diff --git a/lib/matplotlib/testing/compare.py b/lib/matplotlib/testing/compare.py
index 55d13b7a178c..abd4f4f7a953 100644
--- a/lib/matplotlib/testing/compare.py
+++ b/lib/matplotlib/testing/compare.py
@@ -174,8 +174,13 @@ def comparable_formats():
 
 def convert(filename, cache):
    '''
-   Convert the named file into a png file.
-   Returns the name of the created file.
+   Convert the named file into a png file.  Returns the name of the
+   created file.
+
+   If *cache* is True, the result of the conversion is cached in
+   `~/.matplotlib/test_cache/`.  The caching is based on a hash of the
+   exact contents of the input file.  The is no limit on the size of
+   the cache, so it may need to be manually cleared periodically.
    '''
    base, extension = filename.rsplit('.', 1)
    if extension not in converter:
@@ -278,10 +283,6 @@ def compare_images( expected, actual, tol, in_decorator=False ):
 
    actualImage, expectedImage = crop_to_same(actual, actualImage, expected, expectedImage)
 
-   # normalize the images
-   # expectedImage = image_util.autocontrast( expectedImage, 2 )
-   # actualImage = image_util.autocontrast( actualImage, 2 )
-
    # compare the resulting image histogram functions
    expected_version = version.LooseVersion("1.6")
    found_version = version.LooseVersion(np.__version__)