Add exposure module with histogram equalization function

skimage/exposure/__init__.py

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+from exposure import equalize_hist, cumulative_distribution

skimage/exposure/exposure.py

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+import numpy as np
+
+
+__all__ = ['cumulative_distribution', 'equalize_hist']
+
+
+def cumulative_distribution(img, nbins=256):
+    """Return cumulative distribution function (cdf) for the given image.
+
+    Parameters
+    ----------
+    img : array
+        Image array.
+    nbins : int
+        Number of bins for image histogram.
+
+    Returns
+    -------
+    img_cdf : array
+        Values of cumulative distribution function.
+    bin_edges : array
+        Bin edges for cdf. Length is ``len(img_cdf) + 1``.
+
+    References
+    ----------
+    .. [1] http://en.wikipedia.org/wiki/Cumulative_distribution_function
+
+    """
+    hist, bin_edges = np.histogram(img.flat, nbins, density=True)
+    img_cdf = hist.cumsum()
+    return img_cdf, bin_edges
+
+
+def equalize_hist(img, nbins=256, max_intensity=255):
+    """Return image after histogram equalization.
+
+    Parameters
+    ----------
+    img : array
+        Image array.
+    nbins : int
+        Number of bins for image histogram.
+    max_intensity : int
+        Maximum intensity of the returned image.
+
+    Returns
+    -------
+    out : 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
+
+    """
+    cdf, bin_edges = cumulative_distribution(img, nbins)
+    cdf = max_intensity * cdf / cdf[-1]
+    out = np.interp(img.flat, bin_edges[:-1], cdf)
+    return out.reshape(img.shape)
+