[QGIS-Developer] Symbology: multiple simple marker symbols are not correctly aligned on top of each other

[Andreas Neumann]

Hi,

Thanks for the reply. And thanks for confirming that you also noticed 
this issue and have an explanation for what's the cause. Glad that I'm 
not the only one who got annoyed by this ...

In that case - can we do something about the second issue - that larger 
symbols always appears as squares, no matter what shape you choose?

See this screenshot here:

That triangle appears as a square if you enlarge the size of the symbol.

Ideally, QGIS should "auto-scale" symbols down here so that they can be 
represented in that tree widget and still correctly represent the symbol 
shape.

Thanks,

Andreas


Am 06.07.20 um 00:51 schrieb Nyall Dawson:

> On Sat, 4 Jul 2020 at 00:27, Andreas Neumann <a.neumann at carto.net> wrote:
>> Hi,
>>
>> Here is something that bothered me already for many years in QGIS and I think it might be a long standing issue in QGIS symbology:
>>
>> When you place two simple markers on top of each other (without any offset), f.e. two circle symbols on top of each other, they are not correctly aligned/centered on top of each other: The alignment is a tiny bit off, but it looks ugly and it annoys me a lot.
>>
>> The misalignment is not only in the preview dialogue but also visible in the map rendering.
>>
>> In the symbol preview dialog there is a second annoying thing: strokes in the preview are often clipped away, as you can see in the dialog above
>>
>> Can we do something about it these two annoying issues?
> QGIS isn't actually doing anything wrong here -- it's to do with how
> Qt is antialiasing the symbol rendering. For some sizes it's able to
> snap the rendered shape to nice pixel boundaries, but for other
> in-between sizes it isn't, and you get these small shifts. If you use
> a hidpi display, or export the map to a vector format like PDF or SVG,
> you won't see it anymore. Same if you use the magnifier tool to zoom
> into the map.
>
> I'm not sure there's anything we can do here. That said, one
> experiment I played with about a year ago was to upgrade the symbol
> preview picture to use a QgsMapCanvas widget. The symbol is then
> rendered using a geometry of suitable shape, and you can nicely
> interact with the preview including being able to zoom in and out of
> the preview (which makes it easy to verify that the multiple layers
> are indeed correctly centered). It also means that we can synchronize
> the scale and center of the symbol preview canvas widget with the main
> map canvas' scale and center, so that symbol sizes set to map units
> show exactly as they'll appear on the map.
>
> If anyone's interested in playing, heres' the WIP commit:
> https://github.com/nyalldawson/QGIS/commit/db8364d989e8092c6db4ebc88deeb62cad4743d1
> . There's a good couple of hours left to go to get that over the
> finish line though.
>
> Nyall
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