[mapserver-dev] MS RFC 107: Support for the edge of pixel (OGC) extent model in MapServer

[Tamas Szekeres]

Hi Devs,

We had a small discussion here in the codesprint, whether we should apply
this RFC as it stands or not. In fact I understand that the proposed
changes are fairly system wide, but let us review the issues that should
anyway be handled in some way.
I've also discussed with the client who has raised this problem originally,
mainly to refresh my memory related to all aspects. It must also be added
that applying the proposed changes in a local build could indeed solve all
issues they had. So the problems are as follows:

1. Correction code should be applied at the client when using
setExtent/getExtent in mapscript. This is fairly odd for most users who
would prefer to go with the edge of pixel extent model. We could however
provide variants for   setExtent/getExtent which mimics the  edge of pixel
extent model at the interface.

2. The current code regarding to the geographical width yields incorrect
result from the WMS side, which should probably be reverse corrected if the
query has been originated from WMS specifically. Actually the user would
expect to have the map drawn at the specified extent, but mapserver
considers as if a smaller extent has been specified by the user. When using
geo width dependent rendering (MINGEOWIDTH/MAXGEOWIDTH) the layers may
unexpectedly be drawn at a given zoom level where it should not normally be
drawn.

3. The user wants to draw a given extent, but actually a smaller amount of
features are provided in the view. Given a large extent (like a whole
continent) half pixel difference in each directions is not negligible

4. Smaller amount of features are retrieved from the database which the WMS
client may expect. We should probably reverse correct the extent when
setting the spatial filter against the database to get the correct set of
the features drawn on the map.

Lastly to add the other way to look at it is the original
mapserver implementation for this is very, very old and not OGC compliant
which is now the de-facto standard. It was originally done for a specific
purpose as I understand it (integration with ERDAS reading the discussion
related to this).

Perhaps it is time to review this to be in line with OGC standards and the
other mapping engines. Also I don't buy the argument about 'the effect on
all existing map files out there' is a valid one. I don't think users would
notice any difference. Besides the approach suggested have been
configurable in the map file.


Best regards,

Tamas





2014-01-01 15:43 GMT+01:00 Tamas Szekeres <szekerest at gmail.com>:

> Hi Devs,
>
> Happy New Year All!
>
> According to this thread<http://osgeo-org.1560.x6.nabble.com/Questions-regarding-to-the-extent-scale-calculations-in-MapServer-td5091493.html>I've ringed in to write an RFC about the proposed changes.
> Please find RFC-107 for review submitted to the documentation tree:
> http://mapserver.org/development/rfc/ms-rfc-107.html
>
> Let me know about your suggestions.
>
> Best regards,
>
> Tamas
>
>
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