.. post:: 2015-11-29
   :tags: nose, nosetests, image, comparison, matplotlib, tests
   :category: tests
   :author: Tim Cera
   :location: Gainesville, FL
   :language: en

Testing Images
--------------
TLDR; I use matplotlib.testing.compare.compare_images instead of matplotlib.testing.decorators.image_comparison

Searching for image testing led to serveral pages using the "image_comparison" decorator in matplotlib.testing.decorators.

Tried to use it, but since a decorator hides the name of the decorated function, nose didn't see that the name began with "test\_" and should be part of the test suite.

Tried to create a decorator that would use "image_comparison" in the background, but wrap the "test_*"" function to keep it's name correctly visible to nose.  I couldn't figure it out.

After looking at the "image_comparison" decorator code found that the heavy lifting is done by a "compare_images" function.  Was able to use that directly with a much easier to understand test.::

    from matplotlib.testing.compare import compare_images
    from pandas.util.testing import TestCase

    from tstoolbox import tstoolbox

    class TestPlot(TestCase):
        def setUp(self):
            # Data to plot and temporary filename for plot image
            self.df = tstoolbox.read('tests/data_sine.csv')
            fp, self.fname = tempfile.mkstemp(suffix='.png')

        def test_sine(self):
            # Plot using function to test
            plt = tstoolbox.plot(input_ts=self.df, ofilename=None)
            plt.savefig(self.fname)

            # Compare against base version...
            # different versions of matplotlib have slightly different fonts
            # set the tolerance pretty high to account for this problem.
            results = compare_images('tests/baseline_images/test_plot/sine.png',
                                     self.fname,
                                     10)

            if results is None:  # the images compare favorably
                return True

            base, ext = os.path.splitext(self.fname)
            os.remove('%s-%s%s' % (base, 'failed-diff', ext))
            print(results)
            assert False