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Symbol placement inconsistency

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Symbol placement inconsistency

Dave Weaver
MapServer doesn't seem to be entirely consistent with placement of symbols.

Consider these 2 overlapping images:

http://www.screenshots.cc/view_image/5437e415/thayer-east.png
http://www.screenshots.cc/view_image/b0aae448/thayer-west.png

This is causing me problems.  I need to generate lots of map tiles covering
a large area. These tiles need to be able to be displayed next to each other
seamlessly. To avoid partial symbols on the edge of tiles, I'm rendering
tiles too large then trimming off a 256-pixel border around each. However,
when symbols are plotted in different places on adjacent tiles, I'm getting
partial symbols around the edge of my trimmed tiles which *don't* have a
corresponding partial symbol on the adjacent tile. (if you see what I mean).

I'm using MapServer 5.0.0 from the ms4w package.

What can I do to cure my problem?

Thanks,
Dave.
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Re: Symbol placement inconsistency

Hal Mueller-3
Dave, it looks to me like you're seeing effects of collision avoidance  
in label cacheing.  I would expect those artifacts to be more extreme  
near the edges  of the images.

I wonder what would happen if you greatly increased your edge buffer.  
If you need a 4096x4096 tile, then produce a 12288x12288 (3x3) image  
and carve out the central 1/9.  That would reduce the artifacts but  
can't be guaranteed to eliminate them.  I think the only guarantee  
would be to explicitly specify label locations, and turn off the label  
collision avoidance.

I've copied this thread over to the new list, as the lists.umn.edu  
address's demise is imminent.

Hal

On Feb 29, 2008, at 3:52 AM, Dave Weaver wrote:

> MapServer doesn't seem to be entirely consistent with placement of  
> symbols.
>
> Consider these 2 overlapping images:
>
> http://www.screenshots.cc/view_image/5437e415/thayer-east.png
> http://www.screenshots.cc/view_image/b0aae448/thayer-west.png
>
> This is causing me problems.  I need to generate lots of map tiles  
> covering
> a large area. These tiles need to be able to be displayed next to  
> each other
> seamlessly. To avoid partial symbols on the edge of tiles, I'm  
> rendering
> tiles too large then trimming off a 256-pixel border around each.  
> However,
> when symbols are plotted in different places on adjacent tiles, I'm  
> getting
> partial symbols around the edge of my trimmed tiles which *don't*  
> have a
> corresponding partial symbol on the adjacent tile. (if you see what  
> I mean).
>
> I'm using MapServer 5.0.0 from the ms4w package.
>
> What can I do to cure my problem?
>
> Thanks,
> Dave.
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