[Qgis-developer] WCS and Raster improvements for QGIS

[Even Rouault]

Hi Radim,

> - (I know that OS people don't like to here it and  they will argue
> that is is not a way ahead.) WCS is not widespread yet and there are
> various issues with implementation in UMN Mapserver (and other
> servers) and GDAL. Combination of that issues made it for example
> impossible to support WCS 1.1 in QGIS via GDAL WCS provider. In QGIS
> we can use various hacks to get over various issues with servers, for
> example axis switching etc. which would not be accepted in GDAL.

Well, when developing the WFS driver in OGR, I have had to implement various
hacks to support the varying behaviour of the WFS servers in the wild, so I
don't think it is an absolute obstacle. There's probably still work to deal with
axis switching in GDAL. For geographic SRS, the support for axis switching is
already there (I didn't check if it is used in the WCS driver, but it is there
at the OGRSpatialReference level, and it is already used in the WMS and WFS
drivers). I have also a patch pending to deal with projected SRS that have axis
switched, if that's what is missing. If the issue is that some servers don't
behave accordingly to the spec, we could imagine to have an option in the
description XML to pass that information to the driver.

> Then
> there is a question of time, if you look at deployment on Linux, it
> takes months or years to get fixed package into a distribution. In
> last two days, I found for example new issues with both UMN Mapserver
> and GDAL which we can get around in QGIS, but we would need to wait
> years until fixed versions get to distributions.

Not false, but I'm wondering why a new version of QGIS would be packaged faster
than a GDAL one. QGIS could document that QGIS X needs at least GDAL >= Y if it
needs Y for a functionality.

> - question/suggestion, GDAL i using CPLHTTPFetch() to get data from
> server, would it be possible to add a possibility to set user function
> to be used instead of default CPLHTTPFetch()? Something like
> SetHTTPFetchFunction( myHTTPFetch ). That would make it possible to
> use GDAL providers with QgsNetworkAccessManager for example.

I was actually thinking to something like that when reading Marco's email.
Sounds perfectly doable. The only point that could be tricky is if you need a
user_data pointer that is specific to each WCS connexion.

My intention was not to force the use of GDAL, but to know if there were
technical reasons that would prevent from using it. This work could be an
opportunity to make its WCS driver better, for QGIS benefit, and all other
projects that rely on GDAL.

Best regards,

Even