Image diff fixture (draft)

image_diff.py

from PIL import ImageChops, ImageDraw, Image
import pytest
import os
import py.path
import math
import operator

def rms_diff(im1, im2):
    """Calculate the root-mean-square difference between two images
    Taken from: http://snipplr.com/view/757/compare-two-pil-images-in-python/
    """
    h1 = im1.histogram()
    h2 = im2.histogram()

    def mean_sqr(a,b):
        if not a:
            a = 0.0
        if not b:
            b = 0.0
        return (a-b)**2

    return math.sqrt(reduce(operator.add, map(mean_sqr, h1, h2))/(im1.size[0]*im1.size[1]))


def create_image():
    im = Image.new('RGB', (100, 100), (0, 0, 0))
    draw = ImageDraw.Draw(im)
    draw.ellipse((10, 10, 90, 90), outline=(0, 0, 255), 
                 fill=(255, 255, 255))
    return im


class ImageDiff:

    def __init__(self, request):
        self.directory = py.path.local(request.node.fspath.dirname) / request.node.fspath.purebasename
        self.expected_name = (request.node.name + '.png') 
        self.expected_filename = self.directory / self.expected_name

    def check(self, im, max_threshold=0.0):
        __tracebackhide__ = True
        local = py.path.local(os.getcwd()) / self.expected_name
        if not self.expected_filename.check(file=1):
            msg = '\nExpecting image at %s, but it does not exist.\n'
            msg += '-> Generating here: %s'
            im.save(str(local))
            pytest.fail(msg % (self.expected_filename, local))
        else:
            expected = Image.open(str(self.expected_filename))
            rms_value = rms_diff(im, expected)
            if rms_value > max_threshold:
                im.save(str(local))
                msg = '\nrms_value %s > max_threshold of %s.\n'
                msg += 'Obtained image saved at %s'
                pytest.fail(msg % (rms_value, max_threshold, str(local)))

@pytest.fixture
def image_diff(request):        
    return ImageDiff(request)

def test_generated_image(image_diff):        
    im = create_image()
    image_diff.check(im)