[QGIS-Developer] Disappearing -180 / 180 longitude grid lines on certain zoom levels

[Even Rouault]

Yes, seems restricted to reprojection cases

This seems to be an issue with the spatial filter issued to OGR

At the zooms where the lines disappear, there are requests like:

Thread 23 "Thread (pooled)" hit Breakpoint 2, OGR_L_SetSpatialFilterRect 
(hLayer=0x7f81180c90a0, dfMinX=-179.79163612932865, 
dfMinY=-69.446164378986353, dfMaxX=179.90530755284408, 
dfMaxY=78.959077253477474) at ogrlayer.cpp:1223

At the zooms where that work (even when zoomed in), there are like:

Thread 29 "Thread (pooled)" hit Breakpoint 2, OGR_L_SetSpatialFilterRect 
(hLayer=0x7f81180c90a0, dfMinX=-180, dfMinY=-90, dfMaxX=180, dfMaxY=90) at 
ogrlayer.cpp:1223


I haven't looked at the QGIS code that computes this bounding box, but from my 
experience with gdalwarp which has similar challenges, it is tricky to compute 
a source bounding box from a target bounding box, because sometimes the 
coordinates in the target bounding box do not correspond to a physical point 
on Eath, and hence inverse projection fails. So you have to resort to a grid 
sampling approach, but that makes you miss the exact boundaries. So probably 
that a band-aid fix would be to add some ad-hoc logic, like "if the source SRS 
is long/lat, and the computed extent is almost worldwide, then extend it to 
full worlwide (or do not emit a spatial filter at all)"

Even


> With EPSG:4326 it does not seem to happen, but I tried EPSG:3857 and the
> -180,180 grid lines do appear and disappear at different zoom levels. I
> also confirm this with the Robinson projection with the addition that the
> -90, 90 degree latitude lines also appear and disappear.
> 
> Calvin
> 
> On Tue, Aug 28, 2018 at 3:31 PM, Andreas Neumann <a.neumann at carto.net>
> 
> wrote:
> > Hi again,
> > 
> > For some time already I noticed that, depending on the zoom level, the
> > -180 / 180 degree grid lines appear / disappear in QGIS. This is unrelated
> > to the EqualEarth projection just discussed and also appears on other
> > world
> > projections, like the Robinson projection.
> > 
> > Here is the testfile I used with world (countries) and gridlines:
> > http://www.carto.net/neumann/temp/gridlines.gpkg
> > 
> > Can you reproduce this behaviour?
> > 
> > Any idea why this happens and how this could be fixed?
> > 
> > Thanks,
> > 
> > Andreas
> > 
> > 
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