"""
Adapted from "Contrast Limited Adaptive Histogram Equalization" by Karel
Zuiderveld, Graphics Gems IV, Academic Press, 1994.
http://tog.acm.org/resources/GraphicsGems/
Relicensed with permission of the author under the Modified BSD license.
"""
import math
import numbers
import numpy as np
from .._shared.utils import _supported_float_type
from ..color.adapt_rgb import adapt_rgb, hsv_value
from .exposure import rescale_intensity
from ..util import img_as_uint
NR_OF_GRAY = 2**14 # number of grayscale levels to use in CLAHE algorithm
[文档]
@adapt_rgb(hsv_value)
def equalize_adapthist(image, kernel_size=None, clip_limit=0.01, nbins=256):
"""Contrast Limited Adaptive Histogram Equalization (CLAHE).
An algorithm for local contrast enhancement, that uses histograms computed
over different tile regions of the image. Local details can therefore be
enhanced even in regions that are darker or lighter than most of the image.
Parameters
----------
image : (M[, ...][, C]) ndarray
Input image.
kernel_size : int or array_like, optional
Defines the shape of contextual regions used in the algorithm. If
iterable is passed, it must have the same number of elements as
``image.ndim`` (without color channel). If integer, it is broadcasted
to each `image` dimension. By default, ``kernel_size`` is 1/8 of
``image`` height by 1/8 of its width.
clip_limit : float, optional
Clipping limit, normalized between 0 and 1 (higher values give more
contrast).
nbins : int, optional
Number of gray bins for histogram ("data range").
Returns
-------
out : (M[, ...][, C]) ndarray
Equalized image with float64 dtype.
See Also
--------
equalize_hist, rescale_intensity
Notes
-----
* For color images, the following steps are performed:
- The image is converted to HSV color space
- The CLAHE algorithm is run on the V (Value) channel
- The image is converted back to RGB space and returned
* For RGBA images, the original alpha channel is removed.
.. versionchanged:: 0.17
The values returned by this function are slightly shifted upwards
because of an internal change in rounding behavior.
References
----------
.. [1] http://tog.acm.org/resources/GraphicsGems/
.. [2] https://en.wikipedia.org/wiki/CLAHE#CLAHE
"""
float_dtype = _supported_float_type(image.dtype)
image = img_as_uint(image)
image = np.round(rescale_intensity(image, out_range=(0, NR_OF_GRAY - 1))).astype(
np.min_scalar_type(NR_OF_GRAY)
)
if kernel_size is None:
kernel_size = tuple([max(s // 8, 1) for s in image.shape])
elif isinstance(kernel_size, numbers.Number):
kernel_size = (kernel_size,) * image.ndim
elif len(kernel_size) != image.ndim:
raise ValueError(f'Incorrect value of `kernel_size`: {kernel_size}')
kernel_size = [int(k) for k in kernel_size]
image = _clahe(image, kernel_size, clip_limit, nbins)
image = image.astype(float_dtype, copy=False)
return rescale_intensity(image)
def _clahe(image, kernel_size, clip_limit, nbins):
"""Contrast Limited Adaptive Histogram Equalization.
Parameters
----------
image : (M[, ...]) ndarray
Input image.
kernel_size : int or N-tuple of int
Defines the shape of contextual regions used in the algorithm.
clip_limit : float
Normalized clipping limit between 0 and 1 (higher values give more
contrast).
nbins : int
Number of gray bins for histogram ("data range").
Returns
-------
out : (M[, ...]) ndarray
Equalized image.
The number of "effective" graylevels in the output image is set by `nbins`;
selecting a small value (e.g. 128) speeds up processing and still produces
an output image of good quality. A clip limit of 0 or larger than or equal
to 1 results in standard (non-contrast limited) AHE.
"""
ndim = image.ndim
dtype = image.dtype
# pad the image such that the shape in each dimension
# - is a multiple of the kernel_size and
# - is preceded by half a kernel size
pad_start_per_dim = [k // 2 for k in kernel_size]
pad_end_per_dim = [
(k - s % k) % k + int(np.ceil(k / 2.0))
for k, s in zip(kernel_size, image.shape)
]
image = np.pad(
image,
[[p_i, p_f] for p_i, p_f in zip(pad_start_per_dim, pad_end_per_dim)],
mode='reflect',
)
# determine gray value bins
bin_size = 1 + NR_OF_GRAY // nbins
lut = np.arange(NR_OF_GRAY, dtype=np.min_scalar_type(NR_OF_GRAY))
lut //= bin_size
image = lut[image]
# calculate graylevel mappings for each contextual region
# rearrange image into flattened contextual regions
ns_hist = [int(s / k) - 1 for s, k in zip(image.shape, kernel_size)]
hist_blocks_shape = np.array([ns_hist, kernel_size]).T.flatten()
hist_blocks_axis_order = np.array(
[np.arange(0, ndim * 2, 2), np.arange(1, ndim * 2, 2)]
).flatten()
hist_slices = [slice(k // 2, k // 2 + n * k) for k, n in zip(kernel_size, ns_hist)]
hist_blocks = image[tuple(hist_slices)].reshape(hist_blocks_shape)
hist_blocks = np.transpose(hist_blocks, axes=hist_blocks_axis_order)
hist_block_assembled_shape = hist_blocks.shape
hist_blocks = hist_blocks.reshape((math.prod(ns_hist), -1))
# Calculate actual clip limit
kernel_elements = math.prod(kernel_size)
if clip_limit > 0.0:
clim = int(np.clip(clip_limit * kernel_elements, 1, None))
else:
# largest possible value, i.e., do not clip (AHE)
clim = kernel_elements
hist = np.apply_along_axis(np.bincount, -1, hist_blocks, minlength=nbins)
hist = np.apply_along_axis(clip_histogram, -1, hist, clip_limit=clim)
hist = map_histogram(hist, 0, NR_OF_GRAY - 1, kernel_elements)
hist = hist.reshape(hist_block_assembled_shape[:ndim] + (-1,))
# duplicate leading mappings in each dim
map_array = np.pad(hist, [[1, 1] for _ in range(ndim)] + [[0, 0]], mode='edge')
# Perform multilinear interpolation of graylevel mappings
# using the convention described here:
# https://en.wikipedia.org/w/index.php?title=Adaptive_histogram_
# equalization&oldid=936814673#Efficient_computation_by_interpolation
# rearrange image into blocks for vectorized processing
ns_proc = [int(s / k) for s, k in zip(image.shape, kernel_size)]
blocks_shape = np.array([ns_proc, kernel_size]).T.flatten()
blocks_axis_order = np.array(
[np.arange(0, ndim * 2, 2), np.arange(1, ndim * 2, 2)]
).flatten()
blocks = image.reshape(blocks_shape)
blocks = np.transpose(blocks, axes=blocks_axis_order)
blocks_flattened_shape = blocks.shape
blocks = np.reshape(blocks, (math.prod(ns_proc), math.prod(blocks.shape[ndim:])))
# calculate interpolation coefficients
coeffs = np.meshgrid(
*tuple([np.arange(k) / k for k in kernel_size[::-1]]), indexing='ij'
)
coeffs = [np.transpose(c).flatten() for c in coeffs]
inv_coeffs = [1 - c for dim, c in enumerate(coeffs)]
# sum over contributions of neighboring contextual
# regions in each direction
result = np.zeros(blocks.shape, dtype=np.float32)
for iedge, edge in enumerate(np.ndindex(*([2] * ndim))):
edge_maps = map_array[tuple([slice(e, e + n) for e, n in zip(edge, ns_proc)])]
edge_maps = edge_maps.reshape((math.prod(ns_proc), -1))
# apply map
edge_mapped = np.take_along_axis(edge_maps, blocks, axis=-1)
# interpolate
edge_coeffs = np.prod(
[[inv_coeffs, coeffs][e][d] for d, e in enumerate(edge[::-1])], 0
)
result += (edge_mapped * edge_coeffs).astype(result.dtype)
result = result.astype(dtype)
# rebuild result image from blocks
result = result.reshape(blocks_flattened_shape)
blocks_axis_rebuild_order = np.array(
[np.arange(0, ndim), np.arange(ndim, ndim * 2)]
).T.flatten()
result = np.transpose(result, axes=blocks_axis_rebuild_order)
result = result.reshape(image.shape)
# undo padding
unpad_slices = tuple(
[
slice(p_i, s - p_f)
for p_i, p_f, s in zip(pad_start_per_dim, pad_end_per_dim, image.shape)
]
)
result = result[unpad_slices]
return result
def clip_histogram(hist, clip_limit):
"""Perform clipping of the histogram and redistribution of bins.
The histogram is clipped and the number of excess pixels is counted.
Afterwards the excess pixels are equally redistributed across the
whole histogram (providing the bin count is smaller than the cliplimit).
Parameters
----------
hist : ndarray
Histogram array.
clip_limit : int
Maximum allowed bin count.
Returns
-------
hist : ndarray
Clipped histogram.
"""
# calculate total number of excess pixels
excess_mask = hist > clip_limit
excess = hist[excess_mask]
n_excess = excess.sum() - excess.size * clip_limit
hist[excess_mask] = clip_limit
# Second part: clip histogram and redistribute excess pixels in each bin
bin_incr = n_excess // hist.size # average binincrement
upper = clip_limit - bin_incr # Bins larger than upper set to cliplimit
low_mask = hist < upper
n_excess -= hist[low_mask].size * bin_incr
hist[low_mask] += bin_incr
mid_mask = np.logical_and(hist >= upper, hist < clip_limit)
mid = hist[mid_mask]
n_excess += mid.sum() - mid.size * clip_limit
hist[mid_mask] = clip_limit
while n_excess > 0: # Redistribute remaining excess
prev_n_excess = n_excess
for index in range(hist.size):
under_mask = hist < clip_limit
step_size = max(1, np.count_nonzero(under_mask) // n_excess)
under_mask = under_mask[index::step_size]
hist[index::step_size][under_mask] += 1
n_excess -= np.count_nonzero(under_mask)
if n_excess <= 0:
break
if prev_n_excess == n_excess:
break
return hist
def map_histogram(hist, min_val, max_val, n_pixels):
"""Calculate the equalized lookup table (mapping).
It does so by cumulating the input histogram.
Histogram bins are assumed to be represented by the last array dimension.
Parameters
----------
hist : ndarray
Clipped histogram.
min_val : int
Minimum value for mapping.
max_val : int
Maximum value for mapping.
n_pixels : int
Number of pixels in the region.
Returns
-------
out : ndarray
Mapped intensity LUT.
"""
out = np.cumsum(hist, axis=-1).astype(float)
out *= (max_val - min_val) / n_pixels
out += min_val
np.clip(out, a_min=None, a_max=max_val, out=out)
return out.astype(int)
