def generate_url(request, image_id, filter_spec):
    # Get the image
    Image = get_image_model()
    try:
        image = Image.objects.get(id=image_id)
    except Image.DoesNotExist:
        return json_response({"error": "Cannot find image."}, status=404)

    # Check if this user has edit permission on this image
    if not image.is_editable_by_user(request.user):
        return json_response({"error": "You do not have permission to generate a URL for this image."}, status=403)

    # Parse the filter spec to make sure its valid
    try:
        Filter(spec=filter_spec).operations
    except InvalidFilterSpecError:
        return json_response({"error": "Invalid filter spec."}, status=400)

    # Generate url
    signature = generate_signature(image_id, filter_spec)
    url = reverse("wagtailimages_serve", args=(signature, image_id, filter_spec))

    # Get site root url
    try:
        site_root_url = Site.objects.get(is_default_site=True).root_url
    except Site.DoesNotExist:
        site_root_url = Site.objects.first().root_url

    # Generate preview url
    preview_url = reverse("wagtailimages:preview", args=(image_id, filter_spec))

    return json_response({"url": site_root_url + url, "preview_url": preview_url}, status=200)

    def test_get_invalid_signature(self):
        """
        Test that an invalid signature returns a 403 response
        """
        # Generate a signature for the incorrect image id
        signature = generate_signature(self.image.id + 1, 'fill-800x600')

        # Get the image
        response = self.client.get(reverse('wagtailimages_serve', args=(signature, self.image.id, 'fill-800x600')))

        # Check response
        self.assertEqual(response.status_code, 403)

def generate_image_url(image, filter_spec):
    signature = generate_signature(image.id, filter_spec)
    url = reverse('wagtailimages_serve', args=(signature, image.id, filter_spec))

    try:
        site_root_url = Site.objects.get(is_default_site=True).root_url
    except Site.DoesNotExist:
        site_root_url = Site.objects.first().root_url

    image_filename = image.file.name[len('original_images/'):]

    return site_root_url + url + image_filename

    def test_get(self):
        """
        Test a valid GET request to the view
        """
        # Generate signature
        signature = generate_signature(self.image.id, "fill-800x600")

        # Get the image
        response = self.client.get(reverse("wagtailimages_serve", args=(signature, self.image.id, "fill-800x600")))

        # Check response
        self.assertEqual(response.status_code, 200)
        self.assertEqual(response["Content-Type"], "image/png")

    def test_get_invalid_filter_spec(self):
        """
        Test that an invalid filter spec returns a 400 response

        This is very unlikely to happen in reality. A user would have
        to create signature for the invalid filter spec which can't be
        done with Wagtails built in URL generator. We should test it
        anyway though.
        """
        # Generate a signature with the invalid filterspec
        signature = generate_signature(self.image.id, 'bad-filter-spec')

        # Get the image
        response = self.client.get(reverse('wagtailimages_serve', args=(signature, self.image.id, 'bad-filter-spec')))

        # Check response
        self.assertEqual(response.status_code, 400)

def generate_url(request, image_id, filter_spec):
    # Get the image
    Image = get_image_model()
    try:
        image = Image.objects.get(id=image_id)
    except Image.DoesNotExist:
        return JsonResponse({
            'error': "Cannot find image."
        }, status=404)

    # Check if this user has edit permission on this image
    if not permission_policy.user_has_permission_for_instance(request.user, 'change', image):
        return JsonResponse({
            'error': "You do not have permission to generate a URL for this image."
        }, status=403)

    # Parse the filter spec to make sure its valid
    try:
        Filter(spec=filter_spec).operations
    except InvalidFilterSpecError:
        return JsonResponse({
            'error': "Invalid filter spec."
        }, status=400)

    # Generate url
    signature = generate_signature(image_id, filter_spec)
    url = reverse('wagtailimages_serve', args=(signature, image_id, filter_spec))

    # Get site root url
    try:
        site_root_url = Site.objects.get(is_default_site=True).root_url
    except Site.DoesNotExist:
        site_root_url = Site.objects.first().root_url

    # Generate preview url
    preview_url = reverse('wagtailimages:preview', args=(image_id, filter_spec))

    return JsonResponse({'url': site_root_url + url, 'preview_url': preview_url}, status=200)

    def test_get(self):
        """
        This tests that the view responds correctly for a user with edit permissions on this image
        """
        # Get
        response = self.client.get(reverse('wagtailimages:generate_url', args=(self.image.id, 'fill-800x600')))

        # Check response
        self.assertEqual(response.status_code, 200)
        self.assertEqual(response['Content-Type'], 'application/json')

        # Check JSON
        content_json = json.loads(response.content.decode())

        self.assertEqual(set(content_json.keys()), set(['url', 'preview_url']))

        expected_url = 'http://localhost/images/%(signature)s/%(image_id)d/fill-800x600/' % {
            'signature': urlquote(generate_signature(self.image.id, 'fill-800x600').decode(), safe=urlquote_safechars),
            'image_id': self.image.id,
        }
        self.assertEqual(content_json['url'], expected_url)

        expected_preview_url = reverse('wagtailimages:preview', args=(self.image.id, 'fill-800x600'))
        self.assertEqual(content_json['preview_url'], expected_preview_url)

 def test_signature_generation(self):
     self.assertEqual(generate_signature(100, 'fill-800x600'), b'xnZOzQyUg6pkfciqcfRJRosOrGg=')