[fdo-internals] New RFC10 Is Posted

Greg Boone greg.boone at autodesk.com
Tue Sep 11 11:06:56 EDT 2007

Hi Frank,

Yes, the override APIs for each provider are provided to allow clients
to programmatically create and manipulate the content of the provider
specific configuration files used to initiate a provider connection. So,
there is a specific API for GDAL, WMS, SHP, etc. These APIs are not
intended to be shared between providers as they allow a user to specify
data store specific values that map the FDO logical schema onto the
underlying physical or logical implementation. Without such a
configuration, there would be no means for a user to customize how an
FDO logical schema is mapped to an underlying implementation. 

I would also like to clarify the purpose of altering the WMS Overrides
API in order to specify the Image Format, as outlined in RFC10. The
purpose of this method is not to allow a client to control the means in
which the provider (i.e. GDAL API) interprets the raster data returned
from the server, nor is it intended to imply that the provider will
translate the retuned image into the specified format. The purpose of
the function call is to allow a client application to specify the format
in which the WMS server will generate the requested WMS image as the
image is being returned to the WMS provider as a result of an FDO select
request. According to the OGC WMS 1.3 specification, the legal and
appropriate values for the format type are server specific and are
advertised by the provider as a part of the server's capabilities
response. Therefore, it will be the client's responsibility to know the
values that are legal and acceptable for a particular server or to use
the IGetImageFormats custom command as specified in
http://trac.osgeo.org/fdo/wiki/FDORfc6 to retrieve the legal values.
Once the legal vaoues are known, they can be used to populate and create
the WMS configuration document.



-----Original Message-----
From: fdo-internals-bounces at lists.osgeo.org
[mailto:fdo-internals-bounces at lists.osgeo.org] On Behalf Of Frank
Warmerdam (External)
Sent: Tuesday, September 11, 2007 10:37 AM
To: FDO Internals Mail List
Subject: Re: [fdo-internals] New RFC10 Is Posted

Traian Stanev wrote:
>> This is clearly an aspect of FDO I don't understand.  I do not
> understand
>> what override apis are or how it is possible to call custom c++
> methods
>> from a particular provider when we don't even install include files
> for
>> them (to the best of my knowledge).  I guess this "provider specific
>> APIs" stuff is seems foreign to me because I have always tried hard
>> to avoid it in GDAL/OGR.
> Hi Frank, 
> The WMS override headers are there together with the FDO header, so in
> theory they are there. However, I do agree with you that having API
> specific to providers goes against the abstraction that is FDO. It
> the client code have to special case providers, which automatically
> reduces the value of the FDO API abstraction.


I stand corrected!  I presume these overrides are rarely used, is that
As far as I can tell most things that can be manipulated via these
provider apis (overrides I guess...) can also be controlled using the
generic APIs and configuration xml for the GDAL provider.  Is that
the case?  That the override apis are roughly speaking APIs to set and
manipulate the stuff you could set in the configuration file?

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo,

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