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.

calculate mMmPerMapUnit with mapUnitsPerPixel to avoid to have redundant info
fixes #21309

(cherry-picked from a04f91b 7de50a0 3e86163)

Author: signedav

python/core/auto_generated/qgslegendsettings.sip.in

@@ -192,6 +192,24 @@ Sets the legend map ``scale``.
 The ``scale`` value indicates the scale denominator, e.g. 1000.0 for a 1:1000 map.
 
 .. seealso:: :py:func:`mapScale`
+%End
+
+    double mapUnitsPerPixel() const;
+%Docstring
+Returns the factor of map units per pixel for symbols with size given in map units calculated by dpi and mmPerMapUnit
+
+.. seealso:: :py:func:`setMapUnitsPerPixel`
+
+.. versionadded:: 3.6
+%End
+
+    void setMapUnitsPerPixel( double mapUnitsPerPixel );
+%Docstring
+Sets the the mmPerMapUnit calculated by ``mapUnitsPerPixel`` mostly taken from the map settings.
+
+.. seealso:: :py:func:`mapUnitsPerPixel`
+
+.. versionadded:: 3.6
 %End
 
     int dpi() const;

src/core/qgslegendsettings.cpp

@@ -37,6 +37,16 @@ QgsLegendSettings::QgsLegendSettings()
   rstyle( QgsLegendStyle::SymbolLabel ).rfont().setPointSizeF( 12.0 );
 }
 
+double QgsLegendSettings::mapUnitsPerPixel() const
+{
+  return 1 / ( mMmPerMapUnit * ( mDpi / 25.4 ) );
+}
+
+void QgsLegendSettings::setMapUnitsPerPixel( double mapUnitsPerPixel )
+{
+  mMmPerMapUnit = 1 / mapUnitsPerPixel / ( mDpi / 25.4 );
+}
+
 QStringList QgsLegendSettings::evaluateItemText( const QString &text, const QgsExpressionContext &context ) const
 {
   const QString textToRender = QgsExpression::replaceExpressionText( text, &context );

src/core/qgslegendsettings.h

@@ -178,6 +178,20 @@ class CORE_EXPORT QgsLegendSettings
      */
     void setMapScale( double scale ) { mMapScale = scale; }
 
+    /**
+     * Returns the factor of map units per pixel for symbols with size given in map units calculated by dpi and mmPerMapUnit
+     * \see setMapUnitsPerPixel()
+     * \since QGIS 3.6
+     */
+    double mapUnitsPerPixel() const;
+
+    /**
+     * Sets the the mmPerMapUnit calculated by \a mapUnitsPerPixel mostly taken from the map settings.
+     * \see mapUnitsPerPixel()
+     * \since QGIS 3.6
+     */
+    void setMapUnitsPerPixel( double mapUnitsPerPixel );
+
     int dpi() const { return mDpi; }
     void setDpi( int dpi ) { mDpi = dpi; }
 

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();