[Qgis-developer] Adding blending modes for labels, to have 'transparenting' labels?

[Nyall Dawson]

On 1 October 2016 at 19:48, Richard Duivenvoorde <rdmailings at duif.net> wrote:
> Hi All,
>
> I've been contacted by a cartophil (no offence Jan ;-)), which sent me
> the two attached images, and asked me if the labelbuffers you see there
> could be done with QGIS...
>
> To precize:
>
> If you look in lblbuf1.png [0] (Dutch kadastral 1:10000 map) it looks
> like the label 'Arnhemse Broek' is normally buffered on first side. But
> better looking into it, it looks like it is ONLY ('transparently')
> buffered the complement(?) color of the buffer.
> So: text = black, so there is only a complement(?) of black == white
> buffer, IF the buffer(!) is covering a pixel which has the same color as
> the text (black in this case).
> For example: looking at the top of the A in 'Arnhemse Broek' you see
> that red is actually NOT having a buffer, but the little black casing of
> the red, DOES have a buffer..
>
> In lblbuf2.png (swiss topo map) if you look at the height-line/contour
> labels, you see exactly the same: there is a buffer, but only if it is
> over a line with the same color...
>
> Tried some label settings in QGIS with lblbuf3.png as nearest result...
> Taking blue as labeltext, yellow (complement) as buffer color, and
> Addition as blending.
>
> But as you see there the blending takes ALL colors into account, so you
> have a slight buffering everywhere, and yellow artifacts in the red
> pixels....
>
> So diving into the code:
>
> Blending mode is here:
>
> https://github.com/qgis/QGIS/blob/master/src/core/qgspallabeling.cpp#L1009
>
> Going to renderer:
>
> https://github.com/qgis/QGIS/blob/master/src/core/qgspainting.cpp#L6
>
> Going to pretty low level qpainter behaviour:
>
> http://doc.qt.io/qt-5/qpainter.html#setCompositionMode
>
> http://doc.qt.io/qt-5/qpainter.html#CompositionMode-enum
>
> It looks like there are even more modes then are available in the
> Blending dropdown in QGIS.
>
> I'm not so much into color theories, but is it possible that the use
> case above is one of the composition modes which just is not in QGIS
> blending mode dropdown yet?
>
> And if so: which one do you think? I can try to add it myself.
>
> OR is the usecase a custom blending mode (we are wondering then how the
> dutch cadastre and the swiss topo are doing this... because it is NOT
> part of esri apparently too....).

It's not a blending mode - it's a technique called "variable depth
buffers" or "smart halos".

- ArcGIS offers it, but requires precreation of a raster layer (see
https://blogs.esri.com/esri/arcgis/2011/11/28/variable-depth-masking-contour-label-example/).
- It can be done in Mapnik - see http://mapnik.org/news/smart-halos

Hugo has been looking into this for QGIS - see
https://github.com/qgis/QGIS-Enhancement-Proposals/issues/63

Nyall




>
> Thanks for any pointers.
>
> Regards,
>
> Richard Duivenvoorde
>
> [0]
> http://storage3.static.itmages.com/i/16/1001/h_1475315030_8477266_8bd662bb47.png
> [1]
> http://storage7.static.itmages.com/i/16/1001/h_1475315072_4576698_eaebba1bb0.png
> [2]
> http://storage1.static.itmages.com/i/16/1001/h_1475315103_2918295_e2b5874a36.png
>