import PIL.ImageChops

    def test_generate_png_compare(self):

        """

        Tests generated PNG identicon against a set of pre-generated samples.

        """

        # Set-up a list of foreground colours (taken from Sigil). Same as used

        # for reference images.

        foreground = ["rgb(45,79,255)",

                      "rgb(254,180,44)",

                      "rgb(226,121,234)",

                      "rgb(30,179,253)",

                      "rgb(232,77,65)",

                      "rgb(49,203,115)",

                      "rgb(141,69,170)"]

        # Set-up a background colour (taken from Sigil). Same as used for

        # reference images.

        background = "rgb(224,224,224)"

        # Set-up parameters equivalent as used for samples.

        width = 200

        height = 200

        padding = (20, 20, 20, 20)

        # Load the reference images, making sure they're in RGB mode.

        test1_ref = PIL.Image.open("tests/samples/test1.png").convert(mode="RGB")

        test2_ref = PIL.Image.open("tests/samples/test2.png").convert(mode="RGB")

        test3_ref = PIL.Image.open("tests/samples/test3.png").convert(mode="RGB")

        # Set-up the Generator.

        generator = Generator(5, 5, foreground=foreground, background=background)

        # Generate first test identicon.

        raw_image = generator.generate("test1", width, height, padding=padding)

        image_stream = BytesIO(raw_image)

        test1 = PIL.Image.open(image_stream)

        # Generate second test identicon.

        raw_image = generator.generate("test2", width, height, padding=padding)

        image_stream = BytesIO(raw_image)

        test2 = PIL.Image.open(image_stream)

        # Generate third test identicon.

        raw_image = generator.generate("test3", width, height, padding=padding)

        image_stream = BytesIO(raw_image)

        test3 = PIL.Image.open(image_stream)

        # Calculate differences between generated identicons and references.

        diff1 = PIL.ImageChops.difference(test1, test1_ref)

        diff2 = PIL.ImageChops.difference(test2, test2_ref)

        diff3 = PIL.ImageChops.difference(test3, test3_ref)

        # Verify that all the diffs are essentially black (i.e. no differences

        # between generated identicons and reference samples).

        expected_extrema = ((0, 0), (0, 0), (0, 0))

        self.assertEqual(diff1.getextrema(), expected_extrema)

        self.assertEqual(diff2.getextrema(), expected_extrema)

        self.assertEqual(diff3.getextrema(), expected_extrema)