CoordinateSystem: Backwards compatibility

CoordinateSystem used to derive from the deprecated and removed
StateRenderable holding additional transform information for the
renderer. A better approach that is also used in Gaffer, the transform
information is better stored as separate transform information.
Cortex 10.6 also removed all DCC integrations, i.e. IECoreMaya, which
stored, for example Locator transformation on the CoordinateSystem. We
provide a way to still retrieve this information from old SceneCaches,
although not fully transparent and backwards compatible.  Clients of
CoordinateSystem will need to be adjusted if needed.

The `M44fData` is stored as `LegacyTransform` in the `BlindData` of
the CoordinateSystem

Author: yannci

src/IECoreScene/CoordinateSystem.cpp

@@ -34,8 +34,12 @@
 
 #include "IECoreScene/CoordinateSystem.h"
 
+#include "IECore/SimpleTypedData.h"
 #include "IECore/MurmurHash.h"
 
+#include "Imath/ImathMatrix.h"
+#include "Imath/ImathMatrixAlgo.h"
+
 using namespace IECore;
 using namespace IECoreScene;
 using namespace boost;
@@ -104,6 +108,46 @@ void CoordinateSystem::load( LoadContextPtr context )
 	unsigned int v = m_ioVersion;
 	ConstIndexedIOPtr container = context->container( staticTypeName(), v );
 	container->read( g_nameEntry, m_name );
+
+	// CoordinateSystem used to derive from the deprecated and removed StateRenderable
+	// holding additional transform information for the renderer. A better approach that is also
+	// used in Gaffer, the transform information is better stored as separate transform information.
+	// Cortex 10.6 also removed all DCC integrations, i.e. IECoreMaya, which stored, for example
+	// Locator transformation on the CoordinateSystem. We provide a way to still retrieve this
+	// information from old SceneCaches, although not fully transparent and backwards compatible.
+	// Clients of CoordinateSystem will need to be adjusted if needed.
+
+	// We look for a matrix entry somewhere underneath the CoordinateSystem, directly read it from
+	// there instead of loading an object and stash it in the BlindData.
+	const IndexedIO::EntryID matrixEntry( "matrix" );
+	Imath::M44f matrix;
+
+	std::function<void(ConstIndexedIOPtr)> findMatrixEntry = [&]( ConstIndexedIOPtr directory )
+	{
+		if( directory->hasEntry( matrixEntry ) )
+		{
+			float *f = matrix.getValue();
+			directory->read( matrixEntry, f, 16 );
+		}
+
+		IndexedIO::EntryIDList levelDirectories;
+		directory->entryIds( levelDirectories, IndexedIO::EntryType::Directory );
+
+		for ( auto levelDirectory : levelDirectories )
+		{
+			ConstIndexedIOPtr subdirectory = directory->subdirectory( levelDirectory, IndexedIO::MissingBehaviour::NullIfMissing );
+			if( subdirectory )
+			{
+			    findMatrixEntry( subdirectory );
+			}
+		}
+	};
+
+	findMatrixEntry( container );
+	if( matrix != Imath::identity44f )
+	{
+	    blindData()->writable()["LegacyTransform"] = new IECore::M44fData( matrix );
+	}
 }
 
 void CoordinateSystem::hash( MurmurHash &h ) const