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

[Tamas Szekeres]

Thank you all for the information.
I concur with Steve that supporting both of these options controlled by a
parameter in the mapfile would be a good compromise (keeping the current
behaviour as the default).

Best regards,

Tamas



2013/11/27 Daniel Morissette <dmorissette at mapgears.com>

> Oh wait... re-reading my own ticket, we never added the wms_bbox_mode
> metadata that was initially suggested, what was done in the end is that we
> implemented a vendor-specific WMS GetMap param called
> BBOX_PIXEL_IS_POINT=TRUE to switch the pixel model on the fly (works for
> WMS only).
>
> Daniel
>
>
> On 13-11-27 10:37 AM, Daniel Morissette wrote:
>
>> 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
>
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