[feature] Add a QgsScaleCalculator function to calculate canvas width…

… from a given extent, scale, and dpi
qgis · Dec 30, 2021 · 83b5183 · 83b5183
1 parent 6e60731
commit 83b5183
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Showing 5 changed files with 101 additions and 15 deletions.
diff --git a/python/core/auto_generated/qgsscalecalculator.sip.in b/python/core/auto_generated/qgsscalecalculator.sip.in
@@ -65,6 +65,18 @@ Calculate the scale denominator
 :param canvasWidth: Width of the map canvas in pixel (physical) units
 
 :return: scale denominator of current map view, e.g. 1000.0 for a 1:1000 map.
+%End
+
+    QSizeF calculateImageSize( const QgsRectangle &mapExtent, double scale ) const;
+%Docstring
+Calculate the image size in pixel (physical) units
+
+:param mapExtent: :py:class:`QgsRectangle` containing the current map extent
+:param scale: Scale denominator, e.g. 1000.0 for a 1:1000 map
+
+:return: image size
+
+.. versionadded:: 3.24
 %End
 
     double calculateGeographicDistance( const QgsRectangle &mapExtent ) const;

diff --git a/src/core/qgsscalecalculator.cpp b/src/core/qgsscalecalculator.cpp
@@ -49,45 +49,67 @@ QgsUnitTypes::DistanceUnit QgsScaleCalculator::mapUnits() const
 
 double QgsScaleCalculator::calculate( const QgsRectangle &mapExtent, double canvasWidth )  const
 {
+  if ( qgsDoubleNear( canvasWidth, 0. ) || qgsDoubleNear( mDpi, 0.0 ) )
+  {
+    QgsDebugMsg( QStringLiteral( "Can't calculate scale from the input values" ) );
+    return 0;
+  }
+
+  double conversionFactor = 0;
+  double delta = 0;
+  calculateMetrics( mapExtent, delta, conversionFactor );
+
+  const double scale = ( delta * conversionFactor ) / ( static_cast< double >( canvasWidth ) / mDpi );
+  QgsDebugMsgLevel( QStringLiteral( "scale = %1 conversionFactor = %2" ).arg( scale ).arg( conversionFactor ), 4 );
+  return scale;
+}
+
+QSizeF QgsScaleCalculator::calculateImageSize( const QgsRectangle &mapExtent, double scale )  const
+{
+  if ( qgsDoubleNear( scale, 0.0 ) || qgsDoubleNear( mDpi, 0.0 ) )
+  {
+    QgsDebugMsg( QStringLiteral( "Can't calculate image size from the input values" ) );
+    return QSizeF();
+  }
   double conversionFactor = 0;
   double delta = 0;
-  // calculation is based on the map units and extent, the dpi of the
-  // users display, and the canvas width
+
+  calculateMetrics( mapExtent, delta, conversionFactor );
+  const double imageWidth = ( delta * conversionFactor ) / ( static_cast< double >( scale ) ) * mDpi;
+  const double deltaHeight = ( mapExtent.yMaximum() - mapExtent.yMinimum() ) * delta / ( mapExtent.xMaximum() - mapExtent.xMinimum() );
+  const double imageHeight = ( deltaHeight * conversionFactor ) / ( static_cast< double >( scale ) ) * mDpi;
+
+  QgsDebugMsgLevel( QStringLiteral( "imageWidth = %1 imageHeight = %2 conversionFactor = %3" )
+                    .arg( imageWidth ).arg( imageHeight ).arg( conversionFactor ), 4 );
+
+  return QSizeF( imageWidth, imageHeight );
+}
+
+void QgsScaleCalculator::calculateMetrics( const QgsRectangle &mapExtent, double &delta, double &conversionFactor ) const
+{
+  delta = mapExtent.xMaximum() - mapExtent.xMinimum();
   switch ( mMapUnits )
   {
     case QgsUnitTypes::DistanceMeters:
       // convert meters to inches
       conversionFactor = 39.3700787;
-      delta = mapExtent.xMaximum() - mapExtent.xMinimum();
       break;
     case QgsUnitTypes::DistanceFeet:
       conversionFactor = 12.0;
-      delta = mapExtent.xMaximum() - mapExtent.xMinimum();
       break;
     case QgsUnitTypes::DistanceNauticalMiles:
       // convert nautical miles to inches
       conversionFactor = 72913.4;
-      delta = mapExtent.xMaximum() - mapExtent.xMinimum();
       break;
-
     default:
     case QgsUnitTypes::DistanceDegrees:
       // degrees require conversion to meters first
       conversionFactor = 39.3700787;
       delta = calculateGeographicDistance( mapExtent );
       break;
   }
-  if ( qgsDoubleNear( canvasWidth, 0. ) || qgsDoubleNear( mDpi, 0.0 ) )
-  {
-    QgsDebugMsg( QStringLiteral( "Can't calculate scale from the input values" ) );
-    return 0;
-  }
-  const double scale = ( delta * conversionFactor ) / ( static_cast< double >( canvasWidth ) / mDpi );
-  QgsDebugMsgLevel( QStringLiteral( "scale = %1 conversionFactor = %2" ).arg( scale ).arg( conversionFactor ), 4 );
-  return scale;
 }
 
-
 double QgsScaleCalculator::calculateGeographicDistance( const QgsRectangle &mapExtent ) const
 {
   // need to calculate the x distance in meters

diff --git a/src/core/qgsscalecalculator.h b/src/core/qgsscalecalculator.h
@@ -72,6 +72,15 @@ class CORE_EXPORT QgsScaleCalculator
      */
     double calculate( const QgsRectangle &mapExtent, double canvasWidth ) const;
 
+    /**
+     * Calculate the image size in pixel (physical) units
+     * \param mapExtent QgsRectangle containing the current map extent
+     * \param scale Scale denominator, e.g. 1000.0 for a 1:1000 map
+     * \returns image size
+     * \since QGIS 3.24
+     */
+    QSizeF calculateImageSize( const QgsRectangle &mapExtent, double scale ) const;
+
     /**
      * Calculate the distance between two points in geographic coordinates.
      * Used to calculate scale for map views with geographic (decimal degree)
@@ -82,6 +91,9 @@ class CORE_EXPORT QgsScaleCalculator
 
   private:
 
+    //! Calculate the \a delta and \a conversionFactor values based on the provided \a mapExtent.
+    void calculateMetrics( const QgsRectangle &mapExtent, double &delta, double &conversionFactor ) const;
+
     //! dpi member
     double mDpi;
 

diff --git a/tests/src/python/CMakeLists.txt b/tests/src/python/CMakeLists.txt
@@ -291,6 +291,7 @@ ADD_PYTHON_TEST(PyQgsRenderer test_qgsrenderer.py)
 ADD_PYTHON_TEST(PyQgsReport test_qgsreport.py)
 ADD_PYTHON_TEST(PyQgsRulebasedRenderer test_qgsrulebasedrenderer.py)
 ADD_PYTHON_TEST(PyQgsScaleBarRendererRegistry test_qgsscalebarrendererregistry.py)
+ADD_PYTHON_TEST(PyQgsScaleCalculator test_qgsscalecalculator.py)
 ADD_PYTHON_TEST(PyQgsScaleWidget test_qgsscalewidget.py)
 ADD_PYTHON_TEST(PyQgsServerWMSGetFeatureInfo test_qgsserver_wms_getfeatureinfo.py)
 ADD_PYTHON_TEST(PyQgsSingleSymbolRenderer test_qgssinglesymbolrenderer.py)

diff --git a/tests/src/python/test_qgsscalecalculator.py b/tests/src/python/test_qgsscalecalculator.py
@@ -0,0 +1,39 @@
+# -*- coding: utf-8 -*-
+"""QGIS Unit tests for QgsScaleCalculator
+
+.. note:: This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+"""
+__author__ = 'Mathieu Pellerin'
+__date__ = '30/12/2021'
+__copyright__ = 'Copyright 2021, The QGIS Project'
+
+import qgis  # NOQA
+import math
+from qgis.PyQt.QtCore import Qt, QSizeF
+from qgis.PyQt.QtTest import QSignalSpy
+
+from qgis.core import QgsScaleCalculator, QgsRectangle, QgsUnitTypes
+from qgis.testing import start_app, unittest
+
+start_app()
+
+
+class TestQgsScaleWidget(unittest.TestCase):
+
+    def testCalculateImageSize(self):
+        calculator = QgsScaleCalculator()
+
+        calculator.setDpi(96)
+        calculator.setMapUnits(QgsUnitTypes.DistanceMeters)
+
+        extent = QgsRectangle(336609, 1162304, 354942, 1168151)
+        image_size = calculator.calculateImageSize(extent, 65000)
+        self.assertAlmostEqual(image_size.width(), 1066.001, 3)
+        self.assertAlmostEqual(image_size.height(), 339.983, 3)
+
+
+if __name__ == '__main__':
+    unittest.main()