[Qgis-developer] Access labels properties from python?

Larry Shaffer larrys at dakotacarto.com
Wed Feb 5 03:11:57 PST 2014

Hi Régis,

On Tue, Feb 4, 2014 at 1:07 PM, Régis Haubourg <
regis.haubourg at eau-adour-garonne.fr> wrote:

> Hi there,
> I would like to explore the possibilities to extend EasyCustomLabeling to
> handle correctly curved labeling on lines, and maybe also labeling on the
> border of polygons.
> My idea is to make PAL engine compute curved labels, get label path and
> convert it to a linestring, a sort of label path.
> Is there any way in the python API to access label properties once computed
> by PAL? If not, does C++ API allows that?

The label information available, after labeling is completed, is stored in
QgsLabelPostion, used by the interactive labeling tools [0]. These are
retrieved from a search tree via
QgsLabelingEngineInterface's::labelsAtPosition(QgsPoint) and
::labelsWithinRect (QgsRectangle) [1]. All of that should already have

QgsLabelPostion has limited label component info, only the minimum needed
for working with the interactive tools, e.g. just a bounding rectangle and
corner points for geometries.

However, there are several outstanding bugs (and the advent of labeling
styles) that could benefit from a new full label class that maintains as
much information about the label *before* sending only the needed bits to
PAL.  The label class properties would be resolved as needed to reduce
overhead. This also means that if the registered label classes persist
during PAL registration and solution, and rendering, then any new
properties calculated along the way can be appended.

The stored result, accessed like QgsLabelPostion, could possibly contain
all initial settings and resolved properties, as well as any geometries
calculated during rendering, i.e. the "label path" you were referring to.
It also means that additional lightweight geometries can be stored within
it, like a data defined destination point for leader (call out) lines.

I have taken the next week off from my job to work solely on labeling
issues. I'm pretty sure I will not have enough time to do such a large
refactoring before the release, since squashing as many bugs as possible
and finishing a full unit test suite should come first.

[0] http://www.qgis.org/api/classQgsLabelPosition.html
[1] http://www.qgis.org/api/classQgsLabelingEngineInterface.html



> Thanks for your hints.
> Cheers
> Régis
> --
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