Performance transparency

Steve Lime Steve.Lime at DNR.STATE.MN.US
Wed Dec 19 13:31:16 EST 2007

Hi Johan: I've never quantified the performance hit but it is certainly expected. Layer-level transparency works like this:

  - a temporary image is created to render the transparent layer into, it is initialized to be completely transparent
  - rendering proceeds as normal in the temp image
  - the temp image is then "merged" into the base image, so for every non-transparent pixel in the temp image alpha blending is done and the base image pixel is modified
  - the temporary image is discarded

So, the processing time will increase with 1) image size and 2) vector layer density.

There are plans to support feature-level transparency but it won't work in all cases so layer level transparency will remain an option.

So, the results you are seeing are expected although I can't say if double is too much or not.


>>> On 12/19/2007 at 8:49 AM, in message
<D93CFEAA72FA764885CC1FDA94A1D89C035462F2 at>, Hallgren
Johan E <jhhal at WMDATA.COM> wrote:
> Hello Friends
> I a question regarding performance when using a transparent vectorlayer
> on top of a rasterlayer. The output is quite large (1400 * 1000 px) and
> vectordata is a shapefile with polygons. 
> When turning the transparency on the vector layer the performance goes
> down quite much. It takes maybe double as long time to produce the map.
> Is there any knowledge about that?
> /Johan
> ___________________________________
> Johan Hallgren
> WM-data, Logica CMG
> Pelle Bergs backe 3
> Box 1938, 791 19 Falun
> Tel (och mobil): 023-547 46 (int: +46-2354746)
> johan.e.hallgren at 
> <blocked::mailto:johan.e.hallgren at> 
> <blocked::

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