Performance transparency

[Steve Lime]

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.

Steve


>>> On 12/19/2007 at 8:49 AM, in message
<D93CFEAA72FA764885CC1FDA94A1D89C035462F2 at WMRI000166.corp.wmdata.net>, 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
>
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> Johan Hallgren
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