Fix painter scaling and add unit test

src/core/layertree/qgslayertreemodellegendnode.cpp

@@ -946,18 +946,23 @@ QSizeF QgsImageLegendNode::drawSymbol( const QgsLegendSettings &settings, ItemCo
 {
   Q_UNUSED( itemHeight )
 
-  if ( ctx && ctx->painter )
+  if ( ctx && ctx->painter && ctx->context )
   {
-    QImage scaledImg = mImage.scaled( settings.wmsLegendSize().toSize(), Qt::KeepAspectRatio, Qt::FastTransformation );
+    QgsScopedRenderContextScaleToPixels scopedScaleToPixels( *( ctx->context ) );
+    double scaleFactor = ctx->context->scaleFactor();
+    double imgWidth = settings.wmsLegendSize().width() * scaleFactor;
+    double imgHeight = settings.wmsLegendSize().height() * scaleFactor;
+
+    QImage scaledImg = mImage.scaled( QSizeF( imgWidth, imgHeight ).toSize(), Qt::KeepAspectRatio, Qt::SmoothTransformation );
     switch ( settings.symbolAlignment() )
     {
       case Qt::AlignLeft:
       default:
-        ctx->painter->drawImage( QPointF( ctx->columnLeft, ctx->top ), scaledImg );
+        ctx->painter->drawImage( QPointF( ctx->columnLeft * scaleFactor, ctx->top * scaleFactor ), scaledImg );
         break;
 
       case Qt::AlignRight:
-        ctx->painter->drawImage( QPointF( ctx->columnRight - settings.wmsLegendSize().width(), ctx->top ), scaledImg );
+        ctx->painter->drawImage( QPointF( ctx->columnRight * scaleFactor - imgWidth, ctx->top * scaleFactor ), scaledImg );
         break;
     }
   }

tests/src/python/test_qgsserver_wms_getlegendgraphic.py

@@ -1045,6 +1045,19 @@ class TestQgsServerWMSGetLegendGraphic(TestQgsServerWMSTestBase):
         self.assertEqual(node['scaleMaxDenom'], 1000)
         self.assertEqual(node['scaleMinDenom'], 10000)
 
+    def testLegendPlaceholderIcon(self):
+        qs = "?" + "&".join(["%s=%s" % i for i in list({
+            "MAP": self.testdata_path + 'test_project_legend_placeholder_image.qgs',
+            "SERVICE": "WMS",
+            "VERSION": "1.3",
+            "REQUEST": "GetLegendGraphic",
+            "LAYER": "landsat",
+            "FORMAT": "image/png",
+        }.items())])
+
+        r, h = self._result(self._execute_request(qs))
+        self._img_diff_error(r, h, "WMS_GetLegendGraphic_Legend_Placeholder_Icon")
+
 
 if __name__ == '__main__':
     unittest.main()