[Qgis-developer] Labeling of starts and ends of a line

[Larry Shaffer]

Hi,

I have an idea concerning this that was originally cooked up for an
approach to doing multi-line curved labels [0]. I refer to them as 'label
anchors,' which would be for line and polygon features (though they could
be handy for point features). The concept is roughly like so:

* An anchor point is sent into PAL that constrains where a single label
candidate is generated. The point could be data defined or a reference to a
calculate-able point, e.g. midpoint, beginning/end, leftmost/rightmost,
nearest point on on line relative to off-line point, point furthest from
extent center, etc..

* With data defined anchors, a map tool could be made that allows the user
to visualize and dynamically move the anchor along a line or even place the
anchor off the line (e.g., over a nearby related feature, like a
building-to-street relationship). This would allow data defined curved
labels to work like moving text along a path in an illustration program,
and allow for precise control over label placement along a line without
forcing full pinning of the label.

* Additional options could be weighting the above/below line labeling
option (for off-line anchors), and quadrant and offset placement of the
label relative to anchor.

There may be other advantages to sending controllable label anchor points
into PAL. This is different than data defined x/y, which totally skips
chosen placement algorithms and essentially treats the feature geometry as
a point. Right now it is just conceptual, and I'd like to hear some
feedback.

Regards,

Larry

[0] http://hub.qgis.org/issues/4442


On Tue, Mar 5, 2013 at 12:24 PM, Andreas Neumann <a.neumann at carto.net>wrote:

> Hi,
>
> So this sounds like its not happening any time soon. Definitely after QGIS
> 2.0
>
> In this case I will do a helper layer and restrict that line using linear
> referencing. My data is in Postgis and I can use its linear referencing
> functions.
>
> For the long run I would want to get rid of that helper layer and find a
> QGIS only solution.
>
> Thanks for your reply!
>
> Andreas
>
> On 3/5/13 8:13 PM, Martin Dobias wrote:
>
>> Hi Andreas
>>
>> On Tue, Mar 5, 2013 at 2:20 PM, Andreas Neumann <a.neumann at carto.net>
>> wrote:
>>
>>> Hi,
>>>
>>> Maybe Larry, Martin or Marco can answer this:
>>>
>>> I am looking for the possibility to label a line at the start or end
>>> (plus a
>>> given offset from start/end). As far as I know this is not yet possible
>>> now?
>>> Does PAL support this and is it a matter of exposing this option in the
>>> GUI
>>> or is this not yet supported at all?
>>>
>> That's not supported as far as I know. It would be handy to have a
>> functionality in PAL that would allow us to constrain where the label
>> candidates should be created and how they should be prioritized. That
>> would also allow us to decide where to put labels for point features
>> and which positions are preferred.
>>
>> Martin
>>
>
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