Replace ImageMagick-based gamma correction with a own version, update documentation

Author: pnuu

bin/halostack_cli.py

@@ -101,7 +101,9 @@ def halostack_cli(args):
     if not args['no_alignment'] and len(images) > 0:
         view_img = base_img.luminance()
         if isinstance(args['view_gamma'], float):
+            from halostack.image import _scale
             view_img.enhance({'gamma': args['view_gamma']})
+            view_img.img = _scale(view_img.img, bits=8)
         print "\nClick tight area (two opposite corners) for "\
             "reference location.\n"
         args['focus_reference'] = get_two_points(view_img)

doc/source/usage.rst

@@ -235,13 +235,13 @@ Below is an example configuration::
     # average stack from raw/tiff images with view gamma set
     [avg_from_raw]
     avg_stack_file = average.png
-    view_gamma = 1.5
+    view_gamma = 0.45
 
     # average stack from linear raw/tiff images with view gamma set
     # and USM applied to the stack
     [avg_from_raw]
     avg_stack_file = average.png
-    view_gamma = 1.5
+    view_gamma = 0.45
     enhance_stacks = usm:25,2
 
     # B-R processing without stacking
@@ -496,4 +496,19 @@ is equal to::
   -E stretch:0.02,0.98
 
 
+Gamma correction
+++++++++++++++++
+
+Apply gamma correction to the image.  Can be used in either of pre- or
+post-processing.
+
+Syntax::
+
+  - E gamma:0.5
+  - E gamma:2.0
+
+Values less than one makes the image lighter and greather value
+darkens the image.
+
+
 .. _Lefadeux: http://opticsaround.blogspot.fr/2013/03/le-traitement-bleu-moins-rouge-blue.html

halostack/image.py

@@ -289,7 +289,7 @@ def enhance(self, enhancements):
             * ``float``: light source azimuth in degrees [``90``]
             * ``float``: light source elevation in degrees [``45``]
 
-        * ``gamma``: gamma correction using *ImageMagick*
+        * ``gamma``: gamma correction
 
           * possible calls:
 
@@ -752,10 +752,10 @@ def _gamma(self, args):
             return
         if not isinstance(args, list):
             args = [args]
-        self._to_imagemagick()
+        self._to_numpy()
         LOGGER.debug("Apply gamma correction, gamma: %.2lf.", args[0])
-        self.img.gamma(args[0])
-
+        self.img /= self.img.max()
+        self.img **= args[0]
 
 def _blur_worker(data_in):
     '''Worker for blurring rows in parallel.