[backport] SERVER: fix getlegendgraphic with colorramp & JSON

python/core/auto_generated/layertree/qgscolorramplegendnode.sip.in

@@ -55,6 +55,8 @@ Constructor for QgsColorRampLegendNode.
 
     virtual QSizeF drawSymbolText( const QgsLegendSettings &settings, ItemContext *ctx, QSizeF symbolSize ) const;
 
+    virtual QJsonObject exportSymbolToJson( const QgsLegendSettings &settings, const QgsRenderContext &context ) const;
+
 
     void setIconSize( QSize size );
 %Docstring

src/core/layertree/qgscolorramplegendnode.cpp

@@ -25,6 +25,7 @@
 #include "qgsnumericformat.h"
 
 #include <QPalette>
+#include <QBuffer>
 
 QgsColorRampLegendNode::QgsColorRampLegendNode( QgsLayerTreeLayer *nodeLayer, QgsColorRamp *ramp, const QString &minimumLabel, const QString &maximumLabel, QObject *parent )
   : QgsLayerTreeModelLegendNode( nodeLayer, parent )
@@ -476,3 +477,28 @@ QSizeF QgsColorRampLegendNode::drawSymbolText( const QgsLegendSettings &settings
 
   return QSizeF( textWidth, textHeight );
 }
+
+QJsonObject QgsColorRampLegendNode::exportSymbolToJson( const QgsLegendSettings &settings, const QgsRenderContext &context ) const
+{
+  Q_UNUSED( settings );
+  Q_UNUSED( context );
+
+  QJsonObject json;
+
+  const QPixmap icon = data( Qt::DecorationRole ).value<QPixmap>();
+
+  if ( ! icon.isNull() )
+  {
+    const QImage image( icon.toImage() );
+    QByteArray byteArray;
+    QBuffer buffer( &byteArray );
+    image.save( &buffer, "PNG" );
+    const QString base64 = QString::fromLatin1( byteArray.toBase64().data() );
+    json[ QStringLiteral( "icon" ) ] = base64;
+  }
+
+  json [ QStringLiteral( "min" ) ] = mMinimumValue;
+  json [ QStringLiteral( "max" ) ] = mMaximumValue;
+
+  return json;
+}

src/core/layertree/qgscolorramplegendnode.h

@@ -64,6 +64,7 @@ class CORE_EXPORT QgsColorRampLegendNode : public QgsLayerTreeModelLegendNode
     QVariant data( int role ) const override;
     QSizeF drawSymbol( const QgsLegendSettings &settings, ItemContext *ctx, double itemHeight ) const override;
     QSizeF drawSymbolText( const QgsLegendSettings &settings, ItemContext *ctx, QSizeF symbolSize ) const override;
+    QJsonObject exportSymbolToJson( const QgsLegendSettings &settings, const QgsRenderContext &context ) const override;
 
     /**
      * Set the icon \a size, which controls how large the ramp will render in a layer tree widget.

tests/src/python/test_qgsserver_wms_getlegendgraphic.py

@@ -37,6 +37,7 @@
     QgsMarkerSymbol,
     QgsRuleBasedRenderer,
     QgsVectorLayer,
+    QgsRasterLayer,
 )
 
 from qgis.server import (
@@ -45,6 +46,10 @@
     QgsServer,
 )
 
+from utilities import (
+    unitTestDataPath,
+)
+
 # Strip path and content length because path may vary
 RE_STRIP_UNCHECKABLE = br'MAP=[^"]+|Content-Length: \d+'
 RE_ATTRIBUTES = br'[^>\s]+=[^>\s]+'
@@ -1360,6 +1365,31 @@ def testLegendPlaceholderIcon(self):
         r, h = self._result(self._execute_request(qs))
         self._img_diff_error(r, h, "WMS_GetLegendGraphic_Legend_Placeholder_Icon")
 
+    def test_wms_GetLegendGraphic_JSON_raster_color_ramp(self):
+        """Test raster color ramp legend in JSON format"""
+
+        project = QgsProject()
+
+        path = os.path.join(unitTestDataPath('raster'),
+                            'byte.tif')
+
+        layer = QgsRasterLayer(path, "layer1")
+        self.assertTrue(layer.isValid())
+        project.addMapLayers([layer])
+
+        server = QgsServer()
+        request = QgsBufferServerRequest("/?SERVICE=WMS&VERSION=1.30&REQUEST=GetLegendGraphic" +
+                                         "&LAYERS=layer1" +
+                                         "&FORMAT=application/json")
+        response = QgsBufferServerResponse()
+        server.handleRequest(request, response, project)
+        j = json.loads(bytes(response.body()))
+        node = j
+        self.assertEqual(node['nodes'][0]['symbols'][0]['title'], 'Band 1 (Gray)')
+        self.assertEqual(node['nodes'][0]['symbols'][1]['max'], 255)
+        self.assertEqual(node['nodes'][0]['symbols'][1]['min'], 74)
+        self.assertNotEqual(node['nodes'][0]['symbols'][1]['icon'], '')
+
 
 if __name__ == '__main__':
     unittest.main()