[feature] Add WMS/XYZ interpretation type for Terrarium terrain tiles

Allows reading DEM tiles in the Terrarium terrain encoding type (e.g.
the global Mapzen terrain tiles freely available through AWS)

src/providers/wms/qgswmsprovider.cpp

@@ -4844,19 +4844,26 @@ QGISEXTERN QgsProviderMetadata *providerMetadataFactory()
 }
 #endif
 
-Qgis::DataType QgsWmsInterpretationConverter::dataType() const
-{
-  return Qgis::DataType::Float32;
-}
 
 std::unique_ptr<QgsWmsInterpretationConverter> QgsWmsInterpretationConverter::createConverter( const QString &key )
 {
   if ( key == QgsWmsInterpretationConverterMapTilerTerrainRGB::interpretationKey() )
     return std::make_unique<QgsWmsInterpretationConverterMapTilerTerrainRGB>();
+  else if ( key == QgsWmsInterpretationConverterTerrariumRGB::interpretationKey() )
+    return std::make_unique<QgsWmsInterpretationConverterTerrariumRGB>();
 
   return nullptr;
 }
 
+Qgis::DataType QgsWmsInterpretationConverter::dataType() const
+{
+  return Qgis::DataType::Float32;
+}
+
+//
+// QgsWmsInterpretationConverterMapTilerTerrainRGB
+//
+
 void QgsWmsInterpretationConverterMapTilerTerrainRGB::convert( const QRgb &color, float *converted ) const
 {
   int R = qRed( color );
@@ -4879,3 +4886,36 @@ QgsRasterHistogram QgsWmsInterpretationConverterMapTilerTerrainRGB::histogram( i
 {
   return QgsRasterHistogram();
 }
+
+//
+// QgsWmsInterpretationConverterTerrariumRGB
+//
+
+void QgsWmsInterpretationConverterTerrariumRGB::convert( const QRgb &color, float *converted ) const
+{
+  // for description of the "terrarium" format:
+  // https://github.com/tilezen/joerd/blob/master/docs/formats.md
+
+  if ( qAlpha( color ) == 255 )
+  {
+    *converted = qRed( color ) * 256 + qGreen( color ) + qBlue( color ) / 256.f - 32768;
+  }
+  else
+  {
+    *converted = std::numeric_limits<float>::quiet_NaN();
+  }
+}
+
+QgsRasterBandStats QgsWmsInterpretationConverterTerrariumRGB::statistics( int, int, const QgsRectangle &, int, QgsRasterBlockFeedback * ) const
+{
+  QgsRasterBandStats stat;
+  stat.minimumValue = -11000;
+  stat.maximumValue = 9000;
+  stat.statsGathered = QgsRasterBandStats::Min | QgsRasterBandStats::Max;
+  return stat;
+}
+
+QgsRasterHistogram QgsWmsInterpretationConverterTerrariumRGB::histogram( int, int, double, double, const QgsRectangle &, int, bool, QgsRasterBlockFeedback * ) const
+{
+  return QgsRasterHistogram();
+}

src/providers/wms/qgswmsprovider.h

@@ -156,6 +156,30 @@ class QgsWmsInterpretationConverterMapTilerTerrainRGB : public QgsWmsInterpretat
     static QString interpretationKey() {return QStringLiteral( "maptilerterrain" );}
 };
 
+//! Class to convert color to float value following the terrarium terrain RGB interpretation
+class QgsWmsInterpretationConverterTerrariumRGB : public QgsWmsInterpretationConverter
+{
+  public:
+    void convert( const QRgb &color, float *converted ) const override;
+
+    QgsRasterBandStats statistics( int bandNo,
+                                   int stats = QgsRasterBandStats::All,
+                                   const QgsRectangle &extent = QgsRectangle(),
+                                   int sampleSize = 0, QgsRasterBlockFeedback *feedback = nullptr ) const override;
+
+    QgsRasterHistogram histogram( int bandNo,
+                                  int binCount = 0,
+                                  double minimum = std::numeric_limits<double>::quiet_NaN(),
+                                  double maximum = std::numeric_limits<double>::quiet_NaN(),
+                                  const QgsRectangle &extent = QgsRectangle(),
+                                  int sampleSize = 0,
+                                  bool includeOutOfRange = false,
+                                  QgsRasterBlockFeedback *feedback = nullptr ) const override;
+
+    static QString displayName() {return QObject::tr( "Terrarium Terrain RGB" );}
+    static QString interpretationKey() {return QStringLiteral( "terrariumterrain" );}
+};
+
 /**
  *
  * \brief Data provider for OGC WMS layers.

src/providers/wms/qgswmssourceselect.cpp

@@ -1322,6 +1322,7 @@ QgsWmsInterpretationComboBox::QgsWmsInterpretationComboBox( QWidget *parent ): Q
 {
   addItem( tr( "Default" ), QString() );
   addItem( QgsWmsInterpretationConverterMapTilerTerrainRGB::displayName(), QgsWmsInterpretationConverterMapTilerTerrainRGB::interpretationKey() );
+  addItem( QgsWmsInterpretationConverterTerrariumRGB::displayName(), QgsWmsInterpretationConverterTerrariumRGB::interpretationKey() );
 }
 
 void QgsWmsInterpretationComboBox::setInterpretation( const QString &interpretationKey )

tests/src/providers/testqgswmsprovider.cpp

@@ -26,6 +26,8 @@
 #include <qgssinglebandpseudocolorrenderer.h>
 #include <qgsrastershader.h>
 #include <qgsstyle.h>
+#include "qgssinglebandgrayrenderer.h"
+#include "qgsrasterlayer.h"
 
 /**
  * \ingroup UnitTests
@@ -271,6 +273,34 @@ class TestQgsWmsProvider: public QObject
       QVERIFY( imageCheck( "convert_value", mapSettings ) );
     }
 
+    void testTerrariumInterpretation()
+    {
+      QString dataDir( TEST_DATA_DIR );
+
+      QgsXyzConnection xyzConn;
+      xyzConn.interpretation = QgsWmsInterpretationConverterTerrariumRGB::interpretationKey();
+      xyzConn.url = QUrl::fromLocalFile( dataDir + QStringLiteral( "/terrarium_terrain_rgb.png" ) ).toString();
+      QgsRasterLayer layer( xyzConn.encodedUri(), "terrain", "wms" );
+      QVERIFY( layer.isValid() );
+      QVERIFY( layer.dataProvider()->dataType( 1 ) == Qgis::DataType::Float32 );
+
+      QgsSingleBandGrayRenderer *renderer = new QgsSingleBandGrayRenderer( layer.dataProvider(), 1 );
+      QgsContrastEnhancement *e = new QgsContrastEnhancement( Qgis::DataType::Float32 );
+      e->setMinimumValue( -50 );
+      e->setMaximumValue( 50 );
+      e->setContrastEnhancementAlgorithm( QgsContrastEnhancement::StretchToMinimumMaximum );
+      renderer->setContrastEnhancement( e );
+      layer.setRenderer( renderer );
+
+      QgsMapSettings mapSettings;
+      mapSettings.setLayers( QList<QgsMapLayer *>() << &layer );
+      mapSettings.setExtent( layer.extent() );
+      mapSettings.setOutputSize( QSize( 400, 400 ) );
+      mapSettings.setOutputDpi( 96 );
+      mapSettings.setDpiTarget( 48 );
+      QVERIFY( imageCheck( "terrarium_terrain", mapSettings ) );
+    }
+
     bool imageCheck( const QString &testType, QgsMapSettings &mapSettings )
     {
       //use the QgsRenderChecker test utility class to