[mapserver-users] Re: Symbol placement inconsistency

[Steve Lime]

The primary thing going on here is that MapServer computes label placement AFTER
features are clipped to the area of interest. It's a necessary step especially with
linear features as a single label point doesn't always make sense. Since the 
algorithm places the label at the center of the longest segment the position will
often change from map to map with clipped features.

This can cause problems especially with tiled data output (e.g. TileCache or Ka-Map)
so a special processing option was put in place in the development version. If you 
set:

  PROCESSING 'LABEL_NO_CLIP=1'

MapServer will compute label positions before clipping. I know that it works with
polygon data, not sure about linear features.

The metatiling features of TileCache and ka-map to a real good job of minimizing
these problems with linear data.

Steve

>>> On 2/29/2008 at 1:24 PM, in message
<3E2F6FCA-354C-4674-B331-AD1C502F9CC8 at mobilegeographics.com>, Hal Mueller
<hal at mobilegeographics.com> wrote:
> 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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