[Mapserver-users] Mapserver 4.0 as WMS server issuing transparent PNG

[Frank Warmerdam]

Lars V. Nielsen wrote:

> Hi list,
>  
> Today I've (finally) migrated to Mapserver 4.0, and needed to migrate 
> several locally cascaded WMS services too. All but one of these are used 
> to generate backdrops, i.e. not transparent, but a single service is 
> used to publish thematic layers to be shown on top of other layers. So 
> this service needs to issue transparent map images, and therefore I'm 
> using PNG.
>  
> However, only the surrounding area is transparent, not the interior of 
> the polygons features, even though these are set to 50% transparency. 
> What am I doing wrong ?
>  
> It worked in Mapserver 3.6.3 with 8bit PNG's, and all I've done is add a 
> single OUTPUTFORMAT + IMAGETYPE clause to the MAP file used by the 
> service (set to 24bit PNG with RGBA). The layers/data it reads are all 
> locally saved MapInfo TAB tables.

Lars,

Note that with 32bit PNG files (RGB+alpha), a 50% transparency area will
actually be 50% opaque.  I'm not sure how this used to be handled.  The
whole area of alpha blending has been a source of problems since upgrading
to GD 2 and non-eight bit output.  I would suggest you prepare an small
isolated configuration that reproduces the problems, and submit it in
Bugzilla assigned to me, and I will dig into it.

Best regards,

-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Programmer for Rent