[Qgis-developer] arrows on line strings?

[Martin Dobias]

On Jan 31, 2008 5:49 PM, Stefanie Tellex <stefie10 at media.mit.edu> wrote:
>
> Hi,
>
> I would like to draw arrows on line strings, since they represent
> movement vectors and I want to show the direction of movement.  I
> couldn't find any way to do that in the gui or any obvious way in the
> programmatic API.  What's the best way to add this feature or is it
> there already somewhere?

Currently QGIS renderers work in a way that they just set some
properties for painting and let the vector layer to draw the polyline,
so it's not possible to create a renderer which would e.g. render
arrows in the direction of the flow. We should allow renderers to do
arbitrary drawing in future.

Now it will be more complicated to acquire this effect: you can
generate some points (e.g. in the middle of every line segment) and
determine the angle of direction. Such generated vector layer with
points you can load to map canvas and for rendering set a SVG symbol
with an arrow and set rotation field appropriately to the field you've
generated.

Martin