[Qgis-developer] SLD parsing: handling ogc:Literal within CssParameter PR #3996

[Jorge Gustavo Rocha]

Hi devs,

This message introduces the PR #3996 [0] to the developers mailing list.

QGIS fails to load SLD files with valid SLD syntax that uses
<ogc:Literal> to specify the value of a <CssParameter>.

This works:
<CssParameter name="stroke-width">2</CssParameter>

This fails:
<CssParameter name="stroke-width">
	<ogc:Literal>2</ogc:Literal>
</CssParameter>

Both are valid SLD syntax.

I noticed this behaviour when importing styles with the Geoserver
Explorer plugin. I opened an issue on qgis-geoserver-plugin [1]. From
the discussion with Luigi Pirelli, it seems wise to start by solving the
problem on the QGIS side.

So, my plan is to fix QGIS and then also fix the qgis-geoserver-plugin.

I've made my humble first PR to QGIS regarding this issue. I hope I did
it right.

More details

To confirm this behaviour, we can use the default data and styles used
by Geoserver. Several default styles from Geoserver uses this
<ogc:Literal> tag, and that's why so many styles are not parsed correctly.

Let's use the shapefile geoserver/data/data/sf/streams.shp and the style
geoserver/data/styles/simple_streams.sld as an example. Those files can
be downloaded from [5].

1) The style uses <ogc:Literal> tags around <CssParameter> values.

2) The style is valid according to the SLD definition.

xmlstarlet val -s
http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd
simple_streams.sld
simple_streams.sld - valid

3) When the style is loaded in QGIS, these values are not parsed and the
default values are assumed instead.

4) The rendering on Geoserver is [2]. The render on QGIS (right now) is [3].

The same syntax is used in other default Geoserver styles.

Note: The <ogc:Literal> is properly handled when it appears in the
context of a Filter. For example, the following SLD is already properly
imported:

          <ogc:Filter>
            <ogc:PropertyIsBetween>
              <ogc:PropertyName>PERSONS</ogc:PropertyName>
              <ogc:LowerBoundary>
                <ogc:Literal>2000000</ogc:Literal>
              </ogc:LowerBoundary>
              <ogc:UpperBoundary>
                <ogc:Literal>4000000</ogc:Literal>
              </ogc:UpperBoundary>
            </ogc:PropertyIsBetween>
          </ogc:Filter>The problem

The limitation is only when a value of a <CssParameter> parameter is
surrounded by <ogc:Literal>.

The solution

To solve this issue, I've changed
QgsSymbolLayerUtils::getSvgParameterList at
QGIS/src/core/symbology-ng/qgssymbollayerutils.cpp

Instead of just get the value from the firstChild(), I add a test to
check if there is an additional <ogc:Literal> ElementNode. So, the value
can be or not be within a <ogc:Literal> tag.

The rendering on QGIS, after applying the changes proposed, is [4].

[0] https://github.com/qgis/QGIS/pull/3996
[1] https://github.com/boundlessgeo/qgis-geoserver-plugin/issues/58
[2]
http://webgis.di.uminho.pt/~jgr/geoserver%20rendering%20of%20simple_streams.png
[3]
http://webgis.di.uminho.pt/~jgr/qgis%20rendering%20of%20simple_streams.png
[4]
http://webgis.di.uminho.pt/~jgr/qgis%20rendering%20of%20simple_streams%20fixed.png
[5] http://webgis.di.uminho.pt/~jgr/simple_streams.zip

I would love to provide unit tests to confirm the error and to show that
the proposed solution works, but I need to study more deeply how to
write tests. I've just tried to find out a test that already reads SLD
files, but I didn't find any. So, I think I have to write the test from
scratch.

I would like to thank Luigi Pirelli for the initial comments and
pointing me the this approach.

Please be gentle: it is my first PR.

Regards,

J. Gustavo
-- 
Jorge Gustavo Rocha
Departamento de Informática
Universidade do Minho
4710-057 Braga
Tel: +351 253604480
Fax: +351 253604471
Móvel: +351 910333888
skype: nabocudnosor