If using Selenium's Python bindings, you can take a screenshot from Selenium and convert it to OpenCV format like this:

    cv2.imdecode(
        numpy.asarray(
            bytearray(base64.decodestring(driver.get_screenshot_as_base64())),
            dtype=numpy.uint8),
        cv2.CV_LOAD_IMAGE_UNCHANGED)

(where `driver` is your WebDriver object, e.g. `WebDriver.Chrome()`).

Then to match that frame against a previously-captured "template" image, you can use stb-tester's[1] "match" function[2] which allows you to specify things like the region to ignore and tweak the matching sensitivity.

[1] http://stb-tester.com [2] http://stb-tester.com/stb-tester-one/rev2015.1/python-api#st...