Rendering issue due to different DPI between QGISServer and QWC2

Ducarouge commented 3 months ago

Hi,

After an update of a QWC2 instance, we have some rendering issues in a QGIS project containing symbology based on scales rules.

Indeed :

the QWC2 application is based on a 96 DPI display: https://github.com/qgis/qwc2/blob/54844de3298c0e23c3e85bcb6e683ca7a04c54d9/utils/MapUtils.js#L14
QGIS Server seems to respect the OGC specifications with a pixel of 0.28x0.28 mm or 90.71 DPI: (section 7.2.4.6.9 Scale denominators) https://www.ogc.org/standard/wms/

So this is what we get from the same call to QGIS Server with and without the DPI parameter :

With DPI=96 (corresponding to the rendering in qgis desktop and before update in QWC at scale 1/5000):
Without (corresponding to the actual rendering in QWC at scale 1/5000 ):

Is this a bug or there is a way to configure QWC2 (or the QGIS Project) to get the same rendering between QWC2 and QGIS ?

manisandro commented 3 months ago

Does OpenLayers actually send a DPI parameter in the GetMap request?

Ducarouge commented 3 months ago

No, actually, Openlayers send a POST request for the GetMap without this DPI parameter and, i guess, this why we don't have the same rendering.

manisandro commented 3 months ago

DPI:  options.serverType === 'qgis' ? (options.dpi || ConfigUtils.getConfigProp("wmsDpi") || 96) : undefined

Ducarouge commented 3 months ago

When i re-add the DPI with the line above, it works as expected.

For the fix, do i have to do a PR or you do ?

Thank you.

manisandro commented 3 months ago

qgis / qwc2-demo-app