[mapserver-dev] Questions regarding to the extent/scale calculations in MapServer

[Daniel Morissette]

FYI WorldWind also uses this PixelIsPoint model, and even worse, in 
their case they implemented their WMS client code to incorrectly send 
center of pixel coordinates in the WMS BBOX (the WMS spec says that BBOX 
coordinates are the outside of the corner pixels == PixelIsArea).

So in order to allow compatibility with WorldWind when deploying 
MapServer servers for WorldWind use, we added a wms_bbox_mode param that 
does more or less what Steve suggests here, but for WMS only (and it 
indeed breaks WMS compliance).

More info: https://github.com/mapserver/mapserver/issues/4652

Daniel


On 13-11-27 10:27 AM, Stephen Woodbridge wrote:
> Steve L,
>
> If this would be "easy" to change internally would it make sense to
> allow this to be configured in the mapfile. By default the behavior
> would remain the same, but we would have an option in the map object like:
>
> PIXELMODEL POINT|AREA
>
> then it would be easy for people to use what they need?
>
> Food for thought!
>
> -Steve W
>
> On 11/27/2013 10:15 AM, Lime, Steve D (MNIT) wrote:
>> The model was based on how ERDAS represented pixels back when MapServer
>> was first written. I was a satellite image processing guy at the time
>> and that was the initial focus of the software. Early code used the
>> ERDAS C toolkit which reinforced the model. Personally I think the
>> center of a pixel model makes more sense.
>>
>> Changing would probably be **very** disruptive. Not so much within
>> MapServer code since the areas of change are pretty isolated in a few
>> macros and transformations in the OWS code. The change would affect
>> every mapfile that sets or uses scale denominators. Plus, clients would
>> need to be updated and need to be made version aware.
>>
>> Steve
>>
>> *From:*mapserver-dev-bounces at lists.osgeo.org
>> [mailto:mapserver-dev-bounces at lists.osgeo.org] *On Behalf Of *Tamas
>> Szekeres
>> *Sent:* Wednesday, November 27, 2013 8:54 AM
>> *To:* mapserver-dev at lists.osgeo.org
>> *Subject:* [mapserver-dev] Questions regarding to the extent/scale
>> calculations in MapServer
>>
>> Hi All,
>>
>> We've already noticed MapServer use a "center of pixel" representation
>> when doing the extent/scale calculations in the code, which may cause
>> quite some confusion for the users (mostly from the mapscript side)
>> regarding to the behaviour.
>>
>> The most typical issue I've encountered is the complain about "why
>> mapserver modifies my accurate extent specified in setExtent and why
>> MapServer calculates a different scale I can calculate?"
>>
>> The reason of why is in fact that we consider the area coverage of the
>> image is larger than the area coverage of the map extent (half of the
>> pixel size in each directions). But the users (and mostly everyone in
>> the world except MapServer) considers that the area coverage of the
>> image is the same as the area coverage of the map extent.
>>
>> Can someone explain why we do things this way and do we have the chance
>> to get rid of it?
>>
>> We could also eliminate the unnecessary transformations done in the
>> WMS/WCS interface where the extent of the BBOX is considered to be in -
>> let's say - edge of pixel representation and not in center of pixel
>> representation.
>>
>> Best regards,
>>
>> Tamas
>>
>>
>>
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>
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-- 
Daniel Morissette
http://www.mapgears.com/
Provider of Professional MapServer Support since 2000