use scale and mapUnitsPerPixel from map parameters bbox and size

in case bbox and size is given in the GetLegendGraphics request, the size of symbols defined by map units is calculated regarding the scale of that.

fixes #21309

Author: signedav

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -395,7 +395,7 @@ QSizeF QgsSymbolLegendNode::drawSymbol( const QgsLegendSettings &settings, ItemC
   QgsRenderContext context;
   context.setScaleFactor( settings.dpi() / 25.4 );
   context.setRendererScale( settings.mapScale() );
-  context.setMapToPixel( QgsMapToPixel( 1 / ( settings.mmPerMapUnit() * context.scaleFactor() ) ) );
+  context.setMapToPixel( QgsMapToPixel( settings.mapUnitsPerPixel() == 1 ? 1 / ( settings.mmPerMapUnit() * context.scaleFactor() ) : settings.mapUnitsPerPixel() ) );
   context.setForceVectorOutput( true );
   context.setPainter( ctx ? ctx->painter : nullptr );
 

src/core/qgslegendsettings.h

@@ -178,6 +178,18 @@ class CORE_EXPORT QgsLegendSettings
      */
     void setMapScale( double scale ) { mMapScale = scale; }
 
+    /**
+     * Returns the factor of map units per pixel used for symbol sized by map units.
+     * \see setMapScale()
+     */
+    double mapUnitsPerPixel() const { return mMapUnitsPerPixel; }
+
+    /**
+     * Sets the map units per pixel \a mapUnitsPerPixel mostly taken from the map settings.
+     * \see mapUnitsPerPixel()
+     */
+    void setMapUnitsPerPixel( double mapUnitsPerPixel ) { mMapUnitsPerPixel = mapUnitsPerPixel; }
+
     int dpi() const { return mDpi; }
     void setDpi( int dpi ) { mDpi = dpi; }
 
@@ -290,6 +302,9 @@ class CORE_EXPORT QgsLegendSettings
     //! Denominator of map's scale
     double mMapScale = 1;
 
+    //! the map units per pixel given by the map parameters - for symbols with size given in map units
+    double mMapUnitsPerPixel = 1;
+
     //! DPI to be used when rendering legend
     int mDpi = 96;
 };

src/server/services/wms/qgswmsrenderer.cpp

@@ -183,6 +183,16 @@ namespace QgsWms
     std::unique_ptr<QImage> image;
     std::unique_ptr<QPainter> painter;
 
+    // getting scale from bbox
+    if( !mWmsParameters.bbox().isEmpty() )
+    {
+        QgsMapSettings mapSettings;
+        image.reset( createImage( mWmsParameters.widthAsInt(), mWmsParameters.heightAsInt(), false ) );
+        configureMapSettings( image.get(), mapSettings );
+        legendSettings.setMapScale( mapSettings.scale() );
+        legendSettings.setMapUnitsPerPixel( mapSettings.mapUnitsPerPixel() );
+    }
+
     if ( !mWmsParameters.rule().isEmpty() )
     {
       QString rule = mWmsParameters.rule();