[OpenLayers-Users] differences in transparent PNG rendering quality in IE7 & Firefox

[David Herbert]

I am trying to figure out why my OL 2.5 mapping app looks so awful in
IE7 (7.0.5730.13 WinXP) compared to Firefox.  Basically I have an opaque
PNG base layer with a stack of PNGs of varying opacity on top.  All the
images are generated by Geoserver.  What I don't understand is that I
can bring up individual tiles in IE7 directly and they look exactly the
same as in Firefox.  However when viewed as part of the OL stack, for
example a tile with a line feature of width 2px appears with a very thin
black jagged border, and appears *much* more opaque than in FF 2 and
Opera.  This same tile viewed on its own in IE7 exhibits none of these
characteristics, so it's a property of being part of a stack.  The poor
rendering occurs with e.g. just the base layer and the line feature
layer BTW.

The style according to IE7 DebugBar is:

FILTER: alpha(opacity=80); WIDTH: 256px; POSITION: relative; HEIGHT:
256px; alt: OpenLayersDiv1120; opacity: 0.8

Is there something I can do to improve the appearance of layers in IE?
I can attach screenshots of what I mean if that'd help anyone pinpoint
the problem.

Thanks for any help.

David Herbert
British Antarctic Survey.

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