Add QgsMapSettings::computeExtentForScale and QgsMapSettings::computeScaleForExtent

python/core/auto_generated/qgsmapsettings.sip.in

@@ -664,6 +664,39 @@ transform rectangle from output CRS to layer's CRS
 Returns the coordinate transform from layer's CRS to destination CRS
 
 :return: transform - may be invalid if the transform is not needed
+%End
+
+    QgsRectangle computeExtentForScale( const QgsPointXY &center, double scale ) const;
+%Docstring
+Compute the extent such that its ``center`` is at the specified
+position (mapped to the destinatonCrs) and the zoom factor corresponds
+to the specified ``scale``
+
+:param center: the center, in map coordinates
+:param scale: the desired zoom factor (the x part of 1:x)
+
+:return: an extent which can be passed to :py:class:`QgsMapCanvas`.setExtent
+
+.. seealso:: :py:func:`computeScaleForExtent`
+
+.. versionadded:: 3.22
+%End
+
+    double computeScaleForExtent( const QgsRectangle &extent ) const;
+%Docstring
+Compute the scale that corresponds to the specified ``extent``
+
+:param extent: the extent, as passed to :py:func:`QgsMapCanvas.setExtent`
+
+:return: the scale denominator
+
+.. seealso:: :py:func:`computeExtentForScale`
+
+.. note::
+
+   This function does not consider any map rotation
+
+.. versionadded:: 3.22
 %End
 
     QgsRectangle fullExtent() const;

src/core/qgsmapsettings.cpp

@@ -436,6 +436,39 @@ QgsCoordinateTransform QgsMapSettings::layerTransform( const QgsMapLayer *layer
   return QgsCoordinateTransform( layer->crs(), mDestCRS, mTransformContext );
 }
 
+QgsRectangle QgsMapSettings::computeExtentForScale( const QgsPointXY &center, double scale ) const
+{
+  // Output width in inches
+  const double outputWidthInInches = outputSize().width() / outputDpi();
+
+  // Desired visible width (honouring scale)
+  double scaledWidthInInches = outputWidthInInches * scale;
+
+  if ( mapUnits() == QgsUnitTypes::DistanceDegrees )
+  {
+    // Start with some fraction of the current extent around the center
+    double delta = mExtent.width() / 100.;
+    QgsRectangle ext( center.x() - delta, center.y() - delta, center.x() + delta, center.y() + delta );
+    // Get scale at extent, and then scale extent to the desired scale
+    double testScale = mScaleCalculator.calculate( ext, outputSize().width() );
+    ext.scale( scale / testScale );
+    return ext;
+  }
+  else
+  {
+    // Conversion from inches to mapUnits  - this is safe to use, because we know here that the map units AREN'T in degrees
+    double conversionFactor = QgsUnitTypes::fromUnitToUnitFactor( QgsUnitTypes::DistanceFeet, mapUnits() ) / 12;
+
+    double delta = 0.5 * scaledWidthInInches * conversionFactor;
+    return QgsRectangle( center.x() - delta, center.y() - delta, center.x() + delta, center.y() + delta );
+  }
+}
+
+double QgsMapSettings::computeScaleForExtent( const QgsRectangle &extent ) const
+{
+  return mScaleCalculator.calculate( extent, outputSize().width() );
+}
+
 double QgsMapSettings::layerToMapUnits( const QgsMapLayer *layer, const QgsRectangle &referenceExtent ) const
 {
   return layerTransform( layer ).scaleFactor( referenceExtent );

src/core/qgsmapsettings.h

@@ -600,6 +600,28 @@ class CORE_EXPORT QgsMapSettings : public QgsTemporalRangeObject
      */
     QgsCoordinateTransform layerTransform( const QgsMapLayer *layer ) const;
 
+    /**
+     * \brief Compute the extent such that its \a center is at the specified
+     * position (mapped to the destinatonCrs) and the zoom factor corresponds
+     * to the specified \a scale
+     * \param center the center, in map coordinates
+     * \param scale the desired zoom factor (the x part of 1:x)
+     * \returns an extent which can be passed to QgsMapCanvas::setExtent
+     * \see computeScaleForExtent()
+     * \since QGIS 3.22
+     */
+    QgsRectangle computeExtentForScale( const QgsPointXY &center, double scale ) const;
+
+    /**
+     * \brief Compute the scale that corresponds to the specified \a extent
+     * \param extent the extent, as passed to \see QgsMapCanvas::setExtent
+     * \returns the scale denominator
+     * \see computeExtentForScale()
+     * \note This function does not consider any map rotation
+     * \since QGIS 3.22
+     */
+    double computeScaleForExtent( const QgsRectangle &extent ) const;
+
     //! returns current extent of layer set
     QgsRectangle fullExtent() const;
 

tests/src/core/testqgsmapsettings.cpp

@@ -62,6 +62,8 @@ class TestQgsMapSettings: public QObject
     void testRenderedFeatureHandlers();
     void testCustomRenderingFlags();
     void testClippingRegions();
+    void testComputeExtentForScale();
+    void testComputeScaleForExtent();
 
   private:
     QString toString( const QPolygonF &p, int decimalPlaces = 2 ) const;
@@ -624,5 +626,37 @@ void TestQgsMapSettings::testClippingRegions()
   QCOMPARE( settings.clippingRegions().at( 0 ).geometry().asWkt(), QStringLiteral( "Polygon ((10 0, 11 0, 11 1, 10 1, 10 0))" ) ) ;
 }
 
+void TestQgsMapSettings::testComputeExtentForScale()
+{
+  QgsMapSettings settings;
+  settings.setExtent( QgsRectangle( -500., -500., 500., 500. ) ); // Just to ensure settings are valid
+  settings.setDestinationCrs( QgsCoordinateReferenceSystem( "EPSG:3857" ) );
+
+  settings.setOutputSize( QSize( 1000, 1000 ) );
+
+  QgsRectangle rect = settings.computeExtentForScale( QgsPoint( 0, 0 ), 500 );
+
+  //                   [                   output width in inches                   ] * [scale]
+  double widthInches = settings.outputSize().width() / double( settings.outputDpi() ) * 500;
+  double widthMapUnits = widthInches * QgsUnitTypes::fromUnitToUnitFactor( QgsUnitTypes::DistanceFeet, settings.mapUnits() ) / 12;
+  QGSCOMPARENEARRECTANGLE( rect, QgsRectangle( - 0.5 * widthMapUnits, - 0.5 * widthMapUnits, 0.5 * widthMapUnits, 0.5 * widthMapUnits ), 0.0001 );
+
+}
+
+void TestQgsMapSettings::testComputeScaleForExtent()
+{
+  QgsMapSettings settings;
+  settings.setExtent( QgsRectangle( -500., -500., 500., 500. ) ); // Just to ensure settings are valid
+  settings.setDestinationCrs( QgsCoordinateReferenceSystem( "EPSG:3857" ) );
+
+  settings.setOutputSize( QSize( 1000, 1000 ) );
+
+  double scale = settings.computeScaleForExtent( QgsRectangle( -500., -500., 500., 500. ) );
+
+  double widthInches = 1000 * QgsUnitTypes::fromUnitToUnitFactor( settings.mapUnits(), QgsUnitTypes::DistanceFeet ) * 12;
+  double testScale = widthInches * settings.outputDpi() / double( settings.outputSize().width() );
+  QGSCOMPARENEAR( scale, testScale, 0.001 );
+}
+
 QGSTEST_MAIN( TestQgsMapSettings )
 #include "testqgsmapsettings.moc"