text: import warnings
  import numpy as np
  
  from skimage import img_as_float
  from skimage.util.dtype import dtype_range, dtype_limits
  from skimage._shared.utils import deprecated
  
  
  __all__ = ['histogram', 'cumulative_distribution', 'equalize',
             'rescale_intensity', 'adjust_gamma', 'adjust_log', 'adjust_sigmoid']
  
  
  DTYPE_RANGE = dtype_range.copy()
  DTYPE_RANGE.update((d.__name__, limits) for d, limits in dtype_range.items())
  DTYPE_RANGE.update({'uint10': (0, 2**10 - 1),
                      'uint12': (0, 2**12 - 1),
                      'uint14': (0, 2**14 - 1),
                      'bool': dtype_range[np.bool_],
                      'float': dtype_range[np.float64]})
  
  
  def histogram(image, nbins=256):
      """Return histogram of image.
  
      Unlike `numpy.histogram`, this function returns the centers of bins and
      does not rebin integer arrays. For integer arrays, each integer value has
      its own bin, which improves speed and intensity-resolution.
  
      The histogram is computed on the flattened image: for color images, the
      function should be used separately on each channel to obtain a histogram
      for each color channel.
  
      Parameters
      ----------
      image : array
          Input image.
      nbins : int
          Number of bins used to calculate histogram. This value is ignored for
          integer arrays.
  
      Returns
      -------
      hist : array
          The values of the histogram.
      bin_centers : array
          The values at the center of the bins.
  
      Examples
      --------
      >>> from skimage import data, exposure, util
      >>> image = util.img_as_float(data.camera())
      >>> np.histogram(image, bins=2)
      (array([107432, 154712]), array([ 0. ,  0.5,  1. ]))
      >>> exposure.histogram(image, nbins=2)
      (array([107432, 154712]), array([ 0.25,  0.75]))
      """
      sh = image.shape
      if len(sh) == 3 and sh[-1] < 4:
          warnings.warn("This might be a color image. The histogram will be "
                        "computed on the flattened image. You can instead "
                        "apply this function to each color channel.")
  
      # For integer types, histogramming with bincount is more efficient.
      if np.issubdtype(image.dtype, np.integer):
          offset = 0
          if np.min(image) < 0:
              offset = np.min(image)
          hist = np.bincount(image.ravel() - offset)
          bin_centers = np.arange(len(hist)) + offset
  
          # clip histogram to start with a non-zero bin
          idx = np.nonzero(hist)[0][0]
          return hist[idx:], bin_centers[idx:]
      else:
          hist, bin_edges = np.histogram(image.flat, nbins)
          bin_centers = (bin_edges[:-1] + bin_edges[1:]) / 2.
          return hist, bin_centers
  
  
  def cumulative_distribution(image, nbins=256):
      """Return cumulative distribution function (cdf) for the given image.
  
      Parameters
      ----------
      image : array
          Image array.
      nbins : int
          Number of bins for image histogram.
  
      Returns
      -------
      img_cdf : array
          Values of cumulative distribution function.
      bin_centers : array
          Centers of bins.
  
      References
      ----------
      .. [1] http://en.wikipedia.org/wiki/Cumulative_distribution_function
  
      """
      hist, bin_centers = histogram(image, nbins)
      img_cdf = hist.cumsum()
      img_cdf = img_cdf / float(img_cdf[-1])
      return img_cdf, bin_centers
  
  
  @deprecated('equalize_hist')
  def equalize(image, nbins=256):
      return equalize_hist(image, nbins)
  
  
  def equalize_hist(image, nbins=256):
      """Return image after histogram equalization.
  
      Parameters
      ----------
      image : array
          Image array.
      nbins : int
          Number of bins for image histogram.
  
      Returns
      -------
      out : float array
          Image array after histogram equalization.
  
      Notes
      -----
      This function is adapted from [1]_ with the author's permission.
  
      References
      ----------
      .. [1] http://www.janeriksolem.net/2009/06/histogram-equalization-with-python-and.html
      .. [2] http://en.wikipedia.org/wiki/Histogram_equalization
  
      """
      image = img_as_float(image)
      cdf, bin_centers = cumulative_distribution(image, nbins)
      out = np.interp(image.flat, bin_centers, cdf)
      return out.reshape(image.shape)
  
  
  def rescale_intensity(image, in_range=None, out_range=None):
      """Return image after stretching or shrinking its intensity levels.
  
      The image intensities are uniformly rescaled such that the minimum and
      maximum values given by `in_range` match those given by `out_range`.
  
      Parameters
      ----------
      image : array
          Image array.
      in_range : 2-tuple (float, float) or str
          Min and max *allowed* intensity values of input image. If None, the
          *allowed* min/max values are set to the *actual* min/max values in the
          input image. Intensity values outside this range are clipped.
          If string, use data limits of dtype specified by the string.
      out_range : 2-tuple (float, float) or str
          Min and max intensity values of output image. If None, use the min/max
          intensities of the image data type. See `skimage.util.dtype` for
          details. If string, use data limits of dtype specified by the string.
  
      Returns
      -------
      out : array
          Image array after rescaling its intensity. This image is the same dtype
          as the input image.
  
      Examples
      --------
      By default, intensities are stretched to the limits allowed by the dtype:
  
      >>> image = np.array([51, 102, 153], dtype=np.uint8)
      >>> rescale_intensity(image)
      array([  0, 127, 255], dtype=uint8)
  
      It's easy to accidentally convert an image dtype from uint8 to float:
  
      >>> 1.0 * image
      array([  51.,  102.,  153.])
  
      Use `rescale_intensity` to rescale to the proper range for float dtypes:
  
      >>> image_float = 1.0 * image
      >>> rescale_intensity(image_float)
      array([ 0. ,  0.5,  1. ])
  
      To maintain the low contrast of the original, use the `in_range` parameter:
  
      >>> rescale_intensity(image_float, in_range=(0, 255))
      array([ 0.2,  0.4,  0.6])
  
      If the min/max value of `in_range` is more/less than the min/max image
      intensity, then the intensity levels are clipped:
  
      >>> rescale_intensity(image_float, in_range=(0, 102))
      array([ 0.5,  1. ,  1. ])
  
      If you have an image with signed integers but want to rescale the image to
      just the positive range, use the `out_range` parameter:
  
      >>> image = np.array([-10, 0, 10], dtype=np.int8)
      >>> rescale_intensity(image, out_range=(0, 127))
      array([  0,  63, 127], dtype=int8)
  
      """
      dtype = image.dtype.type
  
      if in_range is None:
          imin = np.min(image)
          imax = np.max(image)
      elif in_range in DTYPE_RANGE:
          imin, imax = DTYPE_RANGE[in_range]
      else:
          imin, imax = in_range
  
      if out_range is None or out_range in DTYPE_RANGE:
          out_range = dtype if out_range is None else out_range
          if out_range is None:
              out_range
          omin, omax = DTYPE_RANGE[out_range]
          if imin >= 0:
              omin = 0
      else:
          omin, omax = out_range
  
      image = np.clip(image, imin, imax)
  
      image = (image - imin) / float(imax - imin)
      return dtype(image * (omax - omin) + omin)
  
  
  def _assert_non_negative(image):
  
      if np.any(image < 0):
          raise ValueError('Image Correction methods work correctly only on '
                           'images with non-negative values. Use '
                           'skimage.exposure.rescale_intensity.')
  
  
  def adjust_gamma(image, gamma=1, gain=1):
      """Performs Gamma Correction on the input image.
  
      Also known as Power Law Transform.
      This function transforms the input image pixelwise according to the
      equation ``O = I**gamma`` after scaling each pixel to the range 0 to 1.
  
      Parameters
      ----------
      image : ndarray
          Input image.
      gamma : float
          Non negative real number. Default value is 1.
      gain : float
          The constant multiplier. Default value is 1.
  
      Returns
      -------
      out : ndarray
          Gamma corrected output image.
  
      Notes
      -----
      For gamma greater than 1, the histogram will shift towards left and
      the output image will be darker than the input image.
  
      For gamma less than 1, the histogram will shift towards right and
      the output image will be brighter than the input image.
  
      References
      ----------
      .. [1] http://en.wikipedia.org/wiki/Gamma_correction
  
      """
      _assert_non_negative(image)
      dtype = image.dtype.type
  
      if gamma < 0:
          raise ValueError("Gamma should be a non-negative real number.")
  
      scale = float(dtype_limits(image, True)[1] - dtype_limits(image, True)[0])
  
      out = ((image / scale) ** gamma) * scale * gain
      return dtype(out)
  
  
  def adjust_log(image, gain=1, inv=False):
      """Performs Logarithmic correction on the input image.
  
      This function transforms the input image pixelwise according to the
      equation ``O = gain*log(1 + I)`` after scaling each pixel to the range 0 to 1.
      For inverse logarithmic correction, the equation is ``O = gain*(2**I - 1)``.
  
      Parameters
      ----------
      image : ndarray
          Input image.
      gain : float
          The constant multiplier. Default value is 1.
      inv : float
          If True, it performs inverse logarithmic correction,
          else correction will be logarithmic. Defaults to False.
  
      Returns
      -------
      out : ndarray
          Logarithm corrected output image.
  
      References
      ----------
      .. [1] http://www.ece.ucsb.edu/Faculty/Manjunath/courses/ece178W03/EnhancePart1.pdf
  
      """
      _assert_non_negative(image)
      dtype = image.dtype.type
      scale = float(dtype_limits(image, True)[1] - dtype_limits(image, True)[0])
  
      if inv:
          out = (2 ** (image / scale) - 1) * scale * gain
          return dtype(out)
  
      out = np.log2(1 + image / scale) * scale * gain
      return dtype(out)
  
  
  def adjust_sigmoid(image, cutoff=0.5, gain=10, inv=False):
      """Performs Sigmoid Correction on the input image.
  
      Also known as Contrast Adjustment.
      This function transforms the input image pixelwise according to the
      equation ``O = 1/(1 + exp*(gain*(cutoff - I)))`` after scaling each pixel
      to the range 0 to 1.
  
      Parameters
      ----------
      image : ndarray
          Input image.
      cutoff : float
          Cutoff of the sigmoid function that shifts the characteristic curve
          in horizontal direction. Default value is 0.5.
      gain : float
          The constant multiplier in exponential's power of sigmoid function.
          Default value is 10.
      inv : bool
          If True, returns the negative sigmoid correction. Defaults to False.
  
      Returns
      -------
      out : ndarray
          Sigmoid corrected output image.
  
      References
      ----------
      .. [1] Gustav J. Braun, "Image Lightness Rescaling Using Sigmoidal Contrast
             Enhancement Functions",
             http://www.cis.rit.edu/fairchild/PDFs/PAP07.pdf
  
      """
      _assert_non_negative(image)
      dtype = image.dtype.type
      scale = float(dtype_limits(image, True)[1] - dtype_limits(image, True)[0])
  
      if inv:
          out = (1 - 1 / (1 + np.exp(gain * (cutoff - image/scale)))) * scale
          return dtype(out)
  
      out = (1 / (1 + np.exp(gain * (cutoff - image/scale)))) * scale
      return dtype(out)
  