OWSLib vs shp2img output of MapServer Layer

Question

I'm working on a map layout engine using Python and ReportLab and have come across a problem and was wondering if anyone can give me some insight.

Scenerio:

I'm trying to generate an image using OWSLib (see example code). The layer that I'm trying to render has an opacity of 40%. When I generate the image using the sample code the background is transparent but the features are not but if I generate the image using shp2img -m /www/cgi-bin/mapfiles/40e6b3a8.map -o test.png the features are 40% opaque. Any ideas how I can make this work using OWSLib or if I'm doing something wrong? Thank you for any help.

OWSLIB EXAMPLE CODE

from owslib.wms import WebMapService
wms = WebMapService('http://localhost/cgi-bin/mapserv?MAP=/www/cgi-bin/mapfiles/40e6b3a8.map&', version='1.1.1')
img = wms.getmap(   layers=['vlyr_oqiJw4Ib4U'],
    srs='EPSG:26910',
    bbox=(685400,5616300,689500,5619500),
    size=(800, 600),
    format='image/png',
    transparent=True
)
print(img.geturl())
out = open('c:/temp/test2.png', 'wb')
out.write(img.read())
out.close()

Answer 1

Have you set the OPACITY for the layer in your MapServer map file?

This can also be altered by client applications through a URL setting a value from 0-100. In fact most parts of the map file can be modified.

If you add the following to your query string you should get a transparent image:

&map.layer[LAYERNAME]=OPACITY 80

Which when encoded looks like:

map.layer%5BLAYERNAME%5D%3DOPACITY%2080

I'm not sure how owslib handles additional custom parameters. Unfortunately you can't do something like the following due to the non-standard parameter name:

img = wms.getmap(   layers=['vlyr_oqiJw4Ib4U'],
    srs='EPSG:26910',
    bbox=(685400,5616300,689500,5619500),
    size=(800, 600),
    format='image/png',
    transparent=True,
    map.layer[vlyr_oqiJw4Ib4U]="OPACITY 80"
)

So you will have to add a custom function / event / override to add the OPACITY for a layer.