Add method for a callout to obtain a transform from the original

associated layer's crs to the destination map crs

python/core/auto_generated/callouts/qgscallout.sip.in

@@ -183,9 +183,31 @@ Returns the desired drawing order (stacking) to use while rendering this callout
 The default order is QgsCallout.OrderBelowIndividualLabels.
 %End
 
-    struct QgsCalloutContext
-    {
-      bool allFeaturePartsLabeled;
+    class QgsCalloutContext
+{
+%Docstring
+Contains additional contextual information about the context in which a callout is
+being rendered.
+
+.. versionadded:: 3.10
+%End
+
+%TypeHeaderCode
+#include "qgscallout.h"
+%End
+      public:
+        bool allFeaturePartsLabeled;
+
+        QgsCoordinateReferenceSystem originalFeatureCrs;
+
+        QgsCoordinateTransform originalFeatureToMapTransform( const QgsRenderContext &renderContext ) const;
+%Docstring
+Returns the coordinate transform to convert from the original layer associated with
+the callout to the destination map CRS.
+
+.. versionadded:: 3.20
+%End
+
     };
 
     void render( QgsRenderContext &context, QRectF rect, const double angle, const QgsGeometry &anchor, QgsCalloutContext &calloutContext );

src/core/callouts/qgscallout.cpp

@@ -694,3 +694,13 @@ void QgsManhattanLineCallout::draw( QgsRenderContext &context, QRectF rect, cons
       drawCalloutLine( part );
   }
 }
+
+QgsCoordinateTransform QgsCallout::QgsCalloutContext::originalFeatureToMapTransform( const QgsRenderContext &renderContext ) const
+{
+  if ( !mOriginalFeatureToMapTransform.isValid() )
+  {
+    // lazy initialization, only create if needed...
+    mOriginalFeatureToMapTransform = QgsCoordinateTransform( originalFeatureCrs, renderContext.coordinateTransform().destinationCrs(), renderContext.transformContext() );
+  }
+  return mOriginalFeatureToMapTransform;
+}

src/core/callouts/qgscallout.h

@@ -211,10 +211,30 @@ class CORE_EXPORT QgsCallout
      * \ingroup core
      * \since QGIS 3.10
      */
-    struct CORE_EXPORT QgsCalloutContext
+    class CORE_EXPORT QgsCalloutContext
     {
-      //! TRUE if all parts of associated feature were labeled
-      bool allFeaturePartsLabeled = false;
+      public:
+        //! TRUE if all parts of associated feature were labeled
+        bool allFeaturePartsLabeled = false;
+
+        /**
+         * Contains the CRS of the original feature associated with this callout.
+         *
+         * \since QGIS 3.18
+         */
+        QgsCoordinateReferenceSystem originalFeatureCrs;
+
+        /**
+         * Returns the coordinate transform to convert from the original layer associated with
+         * the callout to the destination map CRS.
+         *
+         * \since QGIS 3.20
+         */
+        QgsCoordinateTransform originalFeatureToMapTransform( const QgsRenderContext &renderContext ) const;
+
+      private:
+        //! Lazy initialized coordinate transform from original feature CRS to map CRS
+        mutable QgsCoordinateTransform mOriginalFeatureToMapTransform;
     };
 
     /**

src/core/labeling/qgsvectorlayerlabelprovider.cpp

@@ -330,6 +330,7 @@ void QgsVectorLayerLabelProvider::drawCallout( QgsRenderContext &context, pal::L
     g.transform( xform.transform() );
     QgsCallout::QgsCalloutContext calloutContext;
     calloutContext.allFeaturePartsLabeled = label->getFeaturePart()->feature()->labelAllParts();
+    calloutContext.originalFeatureCrs = label->getFeaturePart()->feature()->originalFeatureCrs();
     mSettings.callout()->render( context, rect, label->getAlpha() * 180 / M_PI, g, calloutContext );
   }
 }

src/core/qgsmaprendererjob.cpp

@@ -649,6 +649,9 @@ LabelRenderJob QgsMapRendererJob::prepareLabelingJob( QPainter *painter, QgsLabe
   job.context.setLabelingEngine( labelingEngine2 );
   job.context.setExtent( mSettings.visibleExtent() );
   job.context.setFeatureFilterProvider( mFeatureFilterProvider );
+  QgsCoordinateTransform ct;
+  ct.setDestinationCrs( mSettings.destinationCrs() );
+  job.context.setCoordinateTransform( ct );
 
   // if we can use the cache, let's do it and avoid rendering!
   bool hasCache = canUseLabelCache && mCache && mCache->hasCacheImage( LABEL_CACHE_ID );