[Qgis-developer] SLD export not compliant due to unit attribute in the root element

[Andrea Aime]

Hi,
I'm looking into improving the SLD export, and stumbled into a schema
violation
that, if fixed, may cause other regression.
In particular, the root element StyledLayerDescriptor contains a "units"
attribute that
is not part of the SLD/SE specification, and thus makes validation fail:

<?xml version="1.0" encoding="UTF-8"?>
<StyledLayerDescriptor xmlns="http://www.opengis.net/sld" xmlns:ogc="
http://www.opengis.net/ogc" xmlns:xsi="
http://www.w3.org/2001/XMLSchema-instance" version="1.1.0" xmlns:xlink="
http://www.w3.org/1999/xlink" *units="mm" *xsi:schemaLocation="
http://www.opengis.net/sld
http://schemas.opengis.net/sld/1.1.0/StyledLayerDescriptor.xsd" xmlns:se="
http://www.opengis.net/se">
   ...
</StyledLayerDescriptor>

It's in particular the units="mm" element. Checking the git history I found
it has been introduced by this commit:

---------------------

b54b159 - rldhont <rldhont at gmail.com> 23-ott-2015
[BUGFIX] Set default units to mm in exportSld

The default units in QgsMapRenderer is Millimeters. The default units in SLD
 is Pixel but exportSld does not convert all millimeters in pixels.

We notes this bug by comparing default QGS Server rendering and rendering
with
 the SLD generated by QGIS.

To resolve it, we just have to add units to the document element.

---------------------

The above comments seems to suggest something is actually using that
attribute, but I'm not sure what.

Mind, GeoServer can parse the file the same, but if users try to validate
it (there is a validation button in the style editor), they will get the
following error:

"ine 2: cvc-complex-type.3.2.2: Attribute 'units' is not allowed to appear
in element 'StyledLayerDescriptor'."

Also, the attribute per se does nothing in a compliant client, since SLD
1.1 lacks the very concept of "on screen unit", the uom allowed
are pixel (on screen), meter (on ground), feet (on ground).
A SLD export trying to be compliant should convert mm to pixels assuming
some DPI conversion factor (which means,
making many changes around... bummer). I see there is already a uomScale in
the conversion properties, but I see it's not
used for point symbol sizes (as the commit message says.. I actually don't
see it being used anywhere).

Would it be ok to remove that attribute and use uomScale to properly adapt
sizes from mm to px instead?

Cheers
Andrea

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Ing. Andrea Aime
@geowolf
Technical Lead

GeoSolutions S.A.S.
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