diff --git a/MatterControlLib/PartPreviewWindow/SelectedObjectPanel.cs b/MatterControlLib/PartPreviewWindow/SelectedObjectPanel.cs
index 1b5bc7b90..5d6f4bf35 100644
--- a/MatterControlLib/PartPreviewWindow/SelectedObjectPanel.cs
+++ b/MatterControlLib/PartPreviewWindow/SelectedObjectPanel.cs
@@ -368,11 +368,17 @@ namespace MatterHackers.MatterControl.PartPreviewWindow
 				void SetToSolid()
                 {
 					// make sure the render mode is set to shaded or outline
-					if (sceneContext.ViewState.RenderType != RenderOpenGl.RenderTypes.Shaded
-							&& sceneContext.ViewState.RenderType != RenderOpenGl.RenderTypes.Outlines)
-					{
-						// make sure the render mode is set to outline
-						sceneContext.ViewState.RenderType = RenderOpenGl.RenderTypes.Outlines;
+					switch(sceneContext.ViewState.RenderType)
+                    {
+                        case RenderOpenGl.RenderTypes.Shaded:
+                        case RenderOpenGl.RenderTypes.Outlines:
+						case RenderOpenGl.RenderTypes.Polygons:
+							break;
+
+						default:
+							// make sure the render mode is set to outline
+							sceneContext.ViewState.RenderType = RenderOpenGl.RenderTypes.Outlines;
+							break;
 					}
 
 					var currentOutputType = selectedItem.WorldOutputType();
diff --git a/MatterControlLib/PartPreviewWindow/View3D/Actions/CombineObject3D_2.cs b/MatterControlLib/PartPreviewWindow/View3D/Actions/CombineObject3D_2.cs
index a63dcc160..6ce6bc2f6 100644
--- a/MatterControlLib/PartPreviewWindow/View3D/Actions/CombineObject3D_2.cs
+++ b/MatterControlLib/PartPreviewWindow/View3D/Actions/CombineObject3D_2.cs
@@ -142,7 +142,7 @@ namespace MatterHackers.MatterControl.PartPreviewWindow.View3D
             }
             else
             {
-                resultsMesh = Object3D.CombineParticipanets(SourceContainer, participants, cancellationToken, reporter);
+                resultsMesh = Object3D.CombineParticipants(SourceContainer, participants, cancellationToken, reporter);
             }
 
             var resultsItem = new Object3D()
@@ -152,7 +152,7 @@ namespace MatterHackers.MatterControl.PartPreviewWindow.View3D
 
             if (holes != null)
             {
-                var holesMesh = CombineParticipanets(SourceContainer, holes, cancellationToken, null);
+                var holesMesh = CombineParticipants(SourceContainer, holes, cancellationToken, null);
                 if (holesMesh != null)
                 {
                     var holesItem = new Object3D()
diff --git a/Submodules/agg-sharp b/Submodules/agg-sharp
index 8f4e9b969..b64bd80cc 160000
--- a/Submodules/agg-sharp
+++ b/Submodules/agg-sharp
@@ -1 +1 @@
-Subproject commit 8f4e9b969638d3c70ba81ac4f1a2d8faeda8c73b
+Subproject commit b64bd80cccc97bd8c0f9971d72c5b953588f6fa8
diff --git a/Tests/MatterControl.Tests/MatterControl/InteractiveSceneTests.cs b/Tests/MatterControl.Tests/MatterControl/InteractiveSceneTests.cs
index b98063f9f..7aeaf1ecf 100644
--- a/Tests/MatterControl.Tests/MatterControl/InteractiveSceneTests.cs
+++ b/Tests/MatterControl.Tests/MatterControl/InteractiveSceneTests.cs
@@ -36,9 +36,12 @@ using MatterHackers.MatterControl.DesignTools;
 using MatterHackers.MatterControl.DesignTools.Operations;
 using MatterHackers.MatterControl.PartPreviewWindow.View3D;
 using MatterHackers.MatterControl.Tests.Automation;
+using MatterHackers.PolygonMesh;
+using MatterHackers.PolygonMesh.Processors;
 using MatterHackers.VectorMath;
 using NUnit.Framework;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace MatterControl.Tests.MatterControl
@@ -280,6 +283,8 @@ namespace MatterControl.Tests.MatterControl
 		[Test, Category("InteractiveScene")]
 		public async Task SubtractTests()
 		{
+			StartupMC();
+
 			// Subtract has correct number of results
 			{
 				var root = new Object3D();
@@ -287,7 +292,7 @@ namespace MatterControl.Tests.MatterControl
 				var cubeB = await CubeObject3D.Create(20, 20, 20);
 				var offsetCubeB = new TranslateObject3D(cubeB, 10);
 
-				var subtract = new SubtractObject3D();
+				var subtract = new SubtractObject3D_2();
 				subtract.Children.Add(cubeA);
 				subtract.Children.Add(offsetCubeB);
 				subtract.SelectedChildren.Add(offsetCubeB.ID);
@@ -564,6 +569,93 @@ namespace MatterControl.Tests.MatterControl
 			Assert.IsTrue(rootAabb.Equals(new AxisAlignedBoundingBox(new Vector3(-10, -10, -10), new Vector3(10, 10, 11)), .01));
 		}
 
+		[Test]
+		public void CombineTests2()
+        {
+			// overlaping results in simple new mesh
+			{
+				var meshA = PlatonicSolids.CreateCube(10, 10, 10);
+				meshA.Translate(0, 0, 0);
+				var meshB = PlatonicSolids.CreateCube(10, 10, 10);
+				meshB.Translate(0, 5, 0);
+				var mesh = Object3D.CombineParticipants(null,
+					new IObject3D[]
+					{
+						new Object3D() { Mesh = meshA },
+						new Object3D() { Mesh = meshB },
+					},
+					new CancellationToken());
+				Assert.AreEqual(12, mesh.Faces.Count());
+				var aabb = mesh.GetAxisAlignedBoundingBox();
+				Assert.AreEqual(15, aabb.YSize, .001);
+			}
+
+			// multiple overlaping faces all combine
+			{
+				var meshA = PlatonicSolids.CreateCube(10, 10, 10);
+				meshA.Translate(0, 0, 0);
+				var meshB = PlatonicSolids.CreateCube(10, 10, 10);
+				meshB.Translate(0, -3, 0);
+				var meshC = PlatonicSolids.CreateCube(10, 10, 10);
+				meshC.Translate(0, 3, 0);
+				var mesh = Object3D.CombineParticipants(null,
+					new IObject3D[]
+					{
+						new Object3D() { Mesh = meshA },
+						new Object3D() { Mesh = meshB },
+						new Object3D() { Mesh = meshC },
+					},
+					new CancellationToken());
+				Assert.AreEqual(12, mesh.Faces.Count());
+				var aabb = mesh.GetAxisAlignedBoundingBox();
+				Assert.AreEqual(16, aabb.YSize, .001);
+			}
+
+			// a back face against a front face, both are removed
+			{
+				var meshA = PlatonicSolids.CreateCube(10, 10, 10);
+				meshA.Translate(0, -5, 0);
+				var meshB = PlatonicSolids.CreateCube(10, 10, 10);
+				meshB.Translate(0, 5, 0);
+				var mesh = Object3D.CombineParticipants(null,
+					new IObject3D[]
+					{
+						new Object3D() { Mesh = meshA },
+						new Object3D() { Mesh = meshB },
+					},
+					new CancellationToken());
+				Assert.AreEqual(12, mesh.Faces.Count());
+				var aabb = mesh.GetAxisAlignedBoundingBox();
+				Assert.AreEqual(20, aabb.YSize, .001);
+			}
+
+			// multiple overlaping faces all combine
+			{
+				// right side at 0
+				var meshA = PlatonicSolids.CreateCube(10, 5, 10);
+				meshA.Translate(-5, 0, 0);
+				// left side at -5
+				var meshB = PlatonicSolids.CreateCube(10, 5, 10);
+				meshB.Translate(0, 0, 0);
+				// right side at 0
+				var meshC = PlatonicSolids.CreateCube(5, 10, 10);
+				meshC.Translate(-2.5, 0, 0);
+				var mesh = Object3D.CombineParticipants(null,
+					new IObject3D[]
+					{
+						new Object3D() { Mesh = meshA },
+						new Object3D() { Mesh = meshB },
+						new Object3D() { Mesh = meshC },
+					},
+					new CancellationToken());
+				StlProcessing.Save(mesh, @"C:\temp\temp.stl", new CancellationToken());
+				Assert.AreEqual(20, mesh.Faces.Count());
+				var aabb = mesh.GetAxisAlignedBoundingBox();
+				Assert.AreEqual(16, aabb.YSize, .001);
+			}
+
+		}
+
 		[Test, Category("InteractiveScene")]
 		public async Task AlignObjectHasCorrectPositionsOnXAxis()
 		{