A Python script to automatically check WMS services return images, and check images are not blank.

wms_tester.py

"""
Copyright (C) 2012 S. Girvin

Permission is hereby granted, free of charge, to any person obtaining a copy of this software
and associated documentation files (the "Software"), to deal in the Software without restriction,
including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial
portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT
LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
"""

from PIL import Image
from StringIO import StringIO
import requests
from owslib.wms import WebMapService
    
def check_blank(content):
    """
    Uses PIL (Python Imaging Library to check if an image is a single colour - i.e. blank
    Images are loaded via a binary content string

    Checks for blank images based on the answer at:
    http://stackoverflow.com/questions/1110403/how-can-i-check-for-a-blank-image-in-qt-or-pyqt
    """

    im = Image.open(StringIO(content))
    # we need to force the image to load (PIL uses lazy-loading)
    # otherwise get the following error: AttributeError: 'NoneType' object has no attribute 'bands'
    im.load() 
    bands = im.split()

    # check if the image is completely white or black, if other background colours are used
    # these should be accounted for
    is_all_white = all(band.getextrema() == (255, 255)  for band in bands)
    is_all_black = all(band.getextrema() == (0, 0)  for band in bands)

    is_blank = (is_all_black or is_all_white)

    return is_blank

def get_default_parameters():

    params = {
        'TRANSPARENT': 'TRUE',
        'SERVICE': 'WMS',
        'VERSION': '1.1.1',
        'REQUEST': 'GetMap',
        'STYLES': '',
        'FORMAT': 'image/png',
        'WIDTH': '256',
        'HEIGHT': '256'}

    return params

def get_params_and_bounding_box(url, layer_name):

    params = get_default_parameters()
    
    # get bounding box for the layer
    wms = WebMapService(url, version='1.1.1')
    bounds = wms[layer_name].boundingBox

    if bounds is None:
        # some WMS servers only support a WGS84 boundingbox
        bounds = wms[layer_name].boundingBoxWGS84
        crs = 'EPSG:4326'
    else:
        # a bounding box and projection code are returned in the following
        # format: (0.0, 0.0, 500000.0, 500000.0, 'EPSG:29902')
        crs = bounds[4]
        
    bbox = ",".join([str(b) for b in bounds[:4]])

    # set the custom parameters for the layer
    params['LAYERS'] = layer_name
    params['BBOX'] = bbox
    params['SRS'] = crs

    return params

def check_blank_image(url, layer_name):
    """
    Check if the WMS layer at the WMS server specified in the
    URL returns a blank image when at the full extent
    """

    params = get_params_and_bounding_box(url, layer_name)
    resp = requests.get(url, params=params)
    print "The full URL request is '%s'" % resp.url

    # this should be 200
    print "The HTTP status code is: %i" % resp.status_code

    if resp.headers['content-type'] == 'image/png':
        # a PNG image was returned
        is_blank = check_blank(resp.content)
        if is_blank:
            print "A blank image was returned!"
        else:
            print "The image contains data."
    else:
        # if there are errors then these can be printed out here
        print resp.content

def get_layers(url):
    """
    Get a list of all the WMS layers available on the server
    and loop through each one to check if it is blank
    """
    
    wms = WebMapService(url, version='1.1.1')
    layer_names = list(wms.contents)

    for l in layer_names:
        print "Checking '%s'..." % l
        check_blank_image(url, l)
    
if __name__ == "__main__":
    # set the URL of the WMS server here
    url = 'http://demo.mapserver.org/cgi-bin/wms?'
    get_layers(url)