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

[Tamas Szekeres]

I'm keen to create an RFC about this.

Tamas



2013/12/2 Lime, Steve D (MNIT) <Steve.Lime at state.mn.us>

>  Tamas: Do you want to create an enhancement ticket for this?
>
>
>
> 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 10:14 AM
> *To:* mapserver-dev at lists.osgeo.org
> *Subject:* Re: [mapserver-dev] Questions regarding to the extent/scale
> calculations in MapServer
>
>
>
> 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
>
>
>
> _______________________________________________
> mapserver-dev mailing list
> mapserver-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/mapserver-dev
>
>
> _______________________________________________
> mapserver-dev mailing list
> mapserver-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/mapserver-dev
>
>
>
>
>
> --
> Daniel Morissette
> http://www.mapgears.com/
> Provider of Professional MapServer Support since 2000
>
> _______________________________________________
> mapserver-dev mailing list
> mapserver-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/mapserver-dev
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/mapserver-dev/attachments/20131202/73132452/attachment.html>