diff --git a/Changes.md b/Changes.md
index 4c819e4b818..2cfca36eddb 100644
--- a/Changes.md
+++ b/Changes.md
@@ -36,7 +36,9 @@ Improvements
   - Larger backdrops are automatically drawn behind smaller ones, so that nested backdrops will always appear on top.
   - Added a `depth` plug to assign a manual drawing depth for the rare cases where the automatic depth is unwanted.
 - ImageStats : Added `areaSource` plug, allowing area to be driven by the input display window or data window.
-- 3Delight : Added camera overscan support.
+- 3Delight :
+  - Added camera overscan support.
+  - NSI scene description export format is now based on file extension - `.nsi` for binary and `.nsia` for ASCII.
 
 Fixes
 -----
@@ -116,6 +118,7 @@ Breaking Changes
   - Renamed `channels` plug to `shuffles` plug, matching nodes such as ShuffleAttributes and ShufflePrimitiveVariables.
 - ShuffleUI : Removed `nodeMenuCreateCommand()`.
 - ImageStatsUI : Removed `postCreate()`.
+- 3Delight : Changed NSI scene description export with `.nsi` file extension from ASCII to binary (`.nsia` is used for ASCII now).
 
 Build
 -----
diff --git a/python/IECoreDelightTest/RendererTest.py b/python/IECoreDelightTest/RendererTest.py
index c8cc17ff91d..de8d8407d63 100644
--- a/python/IECoreDelightTest/RendererTest.py
+++ b/python/IECoreDelightTest/RendererTest.py
@@ -68,12 +68,12 @@ def testSceneDescription( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" )
+			str( self.temporaryDirectory() / "test.nsia" )
 		)
 
 		r.render()
 
-		self.assertTrue( ( self.temporaryDirectory() / "test.nsi" ).exists() )
+		self.assertTrue( ( self.temporaryDirectory() / "test.nsia" ).exists() )
 
 	def testOutput( self ) :
 
@@ -144,7 +144,7 @@ def testAOVs( self ) :
 			r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 				"3Delight",
 				GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-				str( self.temporaryDirectory() / "test.nsi" )
+				str( self.temporaryDirectory() / "test.nsia" )
 			)
 
 			r.output(
@@ -163,7 +163,7 @@ def testAOVs( self ) :
 			r.render()
 			del r
 
-			nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+			nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 			self.assertIn( "outputLayer:test", nsi )
 			self.assertEqual( nsi["outputLayer:test"]["nodeType"], "outputlayer")
 			for k, v in expected.items() :
@@ -175,7 +175,7 @@ def testMesh( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		r.object(
@@ -187,7 +187,7 @@ def testMesh( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		meshes = { k: v for k, v in nsi.items() if nsi[k]["nodeType"] == "mesh" }
 		self.assertEqual( len( meshes ), 1 )
@@ -228,7 +228,7 @@ def testAnimatedMesh( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		r.object(
@@ -244,7 +244,7 @@ def testAnimatedMesh( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		meshes = { k: v for k, v in nsi.items() if nsi[k]["nodeType"] == "mesh" }
 		self.assertEqual( len( meshes ), 1 )
@@ -265,7 +265,7 @@ def testPoints( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		points = IECoreScene.PointsPrimitive( IECore.V3fVectorData( [ imath.V3f( x ) for x in range( 0, 4 ) ] ) )
@@ -283,7 +283,7 @@ def testPoints( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 		self.__assertInNSI( '"P" "v point" 4 [ 0 0 0 1 1 1 2 2 2 3 3 3 ]', nsi )
 		self.__assertInNSI( '"width" "v float" 4 [ 1 2 3 4 ]', nsi )
 
@@ -292,7 +292,7 @@ def testPointsWithoutWidth( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		points = IECoreScene.PointsPrimitive( IECore.V3fVectorData( [ imath.V3f( x ) for x in range( 0, 4 ) ] ) )
@@ -306,7 +306,7 @@ def testPointsWithoutWidth( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 		self.__assertInNSI( '"width" "float" 1 1', nsi )
 
 	def testCurves( self ) :
@@ -314,7 +314,7 @@ def testCurves( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		curves = IECoreScene.CurvesPrimitive(
@@ -336,7 +336,7 @@ def testCurves( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 		self.__assertInNSI( '"nvertices" "int" 2 [ 4 4 ]', nsi )
 		self.__assertInNSI( '"basis" "string" 1 "b-spline"', nsi )
 		self.__assertInNSI( '"P" "v point" 8 [ 0 0 0 1 1 1 2 2 2 3 3 3 0 0 0 -1 -1 -1 -2 -2 -2 -3 -3 -3 ]', nsi )
@@ -346,7 +346,7 @@ def testDisk( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		r.object(
@@ -358,7 +358,7 @@ def testDisk( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		# 3Delight doesn't have a disk, so we must convert to particles
 		self.__assertInNSI( '"particles"', nsi )
@@ -371,7 +371,7 @@ def testSphere( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		r.object(
@@ -383,7 +383,7 @@ def testSphere( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		# 3Delight doesn't have a sphere, so we must convert to particles
 		self.__assertInNSI( '"particles"', nsi )
@@ -396,7 +396,7 @@ def testEnvironment( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		r.object(
@@ -413,7 +413,7 @@ def testEnvironment( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		self.__assertInNSI( '"environment"', nsi )
 		self.__assertInNSI( '"angle" "double" 1 25', nsi )
@@ -423,7 +423,7 @@ def testAttributes( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		r.attributes( IECore.CompoundObject( {
@@ -435,7 +435,7 @@ def testAttributes( self ) :
 
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		self.__assertInNSI( '"visibility.diffuse" "int" 1 1', nsi )
 		self.__assertInNSI( '"visibility.camera" "int" 1 0', nsi )
@@ -447,7 +447,7 @@ def testCamera( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		r.camera(
@@ -475,7 +475,7 @@ def testCamera( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		self.__assertInNSI( '"fov" "float" 1 90', nsi )
 		self.__assertInNSI( '"resolution" "int[2]" 1 [ 2000 1000 ]', nsi )
@@ -490,7 +490,7 @@ def testObjectInstancing( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		m = IECoreScene.MeshPrimitive.createPlane( imath.Box2f( imath.V2f( -1 ), imath.V2f( 1 ) ) )
@@ -502,7 +502,7 @@ def testObjectInstancing( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		self.assertEqual( nsi.count( '"transform"' ), 2 )
 		self.assertEqual( nsi.count( '"mesh"' ), 1 )
@@ -512,7 +512,7 @@ def testTransform( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		m = IECoreScene.MeshPrimitive.createPlane( imath.Box2f( imath.V2f( -1 ), imath.V2f( 1 ) ) )
@@ -529,7 +529,7 @@ def testTransform( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		self.__assertNotInNSI( 'DeleteAttribute', nsi )
 		self.__assertNotInNSI( 'SetAttribute "untransformed" "transformationmatrix"', nsi )
@@ -544,7 +544,7 @@ def runSubstitutions( text, attributes ):
 			r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 				"3Delight",
 				GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-				str( self.temporaryDirectory() / "test.nsi" ),
+				str( self.temporaryDirectory() / "test.nsia" ),
 			)
 
 			s = IECoreScene.ShaderNetwork( { "output" : IECoreScene.Shader( "testShader", "surface", { "testStringSubstituted" : text } ) }, output = "output" )
@@ -559,7 +559,7 @@ def runSubstitutions( text, attributes ):
 			r.render()
 			del r
 
-			nsi = open( self.temporaryDirectory() / "test.nsi", encoding = "utf-8" ).read()
+			nsi = open( self.temporaryDirectory() / "test.nsia", encoding = "utf-8" ).read()
 			return re.findall( '\n *"testStringSubstituted" "string" 1 "(.*)" \n', nsi )[0]
 
 		self.assertEqual( runSubstitutions( "<attr:test:foo> TEST <attr:test:bar>", {} ), " TEST " )
@@ -573,7 +573,7 @@ def testMessageHandler( self ) :
 		for renderType, fileName, expected in (
 			( RenderType.Batch, "", 2 ),
 			( RenderType.Interactive, "", 2 ),
-			( RenderType.SceneDescription, str( self.temporaryDirectory() / "test.nsi" ), 1 )
+			( RenderType.SceneDescription, str( self.temporaryDirectory() / "test.nsia" ), 1 )
 		) :
 
 			with IECore.CapturingMessageHandler() as fallbackHandler :
@@ -606,7 +606,7 @@ def testOptions( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" )
+			str( self.temporaryDirectory() / "test.nsia" )
 		)
 
 		options = [
@@ -645,7 +645,7 @@ def testOptions( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parse( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parse( self.temporaryDirectory() / "test.nsia" )
 
 		for name, value, type in options :
 			self.__assertInNSI(
@@ -664,7 +664,7 @@ def testVDB( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		vdb = IECoreVDB.VDBObject( ( pathlib.Path( __file__ ).parent / "volumes" / "sphere.vdb" ).as_posix() )
@@ -673,7 +673,7 @@ def testVDB( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		self.assertIn( "test_vdb", nsi )
 		self.assertEqual( nsi["test_vdb"]["nodeType"], "transform" )
@@ -691,7 +691,7 @@ def testShaderAttributes( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		surfaceNetwork = IECoreScene.ShaderNetwork(
@@ -725,7 +725,7 @@ def testShaderAttributes( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		allAttributes = { k: v for k, v in nsi.items() if nsi[k]["nodeType"] == "attributes" }
 		self.assertEqual( len( allAttributes ), 1 )
@@ -759,7 +759,7 @@ def test3DelightSplineParameters( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		os.environ["OSL_SHADER_PATHS"] += os.pathsep + ( pathlib.Path( __file__ ).parent / "shaders" ).as_posix()
@@ -846,7 +846,7 @@ def test3DelightSplineParameters( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		shaders = { k: v for k, v in nsi.items() if nsi[k]["nodeType"] == "shader" }
 		self.assertEqual( len( shaders ), 1 )
@@ -922,7 +922,7 @@ def testGafferSplineParameters( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		network = IECoreScene.ShaderNetwork(
@@ -960,7 +960,7 @@ def testGafferSplineParameters( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		shaders = { k: v for k, v in nsi.items() if nsi[k]["nodeType"] == "shader" }
 		self.assertEqual( len( shaders ), 1 )
@@ -985,7 +985,7 @@ def testUVCoordShaderInserted( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		s = IECoreScene.ShaderNetwork(
@@ -1003,7 +1003,7 @@ def testUVCoordShaderInserted( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		shaders = { k: v for k, v in nsi.items() if nsi[k]["nodeType"] == "shader" }
 
@@ -1020,7 +1020,7 @@ def testUVCoordShaderNotInserted( self ) :
 		r = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		s = IECoreScene.ShaderNetwork(
@@ -1042,7 +1042,7 @@ def testUVCoordShaderNotInserted( self ) :
 		r.render()
 		del r
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		shaders = { k: v for k, v in nsi.items() if nsi[k]["nodeType"] == "shader" }
 
@@ -1064,7 +1064,7 @@ def testCornersAndCreases( self ) :
 		renderer = GafferScene.Private.IECoreScenePreview.Renderer.create(
 			"3Delight",
 			GafferScene.Private.IECoreScenePreview.Renderer.RenderType.SceneDescription,
-			str( self.temporaryDirectory() / "test.nsi" ),
+			str( self.temporaryDirectory() / "test.nsia" ),
 		)
 
 		renderer.object( "testPlane", mesh, renderer.attributes( IECore.CompoundObject() ) )
@@ -1072,7 +1072,7 @@ def testCornersAndCreases( self ) :
 		renderer.render()
 		del renderer
 
-		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsi" )
+		nsi = self.__parseDict( self.temporaryDirectory() / "test.nsia" )
 
 		mesh = next( node for node in nsi.values() if node["nodeType"] == "mesh" )
 		self.assertEqual( mesh["subdivision.creasevertices"], [ 0, 1, 1, 2 ] )
diff --git a/src/IECoreDelight/Renderer.cpp b/src/IECoreDelight/Renderer.cpp
index 20b4d9b1b9a..2ea8893f57f 100644
--- a/src/IECoreDelight/Renderer.cpp
+++ b/src/IECoreDelight/Renderer.cpp
@@ -1087,11 +1087,13 @@ class DelightRenderer final : public IECoreScenePreview::Renderer
 			vector<NSIParam_t> params;
 
 			const char *apistream = "apistream";
+			const char *streamformat = "autonsi";
 			const char *fileNamePtr = fileName.c_str();
 			if( renderType == SceneDescription )
 			{
 				params = {
 					{ "type", &apistream, NSITypeString, 0, 1, 0 },
+					{ "streamformat", &streamformat, NSITypeString, 0, 1, 0 },
 					{ "streamfilename", &fileNamePtr, NSITypeString , 0, 1, 0 }
 				};
 			}