new_contents: """
  ================
  Phase Unwrapping
  ================
  
  Some signals can only be observed modulo 2*pi, and this can also apply to
  two- and three dimensional images. In these cases phase unwrapping is
  needed to recover the underlying, unwrapped signal. In this example we will
  demonstrate an algorithm [1]_ implemented in ``skimage`` at work for such a
  problem.
  """
  
  import numpy as np
  from matplotlib import pyplot as plt
  from skimage import data, img_as_float, color, exposure
  
  
  # Load an image as a floating-point grayscale
  image = color.rgb2gray(img_as_float(data.chelsea()))
  # Scale the image to [0, 4*pi]
  image = exposure.rescale_intensity(image, out_range=(0, 4 * np.pi))
  # Create a phase-wrapped image in the interval [-pi, pi)
  image_wrapped = np.angle(np.exp(1j * image))
  # Perform phase unwrapping
  image_unwrapped = exposure.unwrap(image_wrapped)
  
  # Plotting
  plt.figure()
  plt.gray()    # grayscale colormap as default
  imkwargs = dict(vmin=0, vmax=4 * np.pi)
  plt.subplot(221)
  plt.title('Original')
  plt.imshow(image, **imkwargs)
  plt.colorbar()
  
  plt.subplot(222)
  plt.title('Wrapped phase')
  plt.imshow(image_wrapped, vmin=-np.pi, vmax=np.pi)
  plt.colorbar()
  
  plt.subplot(223)
  plt.title('After phase unwrapping')
  plt.imshow(image_unwrapped)
  plt.colorbar()
  
  plt.subplot(224)
  plt.title('Unwrapped minus original')
  plt.imshow(image_unwrapped - image)
  plt.colorbar()
  
  plt.show()
  
  
  """
  .. image:: PLOT2RST.current_figure
  
  References
  ----------
  
  .. [1] Miguel Arevallilo Herraez, David R. Burton, Michael J. Lalor,
         and Munther A. Gdeisat, "Fast two-dimensional phase-unwrapping
         algorithm based on sorting by reliability following a noncontinuous
         path", Journal Applied Optics, Vol. 41, No. 35, pp. 7437, 2002
  """
  
subject: Add example for phase unwrapping.