[FEATURE] Add option to select kernel shape (quartic, triangular or uniform) in heatmap plugin

Author: nyalldawson

src/plugins/heatmap/heatmap.cpp

@@ -43,6 +43,9 @@
 
 #define NO_DATA -9999
 
+#ifndef M_PI
+#define M_PI 3.14159265358979323846
+#endif
 
 static const QString sName = QObject::tr( "Heatmap" );
 static const QString sDescription = QObject::tr( "Creates a Heatmap raster for the input point vector" );
@@ -109,7 +112,8 @@ void Heatmap::run()
     int rows = d.rows();
     float cellsize = d.cellSizeX(); // or d.cellSizeY();  both have the same value
     float myDecay = d.decayRatio();
-
+    int kernelShape = d.kernelShape();
+       
     // Start working on the input vector
     QgsVectorLayer* inputLayer = d.inputVectorLayer();
 
@@ -259,7 +263,7 @@ void Heatmap::run()
             continue;
           }
 
-          float pixelValue = weight * ( 1 - (( 1 - myDecay ) * distance / myBuffer ) );
+          float pixelValue = weight * calculateKernelValue( distance, myBuffer, kernelShape );
 
           // clearing anamolies along the axes
           if ( xp == 0 && yp == 0 )
@@ -330,6 +334,40 @@ int Heatmap::bufferSize( float radius, float cellsize )
   return buffer;
 }    
 
+float Heatmap::calculateKernelValue( float distance, int bandwidth, int kernelShape )
+{
+  switch (kernelShape) {
+    case HeatmapGui::Triangular:
+      return ( 1 - ( distance / bandwidth ) );
+      
+    case HeatmapGui::Uniform:
+      return uniformKernel( distance, bandwidth );
+      
+    case HeatmapGui::Quartic:
+      return quarticKernel( distance, bandwidth );
+  }
+  return 0;
+  
+}
+
+float Heatmap::uniformKernel( float distance, int bandwidth )
+{
+  // Normalizing constant. Calculated by polar double integrating the kernel function
+  // with radius of 0 to bandwidth and equating area to 1.
+  float k = 2. / (M_PI * (float)bandwidth);
+  
+  // Derived from Wand and Jones (1995), p. 175
+  return k * ( 0.5 / (float)bandwidth);
+}
+
+float Heatmap::quarticKernel( float distance, int bandwidth )
+{
+  // Normalizing constant
+  float k = 16. / (5. * M_PI * pow((float)bandwidth, 2));
+  
+  // Derived from Wand and Jones (1995), p. 175
+  return k * (15. / 16. ) * pow( 1. - pow( distance / (float)bandwidth, 2), 2);
+}
 
 // Unload the plugin by cleaning up the GUI
 void Heatmap::unload()

src/plugins/heatmap/heatmap.h

@@ -85,6 +85,13 @@ class Heatmap: public QObject, public QgisPlugin
     float mapUnitsOf( float meters, QgsCoordinateReferenceSystem crs );
     //! Worker to calculate buffer size in pixels
     int bufferSize( float radius, float cellsize );
+    //! Calculate the value given to a point width a given distance for a specified kernel shape
+    float calculateKernelValue( float distance, int bandwidth, int kernelShape );
+    //! Uniform kernel function
+    float uniformKernel( float distance, int bandwidth );
+    //! Quartic kernel function    
+    float quarticKernel( float distance, int bandwidth );
+    
 
     // MANDATORY PLUGIN PROPERTY DECLARATIONS  .....
 

src/plugins/heatmap/heatmapgui.cpp

@@ -375,6 +375,11 @@ int HeatmapGui::radiusUnit()
   return mRadiusUnitCombo->currentIndex();
 }
 
+int HeatmapGui::kernelShape()
+{
+  return kernelShapeCombo->currentIndex();
+}
+
 float HeatmapGui::decayRatio()
 {
   return mDecayLineEdit->text().toFloat();

src/plugins/heatmap/heatmapgui.h

@@ -35,6 +35,13 @@ class HeatmapGui : public QDialog, private Ui::HeatmapGuiBase
       Meters,
       MapUnits
     };
+    
+    enum kernelShape
+    {
+      Triangular,
+      Uniform,
+      Quartic
+    };
 
     /** Returns whether to apply weighted heat */
     bool weighted();
@@ -47,6 +54,9 @@ class HeatmapGui : public QDialog, private Ui::HeatmapGuiBase
 
     /** Return the radius Unit (meters/map units) */
     int radiusUnit();
+    
+    /** Return the selected kernel shape */
+    int kernelShape();
 
     /** Return the decay ratio */
     float decayRatio();

src/plugins/heatmap/heatmapguibase.ui

@@ -7,7 +7,7 @@
     <x>0</x>
     <y>0</y>
     <width>460</width>
-    <height>382</height>
+    <height>415</height>
    </rect>
   </property>
   <property name="sizePolicy">
@@ -180,21 +180,21 @@
       </item>
       <item row="1" column="0">
        <layout class="QGridLayout" name="gridLayout_2">
-        <item row="0" column="0">
+        <item row="1" column="0">
          <widget class="QCheckBox" name="useRadius">
           <property name="text">
            <string>Use Radius from field</string>
           </property>
          </widget>
         </item>
-        <item row="0" column="1">
+        <item row="1" column="1">
          <widget class="QComboBox" name="radiusFieldCombo">
           <property name="enabled">
            <bool>false</bool>
           </property>
          </widget>
         </item>
-        <item row="0" column="2">
+        <item row="1" column="2">
          <widget class="QComboBox" name="radiusFieldUnitCombo">
           <property name="enabled">
            <bool>false</bool>
@@ -211,34 +211,60 @@
           </item>
          </widget>
         </item>
-        <item row="1" column="0">
+        <item row="2" column="0">
          <widget class="QCheckBox" name="useWeight">
           <property name="text">
            <string>Use Weight from field</string>
           </property>
          </widget>
         </item>
-        <item row="1" column="1" colspan="2">
+        <item row="2" column="1" colspan="2">
          <widget class="QComboBox" name="weightFieldCombo">
           <property name="enabled">
            <bool>false</bool>
           </property>
          </widget>
         </item>
-        <item row="2" column="1" colspan="2">
+        <item row="3" column="1" colspan="2">
          <widget class="QLineEdit" name="mDecayLineEdit">
           <property name="text">
            <string>0.0</string>
           </property>
          </widget>
         </item>
-        <item row="2" column="0">
+        <item row="3" column="0">
          <widget class="QLabel" name="mDecayLabel">
           <property name="text">
            <string>Decay Ratio</string>
           </property>
          </widget>
         </item>
+        <item row="0" column="1" colspan="2">
+         <widget class="QComboBox" name="kernelShapeCombo">
+          <item>
+           <property name="text">
+            <string>Triangular</string>
+           </property>
+          </item>
+          <item>
+           <property name="text">
+            <string>Uniform</string>
+           </property>
+          </item>
+          <item>
+           <property name="text">
+            <string>Quartic</string>
+           </property>
+          </item>          
+         </widget>
+        </item>
+        <item row="0" column="0">
+         <widget class="QLabel" name="kernelShapeLable">
+          <property name="text">
+           <string>Kernel Shape</string>
+          </property>
+         </widget>
+        </item>
        </layout>
       </item>
      </layout>