[BUGFIX][Server] WMS compliance: stretched, distort, increase, decrease

[rldhont]

The commit d44f1eb seems to break some WMS 1.3.0 client and WMS compliancy.
The commit d44f1eb has been written to fix an issue with QGIS WMS Client and to render image like other WMS Server.

This commit has been written to fix issue introduce by d44f1eb.
It is based on MapServer code:

• https://github.com/mapserver/mapserver/blob/master/mapdraw.c#L115
• https://github.com/mapserver/mapserver/blob/master/HISTORY.TXT#L3768

[nyalldawson]

Since this seems to be a tricky calculation I'd suggest adding unit tests to protect this logic.

[rldhont]

@nyalldawson, this commit comes from this discussion d44f1eb#commitcomment-20259007
Tests has to be updated but unit test are not enough. We need to verify that QGIS WMS client and other WMS client are not broken.

[mhugent]

As the ms calculation uses a mixture of both ratios, I don't think it is 100% correct.
But it does not matter. The image distortion is IMHO a bad idea of the OGC spec. We should disable it completely or use the ms calculation in this PR to not have any troubles with WMS clients.

[rldhont]

Thanks @mhugent

@elpaso can you test this PR ?

[elpaso]

@rldhont you read my mind, I've just built your branch ... :)

[elpaso]

@rldhont tested this PR with QGIS server and QGIS client and there is exactly the same problem we noticed in Lyon, in the attached image you can see the same layer (world shapefile EPSG:4326) rendered directly into QGIS (grey outline) and served through QGIS Server (the coloured polygons).
You can see how the grey outline is the correct one.

[elpaso]

@rldhont, I've tested master again without this PR and it seems to me the right behavior. This PR re-introduces the bug that we've seen in Lyon.

[rldhont]

hi @elpaso, can you provide the URL to the data and the URL generated by QGIS desktop to reprodiuce the issue?

[rldhont]

@mhugent can you provide some example which describe the issue with openlayers that @pka has explain on Twitter?

[elpaso]

@rldhont you can easily reproduce it:

1. create a QGIS project with a single shapefile layer EPSG:4326 whole world
2. serve that project through QGIS Server

then ...

1. open QGIS and load the EPSG:4326 whole world shapefile layer directly inside QGIS
2. add the WMS layer created in the previous steps.
3. enjoy the difference

I checked the QGIS Desktop created URLs and they seems correct: both the BBOX and
HEIGHT/WIDTH seem correct.

Example URL generated by QGIS WMS provider (untiled):
?MAP=/opt/qgis-server/projects/world_qt5.qgs&SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&BBOX=-40.42095738741319622,-189,71.67899928492427364,189&CRS=EPSG:4326&WIDTH=1046&HEIGHT=310&LAYERS=world&STYLES=&FORMAT=image/jpeg&DPI=220&MAP_RESOLUTION=220&FORMAT_OPTIONS=dpi:220

See what happens here with current master (not your PR):

See what happens with your PR (same call):

[rldhont]

@elpaso thanks!
@mhugent any idea about the difference?

[sbrunner]

Why should we change the width and height, shouldn't they simply be unchanged?

[rldhont]

@sbrunner the image has to be stretched by the server to be WMS 1.3.0 compliant

[sbrunner]

I don't see that's scaled back here: https://github.com/rldhont/Quantum-GIS/blob/48835dd6f7fbc4a13fe1e598652cbffa7ab45380/src/server/qgswmsserver.cpp#L1496

[rldhont]

@mhugent and @elpaso this issue is due to the fact that the image ratio does not take account of the Change x- and y- of BBOX for WMS 1.3.0 if axis inverted.

I'll enhance the code to do so!

[sbrunner]

Don't you have unwanted double invert? (with the line 1991)

[rldhont]

no crs84 is an invertion of EPSG:4326.
This code has been copy from the copyMapSettings.

[sbrunner]

OK, thanks :-)

[rldhont]

@elpaso and @mhugent the code has been updated to take account of axis invertion.

So with a world countries data we have:

SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&BBOX=-40.42095738741319622,-189,71.67899928492427364,189&CRS=EPSG:4326&WIDTH=1046&HEIGHT=310&LAYERS=countries&STYLES=&FORMAT=image/jpeg&DPI=220&MAP_RESOLUTION=220&FORMAT_OPTIONS=dpi:220

And I have tested with the OGC CITE ratio-aspect request and we have:
without stretched

SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&LAYERS=cite:Streams,cite:Lakes,cite:Ponds,cite:Bridges,cite:RoadSegments,cite:DividedRoutes,cite:Buildings,cite:MapNeatline&STYLES=&CRS=CRS:84&BBOX=-0.005,-0.0025,0.005,0.0025&WIDTH=200&HEIGHT=100&FORMAT=image/jpeg
with stretched

SERVICE=WMS&VERSION=1.3.0&REQUEST=GetMap&LAYERS=cite:Streams,cite:Lakes,cite:Ponds,cite:Bridges,cite:RoadSegments,cite:DividedRoutes,cite:Buildings,cite:MapNeatline&STYLES=&CRS=CRS:84&BBOX=-0.005,-0.0025,0.005,0.0025&WIDTH=200&HEIGHT=200&FORMAT=image/jpeg

Are you agree with the code now ?