diff --git a/lib/qtgraph/CMakeLists.txt b/lib/qtgraph/CMakeLists.txt
index 9ef4870..3bbb66c 100644
--- a/lib/qtgraph/CMakeLists.txt
+++ b/lib/qtgraph/CMakeLists.txt
@@ -6,6 +6,10 @@ set (CXX_FLAGS "-Wall -Wextra -pedantic -Wshadow "
                "-ggdb "
                "--std=c++0x " )
 
+set ( CMAKE_CXX_COMPILER "/usr/bin/g++-4.9.2")
+# set ( CMAKE_CXX_COMPILER "distcc")
+
+
 add_definitions( ${CXX_FLAGS} )
 
 FIND_PACKAGE(Qt4 REQUIRED)
@@ -25,9 +29,10 @@ main.cpp
 edge.cpp
 node.cpp
 graphwidget.cpp
+marching_squares.cpp
 ${qtgraph_HEADERS_MOC} )
 
-target_link_libraries ( qtgraph ${QT_LIBRARIES} xml2 )
+target_link_libraries ( qtgraph ${QT_LIBRARIES} xml2 png)
 
 add_custom_target (clean2
 COMMAND
diff --git a/lib/qtgraph/graphwidget.cpp b/lib/qtgraph/graphwidget.cpp
index 3a4b3c1..1a1f48c 100644
--- a/lib/qtgraph/graphwidget.cpp
+++ b/lib/qtgraph/graphwidget.cpp
@@ -5,7 +5,9 @@
 
 #include <graph/graph.hpp>
 #include <graph/graph_algorithms.hpp>
+
 #include "floats.hpp"
+#include "marching_squares.hpp"
 
 #include <QtGui/QKeyEvent>
 #include <QtGui/QWheelEvent>
@@ -15,6 +17,7 @@
 #include <QtGui/QApplication>
 
 #include <functional>
+#include <cassert>
 
 namespace std {
   template <>
@@ -85,8 +88,11 @@ QList<Edge*> calculateShortestRoute(const QGraphicsScene* scene,
     QGraphicsItem* source_item = scene->itemAt(QPointFFromfloat2(shortestPath[i]));
     QGraphicsItem* destination_item = scene->itemAt(QPointFFromfloat2(shortestPath[i+1]));
     Node* source_node = dynamic_cast<Node*>(source_item);
+    assert(source_node);
     Node* destination_node = dynamic_cast<Node*>(destination_item);
+    assert(destination_node);
     Edge* e = source_node->edgeTo(destination_node);
+    assert(e);
     route.append(e);
   }
   return route;
@@ -99,7 +105,13 @@ GraphWidget::GraphWidget(Graph<float2>* graph, const QString& png_file, QWidget
     : QGraphicsView(p)
     , m_graph(graph)
     , m_background(0)
+    , m_lines()
+    , m_lines_are_visible(true)
 {
+  MarchingSquares ms;
+  ms.ReadImage(png_file.toStdString());
+  m_lines = ms.RunMarchingSquares();
+
   m_background = new QPixmap(png_file);
   if (m_background->isNull())
     throw std::invalid_argument("Could not load image from png file.");
@@ -193,6 +205,11 @@ void GraphWidget::keyPressEvent(QKeyEvent *e)
     }
     case Qt::Key_Space: {
       modifyRoute();
+      break;
+    }
+    case Qt::Key_L: {
+      showLines();
+      break;
     }
     default:
         QGraphicsView::keyPressEvent(e);
@@ -206,7 +223,7 @@ void GraphWidget::modifyRoute()
 
   QList <QGraphicsItem* > selectedItems = scene()->selectedItems();
   if (selectedItems.isEmpty())
-    break;
+    return;
 
   QGraphicsItem* selectedItem = selectedItems.first();
   Node* selectedNode = dynamic_cast<Node*>(selectedItem);
@@ -225,6 +242,53 @@ void GraphWidget::modifyRoute()
   }
 }
 
+void GraphWidget::showLines()
+{
+  m_lines_are_visible = !m_lines_are_visible;
+  if (m_lines_are_visible) {
+    resetGraph();
+    for (std::size_t i = 0; i < m_lines.size(); ++i) {
+       const std::pair<float2, float2> e = m_lines[i];
+       const float2 s = e.first;
+       Node* s_n = 0;
+       if (!contains(*m_graph, s)) {
+        s_n = new Node(this);
+        scene()->addItem(s_n);
+        s_n->setPos(s.x, s.y);
+       } else {
+        QGraphicsItem* gi = scene()->itemAt(QPointFFromfloat2(s));
+        s_n = dynamic_cast<Node*>(gi);
+       }
+       const float2 d = e.second;
+       Node* d_n = 0;
+       if (!contains(*m_graph, d)) {
+        d_n = new Node(this);
+        scene()->addItem(d_n);
+        d_n->setPos(d.x, d.y);
+       } else {
+        QGraphicsItem* gi = scene()->itemAt(QPointFFromfloat2(d));
+        d_n = dynamic_cast<Node*>(gi);
+       }
+       insertEdge(s_n, d_n);
+       m_graph->addEdge(s, d);
+    }
+  }
+}
+
+void GraphWidget::resetGraph()
+{
+  for (QList<Edge*>::iterator it = m_route.begin(); it != m_route.end(); ++it)
+    (*it)->setIsRoute(false);
+
+  m_route.clear();
+  m_graph->clear();
+  QList <QGraphicsItem* > all_items = scene()->items();
+  for (int i = 0; i < all_items.size(); ++i) {
+    scene()->removeItem(all_items.at(i));
+    delete all_items.at(i);
+  }
+}
+
 
 void GraphWidget::removeEdge(Node* selectedNode, Node* nodeUnderMouse)
 {
diff --git a/lib/qtgraph/graphwidget.hpp b/lib/qtgraph/graphwidget.hpp
index 2d8d0eb..7822759 100644
--- a/lib/qtgraph/graphwidget.hpp
+++ b/lib/qtgraph/graphwidget.hpp
@@ -7,7 +7,7 @@ class float2;
 template<class T> class Graph;
 class Node;
 class Edge;
-// class QPixmap;
+class QString;
 
 class GraphWidget : public QGraphicsView
 {
@@ -39,10 +39,14 @@ private:
   void insertEdge(Node* selectedNode, Node* nodeUnderMouse);
   void removeEdge(Node* selectedNode, Node* nodeUnderMouse);
   void modifyRoute();
+  void showLines();
+  void resetGraph();
 
   Graph<float2>* m_graph;
   QPixmap* m_background;
   QList<Edge*> m_route;
+  std::vector< std::pair<float2, float2> > m_lines;
+  bool m_lines_are_visible;
 };
 
 #endif
diff --git a/lib/qtgraph/marching_squares.cpp b/lib/qtgraph/marching_squares.cpp
new file mode 100644
index 0000000..c74a926
--- /dev/null
+++ b/lib/qtgraph/marching_squares.cpp
@@ -0,0 +1,135 @@
+#include "marching_squares.hpp"
+
+#include "floats.hpp"
+
+// #include <errno.h> // for strerror needed by png++/error.hpp
+#include <string.h> // for strerror needed by png++/error.hpp
+
+#include <png++/png.hpp>
+// #include <iostream>
+#include <sstream>
+
+
+MarchingSquares::MarchingSquares()
+  : width_(0)
+  , height_(0)
+  , cells_()
+{
+}
+
+void MarchingSquares::ReadImage(const std::string& filename)
+{
+  png::image<png::gray_pixel> image(filename); // throws std_error(filename);
+
+  width_  = image.get_width();
+  height_ = image.get_height();
+  cells_.reserve(width_ * height_);
+
+  for (size_t y = 0; y < height_; ++y) {
+    const png::image<png::gray_pixel>::row_type& row = image[y];
+    for (size_t x = 0; x < width_; ++x) {
+      CellType c; // map pixel luminance to cell type
+      if (row[x] < 16)        c = SOLID;       // black
+      else if (row[x] >= 240) c = FREE;        // white
+      else                    c = DESTROYABLE; // everything else
+
+      // mark borders as SOLID, such that the entities in the world cannot fall off
+      // if (x == 0 || x == width_ - 1 || y == 0 || y == height_ - 1)
+      //   c = SOLID;
+
+      cells_.push_back(c);
+    }
+  }
+}
+
+
+std::vector< std::pair<float2, float2> >
+MarchingSquares::RunMarchingSquares() {
+
+  std::vector< std::pair<float2, float2> > lines;
+  for (size_t y = 1; y < height_; ++y) {
+    for (size_t x = 1; x < width_; ++x) {
+
+      const CellType quad[4] = {
+        cells_[(y-1) * width_ + (x-1)], cells_[(y-1) * width_ + x],   // TL T
+        cells_[    y * width_ + (x-1)], cells_[    y * width_ + x] }; // R  X
+
+      const int mask = ((quad[0] == FREE) ? 0x0 : 0x1)
+                     | ((quad[1] == FREE) ? 0x0 : 0x2)
+                     | ((quad[2] == FREE) ? 0x0 : 0x4)
+                     | ((quad[3] == FREE) ? 0x0 : 0x8);
+
+      const float2 points[8] = { // clockwise, starting in top-left corner
+        float2(x-1,    y-1   ), // TL 0
+        float2(x-0.5f, y-1   ), // T  1
+        float2(x,      y-1   ), // TR 2
+        float2(x,      y-0.5f), // R  3
+        float2(x,      y     ), // BR 4
+        float2(x-0.5f, y     ), // B  5
+        float2(x-1,    y     ), // BL 6
+        float2(x-1,    y-0.5f)  // L  7
+      };
+
+      // these are the marching squares cases
+      int s = -1, e = -1;
+      switch (mask) {
+        case 0x0: /* all free, nothing to do */      break;
+
+        case 0x1: /* TL       = TL  */ s = 7; e = 1; break;
+        case 0x2: /*   TR     = TR  */ s = 1; e = 3; break;
+        case 0x4: /*     BL   = BL  */ s = 5; e = 7; break;
+        case 0x8: /*       BR = BR  */ s = 3; e = 5; break;
+
+        case 0x3: /* TLTR     = top    */ s = 7; e = 3; break;
+        case 0x5: /* TL  BL   = left   */ s = 1; e = 5; break;
+        case 0xa: /*   TR  BR = right  */ s = 5; e = 1; break;
+        case 0xc: /*     BLBR = bottom */ s = 3; e = 7; break;
+
+        // two lines
+        case 0x6: /*   TRBL   = left desc diagonal */ s = 1 | 5 << 4; e = 3 | 7 << 4; break;
+        case 0x9: /* TL    BR = right asc diagonal */ s = 7 | 3 << 4; e = 1 | 5 << 4; break;
+
+        case 0x7: /* TLTRBL   = BR free */ s = 3; e = 5; break;
+        case 0xb: /* TLTR  BR = BL free */ s = 5; e = 7; break;
+        case 0xd: /* TL  BLBR = TR free */ s = 1; e = 3; break;
+        case 0xe: /*   TRBLBR = TL free */ s = 7; e = 1; break;
+
+        case 0xf: /* all blocked, nothing to do */       break;
+      }
+
+      if (s == -1) {
+        // assert: e == -1
+      }
+      else if ((s & 0xf) == s) {
+        // assert: (e & 0xf) == e
+        lines.push_back(std::pair<float2, float2>(points[s], points[e]));
+      }
+      else {
+        int s1 = s & 0xf, e1 = e & 0xf;
+        int s2 = s >> 4,  e2 = e >> 4;
+        lines.push_back(std::pair<float2, float2>(points[s1], points[e1]));
+        lines.push_back(std::pair<float2, float2>(points[s2], points[e2]));
+      }
+    }
+  }
+
+  // build quadtree and combine lines (there are many, many small pieces around now, combine to longer lines)
+
+  return lines;
+}
+
+std::string
+dumpLinesToHtml(const std::vector< std::pair< float2, float2 > >& lines, int w, int h)
+{
+  std::stringstream ss;
+
+  // dump to HTML/SVG
+  ss << "<!DOCTYPE html>" << std::endl;
+  ss << "<html><body><svg xmlns=\"http://www.w3.org/2000/svg\" version=\"1.1\" viewbox=\"0 0 " << w << " " << h << "\">" << std::endl;
+  for (size_t i = 0; i < lines.size(); ++i) {
+    const std::pair<float2, float2>& l = lines[i];
+    ss << "\t<line x1=\"" << l.first.x << "\" y1=\"" << l.first.y << "\" x2=\"" << l.second.x << "\" y2=\"" << l.second.y << "\" stroke=\"black\" stroke-width=\"0.25\"/>" << std::endl;
+  }
+  ss << "</svg></body></html>" << std::endl;
+  return ss.str();
+}
diff --git a/lib/qtgraph/marching_squares.hpp b/lib/qtgraph/marching_squares.hpp
new file mode 100644
index 0000000..45c8968
--- /dev/null
+++ b/lib/qtgraph/marching_squares.hpp
@@ -0,0 +1,27 @@
+#ifndef WORLD_WORLD_HPP
+#define WORLD_WORLD_HPP
+
+#include <string>
+#include <vector>
+
+class float2;
+
+class MarchingSquares {
+  enum CellType { FREE, SOLID, DESTROYABLE };
+
+public:
+  MarchingSquares();
+
+  void ReadImage(const std::string& filename);
+  std::vector< std::pair<float2, float2> > RunMarchingSquares();
+
+private:
+
+  std::size_t width_;
+  std::size_t height_;
+  std::vector<CellType> cells_;
+};
+
+std::string dumpLinesToHtml(const std::vector< std::pair<float2, float2> >& lines, int w, int h);
+
+#endif // WORLD_WORLD_HPP