From 391b2edad6f12116191bb7bb27f86ffb787549ac Mon Sep 17 00:00:00 2001
From: Ankit Agrawal <aaaagrawal@iitb.ac.in>
Date: Tue, 16 Apr 2013 06:38:11 +0530
Subject: [PATCH] Added a function for Gamma Correction

---
 skimage/exposure/__init__.py            |  4 ++-
 skimage/exposure/exposure.py            | 41 ++++++++++++++++++++++++-
 skimage/exposure/tests/test_exposure.py | 30 ++++++++++++++++++
 3 files changed, 73 insertions(+), 2 deletions(-)

diff --git a/skimage/exposure/__init__.py b/skimage/exposure/__init__.py
index aba05dae..c1789ab5 100644
--- a/skimage/exposure/__init__.py
+++ b/skimage/exposure/__init__.py
@@ -1,5 +1,6 @@
 from .exposure import histogram, equalize, equalize_hist, \
                       rescale_intensity, cumulative_distribution
+
 from ._adapthist import equalize_adapthist
 
 __all__ = ['histogram',
@@ -7,4 +8,5 @@ __all__ = ['histogram',
            'equalize_hist',
            'equalize_adapthist',
            'rescale_intensity',
-           'cumulative_distribution']
+           'cumulative_distribution',
+           'gammaCorrect']
diff --git a/skimage/exposure/exposure.py b/skimage/exposure/exposure.py
index 84462931..7e86c351 100644
--- a/skimage/exposure/exposure.py
+++ b/skimage/exposure/exposure.py
@@ -1,7 +1,7 @@
 import warnings
 import numpy as np
 
-from skimage import img_as_float
+from skimage import img_as_float, img_as_ubyte
 from skimage.util.dtype import dtype_range
 import skimage.color as color
 from skimage.util.dtype import convert
@@ -216,3 +216,42 @@ def rescale_intensity(image, in_range=None, out_range=None):
 
     image = (image - imin) / float(imax - imin)
     return dtype(image * (omax - omin) + omin)
+
+
+def gammaCorrect(image, gamma = 1):
+    """Transform an image according to Gamma Correction also known as the
+    Power Law Transform.
+
+    Ref :: http://en.wikipedia.org/wiki/Gamma_correction
+
+    Parameters
+    ----------
+    image : ndarray
+        Input image.
+    gamma : float
+        Non-negative real number
+
+    Returns
+    -------
+    out : ndarray
+        Image with Gamma Correction on the input image.
+
+    Notes
+    -----
+    This function transforms the input image pixelwise according to the
+    equation O = I**gamma after scaling each pixel in the range 0 to 1.
+
+    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.
+
+    """
+
+    if gamma < 0:
+        return "Gamma should be a non-negative real number"
+    scale = 255.0
+    temp = img_as_ubyte(image)
+    out = ((temp/scale)**gamma)*scale
+    return out
diff --git a/skimage/exposure/tests/test_exposure.py b/skimage/exposure/tests/test_exposure.py
index a5f5aef9..79b866b8 100644
--- a/skimage/exposure/tests/test_exposure.py
+++ b/skimage/exposure/tests/test_exposure.py
@@ -175,3 +175,33 @@ def norm_brightness_err(img1, img2):
 if __name__ == '__main__':
     from numpy import testing
     testing.run_module_suite()
+
+
+# Test Gamma Correction
+# =====================
+
+def test_gammaCorrect_one():
+    """Same image should be returned for gamma equal to one"""
+    image = skimage.data.camera()
+    result = exposure.gammaCorrect(image, 1)
+    assert result.mean() == image.mean()
+    assert result.std() == image.std()
+
+def test_gammaCorrect_zero():
+    """White image should be returned for gamma less than zero"""
+    image = skimage.img_as_float(data.camera())
+    result = exposure.gammaCorrect(image, 0)
+    assert result.mean() == 255
+    assert result.std() == 0
+
+def test_gammaCorrect_less_one():
+    """Output's mean should be greater than input's mean for gamma less than one"""
+    image = skimage.data.camera()
+    result = exposure.gammaCorrect(image, 0.5)
+    assert result.mean() > image.mean()
+
+def test_gammaCorrect_greater_one():
+    """Output's mean should be less than input's mean for gamma greater than one"""
+    image = skimage.data.camera()
+    result = exposure.gammaCorrect(image, 2)
+    assert result.mean() < image.mean()