Add method to recover extent parameter CRS

python/core/processing/qgsprocessingalgorithm.sip

@@ -688,6 +688,14 @@ class QgsProcessingAlgorithm
  :rtype: QgsGeometry
 %End
 
+    QgsCoordinateReferenceSystem parameterAsExtentCrs( const QVariantMap &parameters, const QString &name, QgsProcessingContext &context );
+%Docstring
+ Returns the coordinate reference system associated with an extent parameter value.
+
+.. seealso:: parameterAsExtent()
+ :rtype: QgsCoordinateReferenceSystem
+%End
+
     QgsPointXY parameterAsPoint( const QVariantMap &parameters, const QString &name, QgsProcessingContext &context ) const;
 %Docstring
  Evaluates the parameter with matching ``name`` to a point.

python/core/processing/qgsprocessingparameters.sip

@@ -543,6 +543,7 @@ class QgsProcessingParameters
  reprojected so that it is in the specified ``crs``. In this case the extent of the reproject rectangle will be returned.
 
 .. seealso:: parameterAsExtentGeometry()
+.. seealso:: parameterAsExtentCrs()
  :rtype: QgsRectangle
 %End
 
@@ -557,9 +558,18 @@ class QgsProcessingParameters
  just the extent of the reprojected rectangle).
 
 .. seealso:: parameterAsExtent()
+.. seealso:: parameterAsExtentCrs()
  :rtype: QgsGeometry
 %End
 
+    static QgsCoordinateReferenceSystem parameterAsExtentCrs( const QgsProcessingParameterDefinition *definition, const QVariantMap &parameters, QgsProcessingContext &context );
+%Docstring
+ Returns the coordinate reference system associated with an extent parameter value.
+
+.. seealso:: parameterAsExtent()
+ :rtype: QgsCoordinateReferenceSystem
+%End
+
     static QgsPointXY parameterAsPoint( const QgsProcessingParameterDefinition *definition, const QVariantMap &parameters, QgsProcessingContext &context );
 %Docstring
  Evaluates the parameter with matching ``definition`` to a point.

src/core/processing/qgsprocessingalgorithm.cpp

@@ -550,6 +550,11 @@ QgsCoordinateReferenceSystem QgsProcessingAlgorithm::parameterAsCrs( const QVari
   return QgsProcessingParameters::parameterAsCrs( parameterDefinition( name ), parameters, context );
 }
 
+QgsCoordinateReferenceSystem QgsProcessingAlgorithm::parameterAsExtentCrs( const QVariantMap &parameters, const QString &name, QgsProcessingContext &context )
+{
+  return QgsProcessingParameters::parameterAsCrs( parameterDefinition( name ), parameters, context );
+}
+
 QgsRectangle QgsProcessingAlgorithm::parameterAsExtent( const QVariantMap &parameters, const QString &name, QgsProcessingContext &context, const QgsCoordinateReferenceSystem &crs ) const
 {
   return QgsProcessingParameters::parameterAsExtent( parameterDefinition( name ), parameters, context, crs );

src/core/processing/qgsprocessingalgorithm.h

@@ -670,6 +670,13 @@ class CORE_EXPORT QgsProcessingAlgorithm
     QgsGeometry parameterAsExtentGeometry( const QVariantMap &parameters, const QString &name, QgsProcessingContext &context,
                                            const QgsCoordinateReferenceSystem &crs = QgsCoordinateReferenceSystem() );
 
+    /**
+     * Returns the coordinate reference system associated with an extent parameter value.
+     *
+     * \see parameterAsExtent()
+     */
+    QgsCoordinateReferenceSystem parameterAsExtentCrs( const QVariantMap &parameters, const QString &name, QgsProcessingContext &context );
+
     /**
      * Evaluates the parameter with matching \a name to a point.
      */

src/core/processing/qgsprocessingparameters.cpp

@@ -569,6 +569,25 @@ QgsGeometry QgsProcessingParameters::parameterAsExtentGeometry( const QgsProcess
   return QgsGeometry::fromRect( parameterAsExtent( definition, parameters, context, crs ) );
 }
 
+QgsCoordinateReferenceSystem QgsProcessingParameters::parameterAsExtentCrs( const QgsProcessingParameterDefinition *definition, const QVariantMap &parameters, QgsProcessingContext &context )
+{
+  QVariant val = parameters.value( definition->name() );
+
+  if ( val.canConvert< QgsReferencedRectangle >() )
+  {
+    QgsReferencedRectangle rr = val.value<QgsReferencedRectangle>();
+    if ( rr.crs().isValid() )
+    {
+      return rr.crs();
+    }
+  }
+
+  if ( context.project() )
+    return context.project()->crs();
+  else
+    return QgsCoordinateReferenceSystem();
+}
+
 QgsPointXY QgsProcessingParameters::parameterAsPoint( const QgsProcessingParameterDefinition *definition, const QVariantMap &parameters, QgsProcessingContext &context )
 {
   if ( !definition )

src/core/processing/qgsprocessingparameters.h

@@ -570,6 +570,7 @@ class CORE_EXPORT QgsProcessingParameters
      * reprojected so that it is in the specified \a crs. In this case the extent of the reproject rectangle will be returned.
      *
      * \see parameterAsExtentGeometry()
+     * \see parameterAsExtentCrs()
      */
     static QgsRectangle parameterAsExtent( const QgsProcessingParameterDefinition *definition, const QVariantMap &parameters, QgsProcessingContext &context,
                                            const QgsCoordinateReferenceSystem &crs = QgsCoordinateReferenceSystem() );
@@ -583,10 +584,18 @@ class CORE_EXPORT QgsProcessingParameters
      * just the extent of the reprojected rectangle).
      *
      * \see parameterAsExtent()
+     * \see parameterAsExtentCrs()
      */
     static QgsGeometry parameterAsExtentGeometry( const QgsProcessingParameterDefinition *definition, const QVariantMap &parameters, QgsProcessingContext &context,
         const QgsCoordinateReferenceSystem &crs = QgsCoordinateReferenceSystem() );
 
+    /**
+     * Returns the coordinate reference system associated with an extent parameter value.
+     *
+     * \see parameterAsExtent()
+     */
+    static QgsCoordinateReferenceSystem parameterAsExtentCrs( const QgsProcessingParameterDefinition *definition, const QVariantMap &parameters, QgsProcessingContext &context );
+
     /**
      * Evaluates the parameter with matching \a definition to a point.
      */

tests/src/core/testqgsprocessing.cpp

@@ -1915,13 +1915,17 @@ void TestQgsProcessing::parameterExtent()
   QgsGeometry gExt = QgsProcessingParameters::parameterAsExtentGeometry( def.get(), params, context, QgsCoordinateReferenceSystem( "EPSG:3785" ) );
   QCOMPARE( gExt.exportToWkt( 1 ), QStringLiteral( "Polygon ((11.1 12.2, 13.3 12.2, 13.3 14.4, 11.1 14.4, 11.1 12.2))" ) );
 
+  p.setCrs( QgsCoordinateReferenceSystem( "EPSG:3785" ) );
+  QCOMPARE( QgsProcessingParameters::parameterAsExtentCrs( def.get(), params, context ).authid(), QStringLiteral( "EPSG:3785" ) );
+
   // QgsReferencedRectangle
   params.insert( "non_optional", QgsReferencedRectangle( QgsRectangle( 1.1, 2.2, 3.3, 4.4 ), QgsCoordinateReferenceSystem( "EPSG:4326" ) ) );
   ext = QgsProcessingParameters::parameterAsExtent( def.get(), params, context );
   QGSCOMPARENEAR( ext.xMinimum(), 1.1, 0.001 );
   QGSCOMPARENEAR( ext.xMaximum(), 3.3, 0.001 );
   QGSCOMPARENEAR( ext.yMinimum(),  2.2, 0.001 );
   QGSCOMPARENEAR( ext.yMaximum(), 4.4, 0.001 );
+  QCOMPARE( QgsProcessingParameters::parameterAsExtentCrs( def.get(), params, context ).authid(), QStringLiteral( "EPSG:4326" ) );
 
   // with target CRS
   ext = QgsProcessingParameters::parameterAsExtent( def.get(), params, context, QgsCoordinateReferenceSystem( "EPSG:3785" ) );