add poly files to further test EikMesh, fix issue #257.

VadereGui/src/org/vadere/gui/components/control/simulation/ActionGeneratePNG.java

@@ -47,7 +47,6 @@ public class ActionGeneratePNG extends AbstractAction implements IRendererChange
 			SimpleDateFormat formatter = new SimpleDateFormat(CONFIG.getString("SettingsDialog.dataFormat"));
 			String formattedDate = formatter.format(todaysDate);
 
-
 			File outputFile = new File(Messages.getString("FileDialog.filenamePrefix") + formattedDate + ".png");
 			fileChooser.setSelectedFile(outputFile);
 
@@ -62,6 +61,7 @@ public class ActionGeneratePNG extends AbstractAction implements IRendererChange
 
 				try {
 					ImageIO.write(bi, "png", outputFile);
+					VadereConfig.getConfig().setProperty("SettingsDialog.snapshotDirectory.path", outputFile.getParentFile().getAbsolutePath());
 					logger.info("generate new png: " + outputFile.getAbsolutePath());
 				} catch (IOException e1) {
 					logger.error(e1.getMessage());

VadereGui/src/org/vadere/gui/components/control/simulation/ActionGeneratePoly.java

 package org.vadere.gui.components.control.simulation;
 
+import org.apache.commons.configuration2.Configuration;
 import org.vadere.gui.components.model.DefaultSimulationConfig;
 import org.vadere.gui.components.model.SimulationModel;
 import org.vadere.gui.components.utils.Messages;
 import org.vadere.gui.components.utils.Resources;
 import org.vadere.gui.components.view.SimulationRenderer;
 import org.vadere.gui.postvisualization.PostVisualisation;
+import org.vadere.meshing.mesh.impl.PSLG;
 import org.vadere.meshing.utils.io.poly.PSLGGenerator;
 import org.vadere.state.scenario.Obstacle;
+import org.vadere.util.config.VadereConfig;
 import org.vadere.util.geometry.shapes.VPolygon;
 import org.vadere.util.geometry.shapes.VRectangle;
 import org.vadere.util.logging.Logger;
@@ -21,6 +24,7 @@ import java.io.OutputStreamWriter;
 import java.io.Writer;
 import java.text.SimpleDateFormat;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Date;
 import java.util.List;
 import java.util.prefs.Preferences;
@@ -30,7 +34,7 @@ import javax.swing.*;
 
 public class ActionGeneratePoly extends AbstractAction {
 	private static Logger logger = Logger.getLogger(ActionGeneratePNG.class);
-	private static Resources resources = Resources.getInstance("global");
+	private static final Configuration CONFIG = VadereConfig.getConfig();
 	private final SimulationModel<? extends DefaultSimulationConfig> model;
 
 	public ActionGeneratePoly(final String name, Icon icon, final SimulationRenderer renderer,
@@ -41,10 +45,10 @@ public class ActionGeneratePoly extends AbstractAction {
 
 	@Override
 	public void actionPerformed(final ActionEvent e) {
-		JFileChooser fileChooser = new JFileChooser(Preferences.userNodeForPackage(PostVisualisation.class).get("SettingsDialog.snapshotDirectory.path", "."));
+		JFileChooser fileChooser = new JFileChooser(CONFIG.getString("SettingsDialog.snapshotDirectory.path"));
 
 		Date todaysDate = new java.util.Date();
-		SimpleDateFormat formatter = new SimpleDateFormat(resources.getProperty("SettingsDialog.dataFormat"));
+		SimpleDateFormat formatter = new SimpleDateFormat(CONFIG.getString("SettingsDialog.dataFormat"));
 		String formattedDate = formatter.format(todaysDate);
 
 
@@ -67,17 +71,27 @@ public class ActionGeneratePoly extends AbstractAction {
 
 			List<Obstacle> obstacles = new ArrayList<>(model.getTopography().getObstacles());
 			obstacles.removeAll(model.getTopography().getBoundaryObstacles());
+
+			List<VPolygon> obsShapes = obstacles.stream()
+					.map(obs -> obs.getShape())
+					.map(shape -> new VPolygon(shape))
+					.collect(Collectors.toList());
+
+			// this computes the union of intersecting obstacles.
+			obsShapes = PSLG.constructHoles(obsShapes);
+
+			// this will help to construct a valid non-rectangular bound.
+			List<VPolygon> polygons = PSLG.constructBound(new VPolygon(bound), obsShapes);
+
 			String polyString = PSLGGenerator.toPSLG(
-					new VPolygon(bound),
-					obstacles.stream()
-							.map(obs -> obs.getShape())
-							.map(shape -> new VPolygon(shape))
-							.collect(Collectors.toList()));
+					polygons.get(0),
+					polygons.size() > 1 ? polygons.subList(1, polygons.size()) : Collections.emptyList());
 			try {
 				outputFile.createNewFile();
 				Writer out = new OutputStreamWriter(new FileOutputStream(outputFile), "UTF-8");
 				out.write(polyString);
 				out.flush();
+				VadereConfig.getConfig().setProperty("SettingsDialog.snapshotDirectory.path", outputFile.getParentFile().getAbsolutePath());
 				logger.info("generate new Poly.file: " + outputFile.getAbsolutePath());
 			} catch (IOException e1) {
 				logger.error(e1.getMessage());

VadereGui/src/org/vadere/gui/components/control/simulation/ActionGenerateSVG.java

@@ -47,6 +47,7 @@ public class ActionGenerateSVG extends AbstractAction implements IRendererChange
 			outputFile = fileChooser.getSelectedFile().toString().endsWith(".svg") ? fileChooser.getSelectedFile()
 					: new File(fileChooser.getSelectedFile().toString() + ".svg");
 			svgGenerator.generateSVG(outputFile);
+			VadereConfig.getConfig().setProperty("SettingsDialog.snapshotDirectory.path", outputFile.getParentFile().getAbsolutePath());
 		}
 
 	}

VadereGui/src/org/vadere/gui/components/control/simulation/ActionGenerateTikz.java

@@ -51,6 +51,7 @@ public class ActionGenerateTikz extends AbstractAction implements IRendererChang
 					: new File(fileChooser.getSelectedFile().toString() + ".tex");
 
 			tikzGenerator.generateTikz(outputFile);
+			VadereConfig.getConfig().setProperty("SettingsDialog.snapshotDirectory.path", outputFile.getParentFile().getAbsolutePath());
 		}
 	}
 

VadereGui/src/org/vadere/gui/postvisualization/utils/MovRecorder.java

@@ -31,6 +31,7 @@ public class MovRecorder implements IRecorder {
 	private Rectangle2D.Double imageSize;
 	private Rectangle2D.Double viewport;
 	private boolean finished;
+	private File outputFile;
 
 	public MovRecorder(final PostvisualizationRenderer renderer) {
 		this.model = renderer.getModel();
@@ -76,6 +77,7 @@ public class MovRecorder implements IRecorder {
 			try {
 				this.finished = true;
 				enc.finish();
+				VadereConfig.getConfig().setProperty("SettingsDialog.snapshotDirectory.path", outputFile.getParentFile().getAbsolutePath());
 			} catch (IndexOutOfBoundsException error) {
 				logger.debug("Nothing recorded! " + error.getMessage());
 				throw error;
@@ -108,7 +110,7 @@ public class MovRecorder implements IRecorder {
 		SimpleDateFormat formatter = new SimpleDateFormat(CONFIG.getString("SettingsDialog.dataFormat"));
 			String formattedDate = formatter.format(todaysDate);
 		JFileChooser fileChooser = new JFileChooser(VadereConfig.getConfig().getString("SettingsDialog.snapshotDirectory.path", "."));
-			File outputFile = new File(Messages.getString("FileDialog.filenamePrefix") + formattedDate + ".mov");
+			outputFile = new File(Messages.getString("FileDialog.filenamePrefix") + formattedDate + ".mov");
 			fileChooser.setSelectedFile(outputFile);
 
 			int returnVal = fileChooser.showDialog(null, "Save");

VadereGui/src/org/vadere/gui/projectview/view/DataProcessingView.java

@@ -120,7 +120,7 @@ class DataProcessingView extends JPanel implements IJsonView {
 	private JMenu processorsMenu = new JMenu();
 
 	private TextView buildExpertView() {
-		TextView panel = new TextView("/" + IOUtils.OUTPUT_DIR, "default_directory_outputprocessors", AttributeType.OUTPUTPROCESSOR);
+		TextView panel = new TextView("ProjectView.defaultDirectoryOutputProcessors", AttributeType.OUTPUTPROCESSOR);
 		JMenuBar processorsMenuBar = new JMenuBar();
 		processorsMenu = new JMenu(Messages.getString("Tab.Model.loadTemplateMenu.title"));
 		processorsMenu.setEnabled(isEditable);

VadereGui/src/org/vadere/gui/projectview/view/ScenarioPanel.java

@@ -106,13 +106,13 @@ public class ScenarioPanel extends JPanel implements IProjectChangeListener, Pro
 
 		//Tab
 		attributesSimulationView =
-				new TextView("/attributes", "ProjectView.defaultDirectoryAttributes", AttributeType.SIMULATION);
+				new TextView("ProjectView.defaultDirectoryAttributes", AttributeType.SIMULATION);
 		attributesSimulationView.setScenarioChecker(model);
 
 		tabbedPane.addTab(Messages.getString("Tab.Simulation.title"), attributesSimulationView);
 
 		//Tab
-		attributesModelView = new TextView("/attributes", "ProjectView.defaultDirectoryAttributes", AttributeType.MODEL);
+		attributesModelView = new TextView("ProjectView.defaultDirectoryAttributes", AttributeType.MODEL);
 		attributesModelView.setScenarioChecker(model);
 
 		JMenuBar presetMenuBar = new JMenuBar();
@@ -192,11 +192,11 @@ public class ScenarioPanel extends JPanel implements IProjectChangeListener, Pro
 		attributesModelView.getPanelTop().add(presetMenuBar, 0); // the 0 puts it at the leftmost position instead of the rightmost
 		tabbedPane.addTab(Messages.getString("Tab.Model.title"), attributesModelView);
 
-		topographyFileView = new TextView("/scenarios", "ProjectView.defaultDirectoryScenarios", AttributeType.TOPOGRAPHY);
+		topographyFileView = new TextView("ProjectView.defaultDirectoryScenarios", AttributeType.TOPOGRAPHY);
 		topographyFileView.setScenarioChecker(model);
 		tabbedPane.addTab(Messages.getString("Tab.Topography.title"), topographyFileView);
 
-		eventFileView = new TextView("/attributes", "ProjectView.defaultDirectoryAttributes", AttributeType.EVENT);
+		eventFileView = new TextView( "ProjectView.defaultDirectoryAttributes", AttributeType.EVENT);
 		eventFileView.isEditable(true);
 		tabbedPane.addTab(Messages.getString("Tab.Event.title"), eventFileView);
 

VadereGui/src/org/vadere/gui/projectview/view/TextView.java

@@ -2,9 +2,11 @@ package org.vadere.gui.projectview.view;
 
 import com.fasterxml.jackson.core.JsonProcessingException;
 
+import org.apache.commons.configuration2.Configuration;
 import org.fife.ui.rsyntaxtextarea.RSyntaxTextArea;
 import org.fife.ui.rsyntaxtextarea.SyntaxConstants;
 import org.fife.ui.rsyntaxtextarea.Theme;
+import org.jetbrains.annotations.NotNull;
 import org.vadere.gui.components.utils.Messages;
 import org.vadere.gui.components.utils.Resources;
 import org.vadere.gui.projectview.VadereApplication;
@@ -17,12 +19,14 @@ import org.vadere.state.events.json.EventInfoStore;
 import org.vadere.state.events.presettings.EventPresettings;
 import org.vadere.state.scenario.Topography;
 import org.vadere.state.util.StateJsonConverter;
+import org.vadere.util.config.VadereConfig;
 import org.vadere.util.io.IOUtils;
 import org.vadere.util.logging.Logger;
 
 import java.awt.*;
 import java.awt.event.ActionEvent;
 import java.awt.event.ActionListener;
+import java.io.File;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.prefs.Preferences;
@@ -41,11 +45,12 @@ import javax.swing.filechooser.FileNameExtensionFilter;
 public class TextView extends JPanel implements IJsonView {
 
 	private static Logger logger = Logger.getLogger(TextView.class);
+	private static final Configuration CONFIG = VadereConfig.getConfig();
 
 	private AttributeType attributeType;
-	private String default_folder;
 	private String default_resource;
 
+
 	private JPanel panelTop = new JPanel();
 
 	private static final long serialVersionUID = 3975758744810301970L;
@@ -64,14 +69,15 @@ public class TextView extends JPanel implements IJsonView {
 	private ActionListener saveToFileActionListener = new ActionListener() {
 		@Override
 		public void actionPerformed(ActionEvent e) {
-			String path = IOUtils.chooseJSONFileSave(Messages.getString("TextFileView.btnSaveToFile.text"),
-					Preferences.userNodeForPackage(VadereApplication.class).get(default_resource, default_folder));
+			String path = IOUtils.chooseJSONFileSave(Messages.getString("TextFileView.btnSaveToFile.text"), CONFIG.getString(default_resource));
 
 			if (path == null)
 				return;
 
 			try {
 				IOUtils.writeTextFile(path.endsWith(".json") ? path : path + ".json", txtrTextfiletextarea.getText());
+				File file = new File(path);
+				VadereConfig.getConfig().setProperty(default_resource, file.getParentFile().getAbsolutePath());
 			} catch (IOException e1) {
 				IOUtils.errorBox(e1.getLocalizedMessage(), Messages.getString("SaveFileErrorMessage.title"));
 				logger.error(e1);
@@ -83,15 +89,15 @@ public class TextView extends JPanel implements IJsonView {
 		@Override
 		public void actionPerformed(ActionEvent arg0) {
 			FileFilter filter = new FileNameExtensionFilter("JSON file", "json");
-			String path = IOUtils.chooseFile("Choose file...",
-					Preferences.userNodeForPackage(VadereApplication.class).get(default_resource, default_folder),
-					filter);
+			String path = IOUtils.chooseFile(Messages.getString("ChooseFile.text"), CONFIG.getString(default_resource), filter);
 
 			if (path == null)
 				return;
 
 			try {
 				String content = IOUtils.readTextFile(path);
+				File file = new File(path);
+				VadereConfig.getConfig().setProperty(default_resource, file.getParentFile().getAbsolutePath());
 				txtrTextfiletextarea.setText(content);
 			} catch (IOException e) {
 				logger.error("could not load from file: " + e.getMessage());
@@ -102,8 +108,7 @@ public class TextView extends JPanel implements IJsonView {
 	/**
 	 * Create the panel.
 	 */
-	public TextView(String default_folder, String default_resource, final AttributeType attributeType) {
-	    this.default_folder = default_folder;
+	public TextView(@NotNull final String default_resource, final AttributeType attributeType) {
 		this.default_resource = default_resource;
 		this.attributeType = attributeType;
 		setLayout(new BorderLayout(0, 0));

VadereGui/src/org/vadere/gui/projectview/view/VDialogManager.java

 package org.vadere.gui.projectview.view;
 
 
+import org.apache.commons.configuration2.Configuration;
 import org.vadere.gui.components.utils.Messages;
 import org.vadere.gui.projectview.VadereApplication;
+import org.vadere.util.config.VadereConfig;
 import org.vadere.util.io.IOUtils;
 import org.vadere.util.logging.Logger;
 
@@ -14,6 +16,7 @@ import java.util.prefs.Preferences;
 
 public class VDialogManager {
 
+	private static final Configuration CONFIG = VadereConfig.getConfig();
 	private static Logger logger = Logger.getLogger(VDialogManager.class);
 	private static final FileFilter PROJECT_FILTER = new FileNameExtensionFilter("Vadere Project", "project");
 
@@ -34,12 +37,11 @@ public class VDialogManager {
 	}
 
 	public static String loadProjectDialog() {
-		return IOUtils.chooseFile(Messages.getString("LoadProjectText"),
-				getDefaultDirectory(), PROJECT_FILTER);
+		return IOUtils.chooseFile(Messages.getString("LoadProjectText"), getDefaultDirectory(), PROJECT_FILTER);
 	}
 
 	private static String getDefaultDirectory() {
-		return Preferences.userNodeForPackage(VadereApplication.class).get("ProjectView.defaultDirectory", "/projects");
+		return CONFIG.getString("ProjectView.defaultDirectory");
 	}
 
 	public static int showConfirmDialogWithBodyAndTextArea(String title, String body, String textAreaContent,

VadereMeshing/resources/poly/bridge.poly

 # nVertices dimension nAttributes boundaryMarker
-21 2 0 0
+19 2 0 0
 # vertexId x y
-1 0.500000 0.500000
-2 0.326351 11.349890
-3 50.295924 42.506680
-4 10.068317 15.240938
-5 50.129121 5.936819
-6 49.500000 49.500000
-7 49.500000 0.500000
-8 7.711478 10.030659
-9 0.463889 45.907735
-10 0.500000 49.500000
-11 32.129121 5.936819
-12 29.943119 0.478289
-13 3.888684 20.251412
-14 0.463889 42.907735
-15 1.463889 42.907735
-16 23.463889 45.907735
-17 0.326351 -0.650110
-18 15.459378 12.808810
-19 20.931872 10.544835
-20 29.943119 -1.521711
-21 31.295924 42.506680
+1 10.068317 15.240938
+2 0.500000 12.473684
+3 49.500000 49.500000
+4 49.500000 0.500000
+5 7.711478 10.030659
+6 0.500000 45.907735
+7 0.500000 42.907735
+8 32.129121 5.936819
+9 0.500000 49.500000
+10 3.888684 20.251412
+11 1.463889 42.907735
+12 1.821300 11.082842
+13 23.463889 45.907735
+14 49.500000 5.936819
+15 15.459378 12.808810
+16 20.931872 10.544835
+17 29.892589 0.500000
+18 49.500000 42.506680
+19 31.295924 42.506680
 #
 # nSegments boundaryMarker
-21 0
+19 0
 # lineId vertexId1 vertexId2
-1 7 6
-2 6 10
-3 10 1
-4 1 7
-5 3 21
-6 21 19
-7 19 11
-8 11 5
-9 5 3
-10 20 12
-11 12 8
-12 8 2
-13 2 17
-14 17 20
-15 13 4
-16 4 18
-17 18 16
-18 16 9
-19 9 14
-20 14 15
-21 15 13
+1 4 14
+2 14 8
+3 8 16
+4 16 19
+5 19 18
+6 18 3
+7 3 9
+8 9 6
+9 6 13
+10 13 15
+11 15 1
+12 1 10
+13 10 11
+14 11 7
+15 7 2
+16 2 12
+17 12 5
+18 5 17
+19 17 4
 #
 # nHoles
-3
-# vertexId x y (of a vertex which lies inside the hole)
-1 37.999061 23.296979
-2 11.703993 3.490417
-3 11.505979 32.412715
\ No newline at end of file
+0
+# vertexId x y (of a vertex which lies inside the hole)
\ No newline at end of file

VadereMeshing/src/org/vadere/meshing/WeilerAtherton.java

@@ -25,9 +25,22 @@ import java.util.stream.Collectors;
 
 /**
  * The Weiler-Atherton-Algorithm (https://en.wikipedia.org/wiki/Weiler%E2%80%93Atherton_clipping_algorithm)
- * merges a set of polygons. Note this merging does not support holes that is if there is a hole it will be
- * filled by the merging algorithm. Two polygons will be merged if they overlap. Co-linear and duplicated points,
- * i.e. useless points, will be removed. If polygon A contains polygon B, the result will be equals to polygon A.
+ * enables boolean operations on polygons, i.e.:
+ * <ul>
+ *     <li>
+ *         INTERSECTION of polygon A (subject) and B (clipper).
+ *     </li>
+ *     <li>
+ *         UNION of polygon A (subject) and B (clipper).
+ *     </li>
+ *     <li>
+ *         SUBTRACTION of polygon B (subject) from polygon A (clipper).
+ *     </li>
+ * </ul>
+ * Note that holes are not supported, i.e. if UNION produces a polygon with holes this will be filled.
+ * If SUBTRACTION produces a hole an error will be thrown since {@link VPolygon} do not support holes.
+ * Furthermore, it is assumed that each {@link VPolygon} which is involved is a simple non-self intersecting
+ * polygon. Co-linear and duplicated points, i.e. useless points, will be removed.
  *
  * @author Benedikt Zoennchen
  */
@@ -39,6 +52,13 @@ public class WeilerAtherton {
 	private static final String propNameInside = "inside";
 	private static final String propNameTwin = "twin";
 
+	private enum Operation {
+		INTERSECTION,
+		UNION,
+		SUBTRACTION
+	}
+
+
 	/**
 	 * The default constructor.
 	 *
@@ -287,6 +307,31 @@ public class WeilerAtherton {
 		return newPolygons;
 	}
 
+	public Optional<VPolygon> subtraction() {
+		List<VPolygon> newPolygons = new ArrayList<>();
+		newPolygons.addAll(polygons);
+
+		if(polygons.size() == 0) {
+			return Optional.empty();
+		}
+		else if(polygons.size() == 1) {
+			return Optional.of(polygons.get(0));
+		}
+		else {
+			VPolygon subject = polygons.get(0);
+			List<VPolygon> clippers = polygons.subList(1, polygons.size());
+			for(VPolygon clipper : clippers) {
+				Optional<VPolygon> result = subtraction(subject, clipper);
+				if(result.isPresent()) {
+					subject = result.get();
+				} else {
+					return Optional.empty();
+				}
+			}
+			return Optional.of(subject);
+		}
+	}
+
 	/**
 	 * Executes the Weiler-Atherton-Algorithm for all of its polygons.
 	 *
@@ -330,13 +375,19 @@ public class WeilerAtherton {
 	private List<VPolygon> construct(
 			@NotNull final VPolygon subjectCandidat,
 			@NotNull final VPolygon clippingCandidat,
-			final boolean cap) {
+			final Operation operation) {
 
 		//Predicate<WeilerPoint> startEdgeCondition = cap ? p -> p.isInside() : p -> !p.isInside();
 		VPolygon subject = GeometryUtils.isCCW(subjectCandidat) ? subjectCandidat : subjectCandidat.revertOrder();
-		VPolygon clipping = GeometryUtils.isCCW(clippingCandidat) ? clippingCandidat : clippingCandidat.revertOrder();
-
-
+		VPolygon clipping;
+		switch (operation) {
+			case SUBTRACTION: clipping = GeometryUtils.isCCW(clippingCandidat) ? clippingCandidat.revertOrder() : clippingCandidat; break;
+			case UNION:
+			case INTERSECTION:
+			default: {
+				clipping = GeometryUtils.isCCW(clippingCandidat) ? clippingCandidat : clippingCandidat.revertOrder(); break;
+			}
+		}
 		PMesh subjectMesh = new PMesh();
 		PMesh clippingMesh = new PMesh();
 
@@ -371,7 +422,13 @@ public class WeilerAtherton {
 		List<VPolygon> polygons = new ArrayList<>();
 		PMesh mesh = subjectMesh;
 
-		Set<PHalfEdge> intersectionSet = cap ? subjectEnteringEdges : subjectExitingEdges;
+		Set<PHalfEdge> intersectionSet;
+		switch (operation) {
+			case INTERSECTION: intersectionSet = subjectEnteringEdges; break;
+			case UNION:
+			case SUBTRACTION:
+			default: intersectionSet = subjectExitingEdges; break;
+		};
 
 		// cup will preserve the polyons.
 		if(intersectionSet.isEmpty()) {
@@ -383,27 +440,34 @@ public class WeilerAtherton {
 					.streamPoints(clippingFace)
 					.allMatch(p -> contains(subject, p));
 
-			if(cap) {
-				if(subInClip) {
-					polygons.add(subjectCandidat);
-				}
-				else if(clipInSub) {
-					polygons.add(clippingCandidat);
-				}
-			}
-			else {
-				if(subInClip) {
-					polygons.add(clippingCandidat);
-				}
-				else if(clipInSub) {
-					polygons.add(subjectCandidat);
-				}
-				else {
-					polygons.add(subjectCandidat);
-					polygons.add(clippingCandidat);
-				}
+			switch (operation) {
+				case INTERSECTION: {
+					if(subInClip) {
+						polygons.add(subjectCandidat);
+					}
+					else if(clipInSub) {
+						polygons.add(clippingCandidat);
+					}
+				} break;
+				case UNION: {
+					if(subInClip) {
+						polygons.add(clippingCandidat);
+					}
+					else if(clipInSub) {
+						polygons.add(subjectCandidat);
+					}
+					else {
+						polygons.add(subjectCandidat);
+						polygons.add(clippingCandidat);
+					}
+				} break;
+				case SUBTRACTION:
+				default: {
+					if(clipInSub) {
+						throw new IllegalArgumentException("subtracting a polygon which is contained in its counterpart will produce a polygon with a hole which is not supported.");
+					}
+				} break;
 			}
-
 			return polygons;
 		}
 
@@ -558,7 +622,7 @@ public class WeilerAtherton {
 	}
 
 	public List<VPolygon> cap(@NotNull final VPolygon subjectCandidat, @NotNull final VPolygon clippingCandidat) {
-		return construct(subjectCandidat, clippingCandidat, true);
+		return construct(subjectCandidat, clippingCandidat, Operation.INTERSECTION);
 	}
 
 	//private PVertex<VPoint, Object, Object> getTwin(@NotNull final PVertex<VPoint, Object, Object> v, PMesh<VPoint, Object, >)
@@ -571,6 +635,18 @@ public class WeilerAtherton {
 	 * @return a pair of polygon where the second element is null if the polygons got merged. This is not the case if there do not overlap.
 	 */
 	public List<VPolygon> cup(@NotNull final VPolygon subjectCandidat, @NotNull final VPolygon clippingCandidat) {
-		return construct(subjectCandidat, clippingCandidat, false);
+		return construct(subjectCandidat, clippingCandidat, Operation.UNION);
+	}
+
+	public Optional<VPolygon> subtraction(@NotNull final VPolygon subjectCandidat, @NotNull final VPolygon clippingCandidat) {
+		List<VPolygon> result = construct(subjectCandidat, clippingCandidat, Operation.SUBTRACTION);
+		assert result.size() >= 1;
+		if(result.size() == 1) {
+			return Optional.of(result.get(0));
+		} else if(result.isEmpty()) {
+			return Optional.empty();
+		} else{
+			throw new IllegalStateException("subtraction of two polygons should never produce more than one polygon.");
+		}
 	}
 }