fixed #627 env vars lost in Popen processes

tests/utils.py

@@ -0,0 +1,18 @@
+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+
+from PIL import Image, ImageChops, ImageStat
+
+
+def get_image_diff(image0: Image.Image, image1: Image.Image) -> float:
+    """
+    calculate the percentage of differences between two images
+
+    :param image0 Image.Image: the first frame
+    :param image1 Image.Image: the second frame
+    :rtype float: the percent difference between the two images
+    """
+    difference = ImageChops.difference(image0, image1)
+    difference_stat = ImageStat.Stat(difference)
+    percent_diff = sum(difference_stat.mean) / (len(difference_stat.mean) * 255) * 100
+    return percent_diff