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

[Richard Duivenvoorde]

Not sure if it is related, but  aren't these actually dateline related
issues? Would be cool to tackle the whole dateline issue too :-)

Maybe these are related (and having some test cases):

https://issues.qgis.org/issues/13380

https://issues.qgis.org/issues/19626
https://issues.qgis.org/issues/597

In the latter there is a link to the grass 'solution'.
I had a short look into it, my feeling was that you use 'virtual' worlds
on both sides of the dateline to be able to paint the right lines.
If you then show them (like googlemaps does when you zoom out a lot), OR
cut them out to only show one world is up to you. Though I'm not sure
how to cut out a not rectangulare projection easy :-)

Regards,

Richard Duivenvoorde


On 08/29/2018 09:58 PM, Even Rouault wrote:
> On mercredi 29 août 2018 21:50:25 CEST Andreas Neumann wrote:
>> Hi Even,
>>
>> Thank you very much for the detailed analysis of the spatial filter
>> issue we have here, that causes the disappearing grid lines.
>>
>> Would you be interested to work on fixing this issue on our bug fixing
>> program? This problem annoyed me for quite some time. And it appears on
>> different world projections.
> 
> I might have a look at this, if nobody else more familiar with the relevant 
> code looks at it. Is there a ticket filed about that ?
> 
> Even
> 
>>
>> Thanks,
>>
>> Andreas
>>
>> Am 28.08.2018 um 22:19 schrieb 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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> 
>