return magnitude multiplier based on units set

python/PyQt6/core/auto_generated/mesh/qgsmeshrenderersettings.sip.in

@@ -448,7 +448,8 @@ Represents a mesh renderer settings for vector datasets displayed with wind barb
 
     double magnitudeMultiplier() const;
 %Docstring
-Returns the multiplier for the magnitude to convert it to knots
+Returns the multiplier for the magnitude to convert it to knots, according to the units set with :py:func:`~QgsMeshRendererVectorWindBarbSettings.setMagnitudeUnits`
+A custom multiplier can be set with :py:func:`~QgsMeshRendererVectorWindBarbSettings.setMagnitudeMultiplier` for the case when units are set to OtherUnit
 %End
 
     void setMagnitudeMultiplier( double magnitudeMultiplier );

python/core/auto_generated/mesh/qgsmeshrenderersettings.sip.in

@@ -448,7 +448,8 @@ Represents a mesh renderer settings for vector datasets displayed with wind barb
 
     double magnitudeMultiplier() const;
 %Docstring
-Returns the multiplier for the magnitude to convert it to knots
+Returns the multiplier for the magnitude to convert it to knots, according to the units set with :py:func:`~QgsMeshRendererVectorWindBarbSettings.setMagnitudeUnits`
+A custom multiplier can be set with :py:func:`~QgsMeshRendererVectorWindBarbSettings.setMagnitudeMultiplier` for the case when units are set to OtherUnit
 %End
 
     void setMagnitudeMultiplier( double magnitudeMultiplier );

src/core/mesh/qgsmeshrenderersettings.cpp

@@ -782,7 +782,22 @@ QDomElement QgsMeshRendererVectorWindBarbSettings::writeXml( QDomDocument &doc )
 
 double QgsMeshRendererVectorWindBarbSettings::magnitudeMultiplier() const
 {
-  return mMagnitudeMultiplier;
+  switch ( mMagnitudeUnits )
+  {
+    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::Knots:
+      return 1.0;
+    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::MetersPerSecond:
+      return 3600.0 / 1852.0;
+    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::KilometersPerHour:
+      return 1.0 / 1.852;
+    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::MilesPerHour:
+      return 1.609344 / 1.852;
+    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::FeetPerSecond:
+      return 3600.0 / 1.852 / 5280.0 * 1.609344 ;
+    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::OtherUnit:
+      return mMagnitudeMultiplier;
+  }
+  return 1.0; // should not reach
 }
 
 void QgsMeshRendererVectorWindBarbSettings::setMagnitudeMultiplier( double magnitudeMultiplier )

src/core/mesh/qgsmeshrenderersettings.h

@@ -419,7 +419,8 @@ class CORE_EXPORT QgsMeshRendererVectorWindBarbSettings
     };
 
     /**
-     * Returns the multiplier for the magnitude to convert it to knots
+     * Returns the multiplier for the magnitude to convert it to knots, according to the units set with setMagnitudeUnits()
+     * A custom multiplier can be set with setMagnitudeMultiplier() for the case when units are set to OtherUnit
      */
     double magnitudeMultiplier() const;
 

src/gui/mesh/qgsmeshrenderervectorsettingswidget.cpp

@@ -50,6 +50,7 @@ QgsMeshRendererVectorSettingsWidget::QgsMeshRendererVectorSettingsWidget( QWidge
   mTracesMaxLengthSpinBox->setClearValue( 100.0 );
 
   mWindBarbLengthSpinBox->setClearValue( 10.0 );
+  mWindBarbMagnitudeMultiplierSpinBox->setValue( 1.0 );
   mWindBarbMagnitudeMultiplierSpinBox->setClearValue( 1.0 );
 
   connect( mColorWidget, &QgsColorButton::colorChanged, this, &QgsMeshRendererVectorSettingsWidget::widgetChanged );
@@ -293,9 +294,9 @@ void QgsMeshRendererVectorSettingsWidget::syncToLayer( )
   // Wind Barb settings
   const QgsMeshRendererVectorWindBarbSettings windBarbSettings = settings.windBarbSettings();
   mWindBarbLengthSpinBox->setValue( windBarbSettings.shaftLength() );
-  mWindBarbMagnitudeMultiplierSpinBox->setValue( windBarbSettings.magnitudeMultiplier() );
   mWindBarbUnitsComboBox->setCurrentIndex( static_cast<int>( windBarbSettings.magnitudeUnits() ) );
-  onWindBarbUnitsChanged( static_cast<int>( windBarbSettings.magnitudeUnits() ) );
+  if ( windBarbSettings.magnitudeUnits() == QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::OtherUnit )
+    mWindBarbMagnitudeMultiplierSpinBox->setValue( windBarbSettings.magnitudeMultiplier() );
 }
 
 void QgsMeshRendererVectorSettingsWidget::onSymbologyChanged( int currentIndex )
@@ -333,30 +334,11 @@ void QgsMeshRendererVectorSettingsWidget::onWindBarbUnitsChanged( int currentInd
 {
   const QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit units =
     static_cast<QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit>( currentIndex );
-  double multiplier;
-  switch ( units )
-  {
-    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::Knots:
-    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::OtherUnit:
-      multiplier = 1.0;
-      break;
-    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::MetersPerSecond:
-      multiplier = 3600.0 / 1852.0;
-      break;
-    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::KilometersPerHour:
-      multiplier = 1.852;
-      break;
-    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::MilesPerHour:
-      multiplier = 1.609344 / 1.852;
-      break;
-    case QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::FeetPerSecond:
-      multiplier = 3600.0 / 1.852 / 5280.0 * 1.609344 ;
-      break;
-  }
 
   mWindBarbMagnitudeMultiplierLabel->setVisible( units == QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::OtherUnit );
   mWindBarbMagnitudeMultiplierSpinBox->setVisible( units == QgsMeshRendererVectorWindBarbSettings::WindSpeedUnit::OtherUnit );
-  mWindBarbMagnitudeMultiplierSpinBox->setValue( multiplier );
+
+  emit widgetChanged();
 }
 
 void QgsMeshRendererVectorSettingsWidget::onColoringMethodChanged()