Transparency in JPGs on windows

[Gabe]

Hi


We are running several instances of mapserver on Linux and Windows.

I have some maps that display a tiled aerial photography layer and some
other vector layers.  The vector layers are polygons displayed using a
symbol with a transparent background.  While the photography is switched off
the map is produced as a png and all works as expected.  However when the
photography layer is on (and the image is produced as a jpg) the vector
layers no longer have any transparency and use instead the map background
colour.

That happens on the windows boxes but not on the linux boxes.  The
installations are as similar as possible and this effect has been consistent
since about mapserver 4.8 (now using 4.10.2 and 5.0.0)

Is there a fix for this ?

Gabe