Use Even's code suggestions + add tests

src/core/providers/gdal/qgsgdalprovider.cpp

@@ -2999,20 +2999,23 @@ void QgsGdalProvider::initBaseDataset()
   {
     QgsLogger::warning( QStringLiteral( "Creating Warped VRT." ) );
 
+    // Add alpha band to the output VRT dataset if it does not exist already
+    // so that pixels in empty regions (e.g. when the raster is rotated) will be transparent
+    gdal::warp_options_unique_ptr psWarpOptions( GDALCreateWarpOptions() );
+    if ( GDALGetRasterColorInterpretation( GDALGetRasterBand( mGdalBaseDataset, GDALGetRasterCount( mGdalBaseDataset ) ) ) != GCI_AlphaBand )
+    {
+      psWarpOptions->nDstAlphaBand = GDALGetRasterCount( mGdalBaseDataset ) + 1;
+    }
+
     if ( GDALGetMetadata( mGdalBaseDataset, "RPC" ) )
     {
       mGdalDataset =
         QgsGdalUtils::rpcAwareAutoCreateWarpedVrt( mGdalBaseDataset, nullptr, nullptr,
-            GRA_NearestNeighbour, 0.2, nullptr );
+            GRA_NearestNeighbour, 0.2, psWarpOptions.get() );
       mGdalTransformerArg = QgsGdalUtils::rpcAwareCreateTransformer( mGdalBaseDataset );
     }
     else
     {
-      // Add alpha band to the output VRT dataset so that pixels in empty regions
-      // (e.g. when the raster is rotated) will be transparent
-      gdal::warp_options_unique_ptr psWarpOptions( GDALCreateWarpOptions() );
-      psWarpOptions->nDstAlphaBand = GDALGetRasterCount( mGdalBaseDataset ) + 1;
-
       mGdalDataset =
         GDALAutoCreateWarpedVRT( mGdalBaseDataset, nullptr, nullptr,
                                  GRA_NearestNeighbour, 0.2, psWarpOptions.get() );

tests/src/core/testqgsrasterlayer.cpp

@@ -100,6 +100,7 @@ class TestQgsRasterLayer : public QObject
     void testRefreshRendererIfNeeded();
     void sample();
     void testTemporalProperties();
+    void rotatedRaster();
 
 
   private:
@@ -1040,5 +1041,24 @@ void TestQgsRasterLayer::testTemporalProperties()
   QCOMPARE( temporalProperties->fixedTemporalRange().end(), dateTimeRange.end() );
 }
 
+void TestQgsRasterLayer::rotatedRaster()
+{
+  mMapSettings->setExtent( QgsRectangle( 994, 922, 1174, 1102 ) );
+
+  std::unique_ptr< QgsRasterLayer> rgb = std::make_unique< QgsRasterLayer >( mTestDataDir + "raster/rotated_rgb.png",
+                                         QStringLiteral( "rgb" ) );
+  QVERIFY( rgb->isValid() );
+
+  mMapSettings->setLayers( QList<QgsMapLayer *>() << rgb.get() );
+  QVERIFY( render( QStringLiteral( "raster_rotated_rgb" ) ) );
+
+  std::unique_ptr< QgsRasterLayer> rgba = std::make_unique< QgsRasterLayer >( mTestDataDir + "raster/rotated_rgba.png",
+                                          QStringLiteral( "rgba" ) );
+  QVERIFY( rgba->isValid() );
+
+  mMapSettings->setLayers( QList<QgsMapLayer *>() << rgba.get() );
+  QVERIFY( render( QStringLiteral( "raster_rotated_rgba" ) ) );
+}
+
 QGSTEST_MAIN( TestQgsRasterLayer )
 #include "testqgsrasterlayer.moc"

tests/testdata/raster/rotated_rgb.pngw

@@ -0,0 +1,6 @@
+0.72744
+0.97092
+0.97092
+-0.7274
+1000
+1000

tests/testdata/raster/rotated_rgba.pngw

@@ -0,0 +1,6 @@
+0.72744
+0.97092
+0.97092
+-0.7274
+1000
+1000