[OpenLayers-Users] transparent overlay and IE

[Christopher Schmidt]

On Fri, Feb 02, 2007 at 02:58:18PM +0100, dome febbo wrote:
> I can see the example page http://labs.metacarta.com/wms-c/demo.html,
> but with  TIGER Overlay I can't see the baselayer in IE6.
> In OpenLayers.Util.js, there is a method
> "OpenLayers.Util.modifyAlphaImageDiv" with "progid" filter but maybe
> there is a problem?

You can turn on alpha transparency for PNG images in WMS via an 'alpha'
option on the layer:
  OL.Layer.WMS("WMS", "http://example.com/wms.cgi", {'layers':'foo'},
  {'alpha':true})

However, doing this is likely to slow Internet Explorer to a crawl.
Making many large images transparent is simply not supported in IE, and
is therefore off by default. 

If you need transparent WMS images, I recommend using some format that
IE supports transparency for natively (GIF or PNG with single-bit
transparency instead of alpha transparency). If you're going to use the
alpha option, you should test it in IE and ensure that  the behavior
matches the performance you need for your application.

Regards,
-- 
Christopher Schmidt
MetaCarta