[Community] Transparent Polygons

[Sean Gillies]

On Apr 26, 2006, at 6:46 AM, Ludwig M Brinckmann wrote:

> I have a number of polygon layers that stack on top of each other.  
> With the new SVN version (pre-0.6) I do not seem to be able to make  
> the interior of them transparent any more, meaning that top layers  
> always black out lower layers.
>
> I assume that the fill opacity box in the polygon symbolizer is  
> supposed to deal with it, but neither setting it to 0 nor to 1 (I  
> am never 100% sure which way it works) seems to make the interior  
> transparent.
>
> Am I missing something?
>
> Ludwig

Your understanding is correct, Ludwig. There is a problem, however,  
with MapServer as a renderer: MapServer can only deal with opacity at  
the layer level, not at the symbol level. This also means that you  
can not have different opacity values for polygon fill and stroke.  
The stroke opacity is the dominant parameter, if it is 1.0 you won't  
notice any difference when you change fill opacity. When we get a  
better renderer (mapnik, maybe?), this limitation can be removed.

For now there are two work-arounds:

- adjust your polygon stroke color so that you get the desired effect  
even with some opacity < 1, or

- separate layers for transparent polygon fill and opaque polygon  
outlines (stroke)

Hope this helps,
Sean

---
Sean Gillies
http://zcologia.com