diff --git a/demos/mandelbrot/CMakeLists.txt b/demos/mandelbrot/CMakeLists.txt
new file mode 100644
index 0000000..5c500e0
--- /dev/null
+++ b/demos/mandelbrot/CMakeLists.txt
@@ -0,0 +1,21 @@
+find_package(Qt4 REQUIRED)
+
+set(CMAKE_INCLUDE_CURRENT_DIR ON)
+
+if (CMAKE_COMPILER_IS_GNUCXX)
+   set ( CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2")
+   add_definitions ( "-DNDEBUG" )
+endif (CMAKE_COMPILER_IS_GNUCXX)
+
+include_directories( ${QT_INCLUDE_DIR} )
+
+set(mandelbrot_SRCS
+    mandelbrot.cpp
+)
+
+qt4_automoc(${mandelbrot_SRCS})
+
+add_executable(mandelbrot ${mandelbrot_SRCS})
+add_dependencies(demos mandelbrot)
+
+target_link_libraries(mandelbrot ${QT_QTCORE_LIBRARY} ${QT_QTGUI_LIBRARY})
diff --git a/demos/mandelbrot/README b/demos/mandelbrot/README
new file mode 100644
index 0000000..a451d65
--- /dev/null
+++ b/demos/mandelbrot/README
@@ -0,0 +1,10 @@
+*** Mandelbrot demo ***
+
+Controls:
+* Left mouse button to center view at a point.
+* Drag vertically with left mouse button to zoom in and out.
+
+Be sure to enable SSE2 or AltiVec to improve performance.
+
+The number of iterations, and the choice between single and double precision, are
+determined at runtime depending on the zoom level.
diff --git a/demos/mandelbrot/mandelbrot.cpp b/demos/mandelbrot/mandelbrot.cpp
new file mode 100644
index 0000000..5d575d5
--- /dev/null
+++ b/demos/mandelbrot/mandelbrot.cpp
@@ -0,0 +1,213 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2008 Benoit Jacob <jacob.benoit.1@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#include "mandelbrot.h"
+#include <iostream>
+#include<QtGui/QPainter>
+#include<QtGui/QImage>
+#include<QtGui/QMouseEvent>
+#include<QtCore/QTime>
+
+void MandelbrotWidget::resizeEvent(QResizeEvent *)
+{
+  if(size < width() * height())
+  {
+    std::cout << "reallocate buffer" << std::endl;
+    size = width() * height();
+    if(buffer) delete[]buffer;
+    buffer = new unsigned char[4*size];
+  }
+}
+
+template<typename T> struct iters_before_test { enum { ret = 8 }; };
+template<> struct iters_before_test<double> { enum { ret = 16 }; };
+
+template<typename Real> void MandelbrotThread::render(int img_width, int img_height)
+{
+  enum { packetSize = Eigen::internal::packet_traits<Real>::size }; // number of reals in a Packet
+  typedef Eigen::Array<Real, packetSize, 1> Packet; // wrap a Packet as a vector
+
+  enum { iters_before_test = iters_before_test<Real>::ret };
+  max_iter = (max_iter / iters_before_test) * iters_before_test;
+  const int alignedWidth = (img_width/packetSize)*packetSize;
+  unsigned char *const buffer = widget->buffer;
+  const double xradius = widget->xradius;
+  const double yradius = xradius * img_height / img_width;
+  const int threadcount = widget->threadcount;
+  typedef Eigen::Array<Real, 2, 1> Vector2;
+  Vector2 start(widget->center.x() - widget->xradius, widget->center.y() - yradius);
+  Vector2 step(2*widget->xradius/img_width, 2*yradius/img_height);
+  total_iter = 0;
+
+  for(int y = id; y < img_height; y += threadcount)
+  {
+    int pix = y * img_width;
+
+    // for each pixel, we're going to do the iteration z := z^2 + c where z and c are complex numbers, 
+    // starting with z = c = complex coord of the pixel. pzi and pzr denote the real and imaginary parts of z.
+    // pci and pcr denote the real and imaginary parts of c.
+
+    Packet pzi_start, pci_start;
+    for(int i = 0; i < packetSize; i++) pzi_start[i] = pci_start[i] = start.y() + y * step.y();
+
+    for(int x = 0; x < alignedWidth; x += packetSize, pix += packetSize)
+    {
+      Packet pcr, pci = pci_start, pzr, pzi = pzi_start, pzr_buf;
+      for(int i = 0; i < packetSize; i++) pzr[i] = pcr[i] = start.x() + (x+i) * step.x();
+
+      // do the iterations. Every iters_before_test iterations we check for divergence,
+      // in which case we can stop iterating.
+      int j = 0;
+      typedef Eigen::Matrix<int, packetSize, 1> Packeti;
+      Packeti pix_iter = Packeti::Zero(), // number of iteration per pixel in the packet
+              pix_dont_diverge; // whether or not each pixel has already diverged
+      do
+      {
+        for(int i = 0; i < iters_before_test/4; i++) // peel the inner loop by 4
+        {
+#         define ITERATE \
+            pzr_buf = pzr; \
+            pzr = pzr.square(); \
+            pzr -= pzi.square(); \
+            pzr += pcr; \
+            pzi = (2*pzr_buf)*pzi; \
+            pzi += pci;
+          ITERATE ITERATE ITERATE ITERATE
+        }
+        pix_dont_diverge = ((pzr.square() + pzi.square())
+                           .eval() // temporary fix as what follows is not yet vectorized by Eigen
+                           <= Packet::Constant(4))
+                                // the 4 here is not a magic value, it's a math fact that if
+                                // the square modulus is >4 then divergence is inevitable.
+                           .template cast<int>();
+        pix_iter += iters_before_test * pix_dont_diverge;
+        j++;
+        total_iter += iters_before_test * packetSize;
+      }
+      while(j < max_iter/iters_before_test && pix_dont_diverge.any()); // any() is not yet vectorized by Eigen
+
+      // compute pixel colors
+      for(int i = 0; i < packetSize; i++)
+      {
+        buffer[4*(pix+i)] = 255*pix_iter[i]/max_iter;
+        buffer[4*(pix+i)+1] = 0;
+        buffer[4*(pix+i)+2] = 0;
+      }
+    }
+
+    // if the width is not a multiple of packetSize, fill the remainder in black
+    for(int x = alignedWidth; x < img_width; x++, pix++)
+      buffer[4*pix] = buffer[4*pix+1] = buffer[4*pix+2] = 0;
+  }
+  return;
+}
+
+void MandelbrotThread::run()
+{
+  setTerminationEnabled(true);
+  double resolution = widget->xradius*2/widget->width();
+  max_iter = 128;
+  if(resolution < 1e-4f) max_iter += 128 * ( - 4 - std::log10(resolution));
+  int img_width = widget->width()/widget->draft;
+  int img_height = widget->height()/widget->draft;
+  single_precision = resolution > 1e-7f;
+
+  if(single_precision)
+    render<float>(img_width, img_height);
+  else
+    render<double>(img_width, img_height);
+}
+
+void MandelbrotWidget::paintEvent(QPaintEvent *)
+{
+  static float max_speed = 0;
+  long long total_iter = 0;
+
+  QTime time;
+  time.start();
+  for(int th = 0; th < threadcount; th++)
+    threads[th]->start(QThread::LowPriority);
+  for(int th = 0; th < threadcount; th++)
+  {
+    threads[th]->wait();
+    total_iter += threads[th]->total_iter;
+  }
+  int elapsed = time.elapsed();
+
+  if(draft == 1)
+  {
+    float speed = elapsed ? float(total_iter)*1000/elapsed : 0;
+    max_speed = std::max(max_speed, speed);
+    std::cout << threadcount << " threads, "
+              << elapsed << " ms, "
+              << speed << " iters/s (max " << max_speed << ")" << std::endl;
+    int packetSize = threads[0]->single_precision
+                   ? int(Eigen::internal::packet_traits<float>::size)
+                   : int(Eigen::internal::packet_traits<double>::size);
+    setWindowTitle(QString("resolution ")+QString::number(xradius*2/width(), 'e', 2)
+                  +QString(", %1 iterations per pixel, ").arg(threads[0]->max_iter)
+                  +(threads[0]->single_precision ? QString("single ") : QString("double "))
+                  +QString("precision, ")
+                  +(packetSize==1 ? QString("no vectorization")
+                                  : QString("vectorized (%1 per packet)").arg(packetSize)));
+  }
+  
+  QImage image(buffer, width()/draft, height()/draft, QImage::Format_RGB32);
+  QPainter painter(this);
+  painter.drawImage(QPoint(0, 0), image.scaled(width(), height()));
+
+  if(draft>1)
+  {
+    draft /= 2;
+    setWindowTitle(QString("recomputing at 1/%1 resolution...").arg(draft));
+    update();
+  }
+}
+
+void MandelbrotWidget::mousePressEvent(QMouseEvent *event)
+{
+  if( event->buttons() & Qt::LeftButton )
+  {
+    lastpos = event->pos();
+    double yradius = xradius * height() / width();
+    center = Eigen::Vector2d(center.x() + (event->pos().x() - width()/2) * xradius * 2 / width(),
+                             center.y() + (event->pos().y() - height()/2) * yradius * 2 / height());
+    draft = 16;
+    for(int th = 0; th < threadcount; th++)
+      threads[th]->terminate();
+    update();
+  }
+}
+
+void MandelbrotWidget::mouseMoveEvent(QMouseEvent *event)
+{
+  QPoint delta = event->pos() - lastpos;
+  lastpos = event->pos();
+  if( event->buttons() & Qt::LeftButton )
+  {
+    double t = 1 + 5 * double(delta.y()) / height();
+    if(t < 0.5) t = 0.5;
+    if(t > 2) t = 2;
+    xradius *= t;
+    draft = 16;
+    for(int th = 0; th < threadcount; th++)
+      threads[th]->terminate();
+    update();
+  }
+}
+
+int main(int argc, char *argv[])
+{
+  QApplication app(argc, argv);
+  MandelbrotWidget w;
+  w.show();
+  return app.exec();
+}
+
+#include "mandelbrot.moc"
diff --git a/demos/mandelbrot/mandelbrot.h b/demos/mandelbrot/mandelbrot.h
new file mode 100644
index 0000000..a687fd0
--- /dev/null
+++ b/demos/mandelbrot/mandelbrot.h
@@ -0,0 +1,71 @@
+// This file is part of Eigen, a lightweight C++ template library
+// for linear algebra.
+//
+// Copyright (C) 2008 Benoit Jacob <jacob.benoit.1@gmail.com>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef MANDELBROT_H
+#define MANDELBROT_H
+
+#include <Eigen/Core>
+#include <QtGui/QApplication>
+#include <QtGui/QWidget>
+#include <QtCore/QThread>
+
+class MandelbrotWidget;
+
+class MandelbrotThread : public QThread
+{
+    friend class MandelbrotWidget;
+    MandelbrotWidget *widget;
+    long long total_iter;
+    int id, max_iter;
+    bool single_precision;
+
+  public:
+    MandelbrotThread(MandelbrotWidget *w, int i) : widget(w), id(i) {}
+    void run();
+    template<typename Real> void render(int img_width, int img_height);
+};
+
+class MandelbrotWidget : public QWidget
+{
+    Q_OBJECT
+
+    friend class MandelbrotThread;
+    Eigen::Vector2d center;
+    double xradius;
+    int size;
+    unsigned char *buffer;
+    QPoint lastpos;
+    int draft;
+    MandelbrotThread **threads;
+    int threadcount;
+
+  protected:
+    void resizeEvent(QResizeEvent *);
+    void paintEvent(QPaintEvent *);
+    void mousePressEvent(QMouseEvent *event);
+    void mouseMoveEvent(QMouseEvent *event);
+
+  public:
+    MandelbrotWidget() : QWidget(), center(0,0), xradius(2),
+                         size(0), buffer(0), draft(16)
+    {
+      setAutoFillBackground(false);
+      threadcount = QThread::idealThreadCount();
+      threads = new MandelbrotThread*[threadcount];
+      for(int th = 0; th < threadcount; th++) threads[th] = new MandelbrotThread(this, th);
+    }
+    ~MandelbrotWidget()
+    {
+      if(buffer) delete[]buffer;
+      for(int th = 0; th < threadcount; th++) delete threads[th];
+      delete[] threads;
+    }
+};
+
+#endif // MANDELBROT_H