1] // (n_y * n_z),
(edges[1] % (n_y * n_z)) // n_z,
(edges[1] % (n_y * n_z)) % n_z])
return gradient
# XXX: Why mask the image after computing the weights?
def _mask_edges_weights(mask, edges, weights=None):
"""Apply a mask to edges (weighted or not)"""
inds = np.arange(mask.size)
inds = inds[mask.ravel()]
ind_mask = np.logical_and(np.in1d(edges[0], inds),
np.in1d(edges[1], inds))
edges = edges[:, ind_mask]
if weights is not None:
weights = weights[ind_mask]
if len(edges.ravel()):
maxval = edges.max()
else:
maxval = 0
order = np.searchsorted(np.unique(edges.ravel()), np.arange(maxval + 1))
edges = order[edges]
if weights is None:
return edges
else:
return edges, weights
def _to_graph(n_x, n_y, n_z, mask=None, img=None,
return_as=sparse.coo_matrix, dtype=None):
"""Auxiliary function for img_to_graph and grid_to_graph
"""
edges = _make_edges_3d(n_x, n_y, n_z)
if dtype is None:
if img is None:
dtype = np.int
else:
dtype = img.dtype
if img is not None:
img = np.atleast_3d(img)
weights = _compute_gradient_3d(edges, img)
if mask is not None:
edges, weights = _mask_edges_weights(mask, edges, weights)
diag = img.squeeze()[mask]
else:
diag = img.ravel()
n_voxels = diag.size
else:
if mask is not None:
mask = mask.astype(dtype=np.bool, copy=False)
mask = np.asarray(mask, dtype=np.bool)
edges = _mask_edges_weights(mask, edges)
n_voxels = np.sum(mask)
else:
n_voxels = n_x * n_y * n_z
weights = np.ones(edges.shape[1], dtype=dtype)
diag = np.ones(n_voxels, dtype=dtype)
diag_idx = np.arange(n_voxels)
i_idx = np.hstack((edges[0], edges[1]))
j_idx = np.hstack((edges[1], edges[0]))
graph = sparse.coo_matrix((np.hstack((weights, weights, diag)),
(np.hstack((i_idx, diag_idx)),
np.hstack((j_idx, diag_idx)))),
(n_voxels, n_voxels),
dtype=dtype)
if return_as is np.ndarray:
return graph.toarray()
return return_as(graph)
def img_to_graph(img, mask=None, return_as=sparse.coo_matrix, dtype=None):
"""Graph of the pixel-to-pixel gradient connections
Edges are weighted with the gradient values.
Read more in the :ref:`User Guide <image_feature_extraction>`.
Parameters
----------
img : ndarray, 2D or 3D
2D or 3D image
mask : ndarray of booleans, optional
An optional mask of the image, to consider only part of the
pixels.
return_as : np.ndarray or a sparse matrix class, optional
The class to use to build the returned adjacency matrix.
dtype : None or dtype, optional
The data of the returned sparse matrix. By default it is the
dtype of img
Notes
-----
For scikit-learn versions 0.14.1 and prior, return_as=np.ndarray was
handled by returning a dense np.matrix instance. Going forward, np.ndarray
returns an np.ndarray, as expected.
For compatibility, user code relying on this method should wrap its
calls in ``np.asarray`` to avoid type issues.
"""
img = np.atleast_3d(img)
n_x, n_y, n_z = img.shape
return _to_graph(n_x, n_y, n_z, mask, img, return_as, dtype)
def grid_to_graph(n_x, n_y, n_z=1, mask=None, return_as=sparse.coo_matrix,
dtype=np.int):
"""Graph of the pixel-to-pixel connections
Edges exist if 2 voxels are connected.
Parameters
----------
n_x : int
Dimension in x axis
n_y : int
Dimension in y axis
n_z : int, optional, default 1
Dimension in z axis
mask : ndarray of booleans, optional
An optional mask of the image, to consider only part of the
pixels.
return_as : np.ndarray or a sparse matrix class, optional
The class to use to build the returned adjacency matrix.
dtype : dtype, optional, default int
The data of the returned sparse matrix. By default it is int
Notes
-----
For scikit-learn versions 0.14.1 and prior, return_as=np.ndarray was
handled by returning a dense np.matrix instance. Going forward, np.ndarray
returns an np.ndarray, as expected.
For compatibility, user code relying on this method should wrap its
calls in ``np.asarray`` to avoid type issues.
"""
return _to_graph(n_x, n_y, n_z, mask=mask, return_as=return_as,
dtype=dtype)
###############################################################################
# From an image to a set of small image patches
def _compute_n_patches(i_h, i_w, p_h, p_w, max_patches=None):
"""Compute the number of patches that will be extracted in an image.
Read more in the :ref:`User Guide <image_feature_extraction>`.
Parameters
----------
i_h : int
The image height
i_w : int
The image with
p_h : int
The height of a patch
p_w : int
The width of a patch
max_patches : integer or float, optional default is None
The maximum number of patches to extract. If max_patches is a float
between 0 and 1, it is taken to be a proportion of the total number
of patches.
"""
n_h = i_h - p_h + 1
n_w = i_w - p_w + 1
all_patches = n_h * n_w
if max_patches:
if (isinstance(max_patches, (numbers.Integral))
and max_patches < all_patches):
return max_patches
elif (isinstance(max_patches, (numbers.Real))
and 0 < max_patches < 1):
return int(max_patches * all_patches)
else:
raise ValueError("Invalid value for max_patches: %r" % max_patches)
else:
return all_patches
def extract_patches(arr, patch_shape=8, extraction_step=1):
"""Extracts patches of any n-dimensional array in place using strides.
Given an n-dimensional array it will return a 2n-dimensional array with
the first n dimensions indexing patch position and the last n indexing
the patch content. This operation is immediate (O(1)). A reshape
performed on the first n dimensions will cause numpy to copy data, leading
to a list of extracted patches.
Read more in the :ref:`User Guide <image_feature_extraction>`.
Parameters
----------
arr : ndarray
n-dimensional array of which patches are to be extracted
patch_shape : integer or tuple of length arr.ndim
Indicates the shape of the patches to be extracted. If an
integer is given, the shape will be a hypercube of
sidelength given by its value.
extraction_step : integer or tuple of length arr.ndim
Indicates step size at which extraction shall be performed.
If integer is given, then the step is uniform in all dimensions.
Returns
-------
patches : strided ndarray
2n-dimensional array indexing patches on first n dimensions and
containing patches on the last n dimensions. These dimensions
are fake, but this way no data is copied. A simple reshape invokes
a copying operation to obtain a list of patches:
result.reshape([-1] + list(patch_shape))
"""
arr_ndim = arr.ndim
if isinstance(patch_shape, numbers.Number):
patch_shape = tuple([patch_shape] * arr_ndim)
if isinstance(extraction_step, numbers.Number):
extraction_step = tuple([extraction_step] * arr_ndim)
patch_strides = arr.strides
slices = [slice(None, None, st) for st in extraction_step]
indexing_strides = arr[slices].strides
patch_indices_shape = ((np.array(arr.shape) - np.array(patch_shape)) //
np.array(extraction_step)) + 1
shape = tuple(list(patch_indices_shape) + list(patch_shape))
strides = tuple(list(indexing_strides) + list(patch_strides))
patches = as_strided(arr, shape=shape, strides=strides)
return patches
def extract_patches_2d(image, patch_size, max_patches=None, random_state=None):
"""Reshape a 2D image into a collection of patches
The resulting patches are allocated in a dedicated array.
Read more in the :ref:`User Guide <image_feature_extraction>`.
Parameters
----------
image : array, shape = (image_height, image_width) or
(image_height, image_width, n_channels)
The original image data. For color images, the last dimension specifies
the channel: a RGB image would have `n_channels=3`.
patch_size : tuple of ints (patch_height, patch_width)
the dimensions of one patch
max_patches : integer or float, optional default is None
The maximum number of patches to extract. If max_patches is a float
between 0 and 1, it is taken to be a proportion of the total number
of patches.
random_state : int, RandomState instance or None, optional (default=None)
Pseudo number generator state used for random sampling to use if
`max_patches` is not None. If int, random_state is the seed used by
the random number generator; If RandomState instance, random_state is
the random number generator; If None, the random number generator is
the RandomState instance used by `np.random`.
Returns
-------
patches : array, shape = (n_patches, patch_height, patch_width) or
(n_patches, patch_height, patch_width, n_channels)
The collection of patches extracted from the image, where `n_patches`
is either `max_patches` or the total number of patches that can be
extracted.
Examples
--------
>>> from sklearn.feature_extraction import image
>>> one_image = np.arange(16).reshape((4, 4))
>>> one_image
array([[ 0, 1, 2, 3],
[ 4, 5, 6, 7],
[ 8, 9, 10, 11],
[12, 13, 14, 15]])
>>> patches = image.extract_patches_2d(one_image, (2, 2))
>>> print(patches.shape)
(9, 2, 2)
>>> patches[0]
array([[0, 1],
[4, 5]])
>>> patches[1]
array([[1, 2],
[5, 6]])
>>> patches[8]
array([[10, 11],
[14, 15]])
"""
i_h, i_w = image.shape[:2]
p_h, p_w = patch_size
if p_h > i_h:
raise ValueError("Height of the patch should be less than the height"
" of the image.")
if p_w > i_w:
raise ValueError("Width of the patch should be less than the width"
" of the image.")
image = check_array(image, allow_nd=True)
image = image.reshape((i_h, i_w, -1))
n_colors = image.shape[-1]
extracted_patches = extract_patches(image,
patch_shape=(p_h, p_w, n_colors),
extraction_step=1)
n_patches = _compute_n_patches(i_h, i_w, p_h, p_w, max_patches)
if max_patches:
rng = check_random_state(random_state)
i_s = rng.randint(i_h - p_h + 1, size=n_patches)
j_s = rng.randint(i_w - p_w + 1, size=n_patches)
patches = extracted_patches[i_s, j_s, 0]
else:
patches = extracted_patches
patches = patches.reshape(-1, p_h, p_w, n_colors)
# remove the color dimension if useless
if patches.shape[-1] == 1:
return patches.reshape((n_patches, p_h, p_w))
else:
return patches
def reconstruct_from_patches_2d(patches, image_size):
"""Reconstruct the image from all of its patches.
Patches are assumed to overlap and the image is constructed by filling in
the patches from left to right, top to bottom, averaging the overlapping
regions.
Read more in the :ref:`User Guide <image_feature_extraction>`.
Parameters
----------
patches : array, shape = (n_patches, patch_height, patch_width) or
(n_patches, patch_height, patch_width, n_channels)
The complete set of patches. If the patches contain colour information,
channels are indexed along the last dimension: RGB patches would
have `n_channels=3`.
image_size : tuple of ints (image_height, image_width) or
(image_height, image_width, n_channels)
the size of the image that will be reconstructed
Returns
-------
image : array, shape = image_size
the reconstructed image
"""
i_h, i_w = image_size[:2]
p_h, p_w = patches.shape[1:3]
img = np.zeros(image_size)
# compute the dimensions of the patches array
n_h = i_h - p_h + 1
n_w = i_w - p_w + 1
for p, (i, j) in zip(patches, product(range(n_h), range(n_w))):
img[i:i + p_h, j:j + p_w] += p
for i in range(i_h):
for j in range(i_w):
# divide by the amount of overlap
# XXX: is this the most efficient way? memory-wise yes, cpu wise?
img[i, j] /= float(min(i + 1, p_h, i_h - i) *
min(j + 1, p_w, i_w - j))
return img
class PatchExtractor(BaseEstimator):
"""Extracts patches from a collection of images
Read more in the :ref:`User Guide <image_feature_extraction>`.
Parameters
----------
patch_size : tuple of ints (patch_height, patch_width)
the dimensions of one patch
max_patches : integer or float, optional default is None
The maximum number of patches per image to extract. If max_patches is a
float in (0, 1), it is taken to mean a proportion of the total number
of patches.
random_state : int, RandomState instance or None, optional (default=None)
If int, random_state is the seed used by the random number generator;
If RandomState instance, random_state is the random number generator;
If None, the random number generator is the RandomState instance used
by `np.random`.
"""
def __init__(self, patch_size=None, max_patches=None, random_state=None):
self.patch_size = patch_size
self.max_patches = max_patches
self.random_state = random_state
def fit(self, X, y=None):
"""Do nothing and return the estimator unchanged
This method is just there to implement the usual API and hence
work in pipelines.
"""
return self
def transform(self, X):
"""Transforms the image samples in X into a matrix of patch data.
Parameters
----------
X : array, shape = (n_samples, image_height, image_width) or
(n_samples, image_height, image_width, n_channels)
Array of images from which to extract patches. For color images,
the last dimension specifies the channel: a RGB image would have
`n_channels=3`.
Returns
-------
patches : array, shape = (n_patches, patch_height, patch_width) or
(n_patches, patch_height, patch_width, n_channels)
The collection of patches extracted from the images, where
`n_patches` is either `n_samples * max_patches` or the total
number of patches that can be extracted.
"""
self.random_state = check_random_state(self.random_state)
n_images, i_h, i_w = X.shape[:3]
X = np.reshape(X, (n_images, i_h, i_w, -1))
n_channels = X.shape[-1]
if self.patch_size is None:
patch_size = i_h // 10, i_w // 10
else:
patch_size = self.patch_size
# compute the dimensions of the patches array
p_h, p_w = patch_size
n_patches = _compute_n_patches(i_h, i_w, p_h, p_w, self.max_patches)
patches_shape = (n_images * n_patches,) + patch_size
if n_channels > 1:
patches_shape += (n_channels,)
# extract the patches
patches = np.empty(patches_shape)
for ii, image in enumerate(X):
patches[ii * n_patches:(ii + 1) * n_patches] = extract_patches_2d(
image, patch_size, self.max_patches, self.random_state)
return patches