diff --git a/Code/hull.cpp b/Code/hull.cpp
index 25249b6ccbbc01eb20c197c883d7a27f7d2b8811..00c06bab9e628e69ee25b88ce38cd8d2361d6684 100644
--- a/Code/hull.cpp
+++ b/Code/hull.cpp
@@ -40,14 +40,18 @@ struct vec {
 
 //}}}
 
+int sign(float x) {
+  if(x > 0) return 1 ;
+  if(x < 0) return -1 ;
+  return 0 ;
+}
+
 int orient(const vec& v0, const vec& v1) {
   //determinant
   float d = v0.x*v1.y - v0.y*v1.x ;
 
   //encode as sign
-  if(d > 0) return 1 ;
-  if(d < 0) return -1 ;
-  return 0 ;
+  return sign(d) ;
 }
 
 struct vec_compare {
@@ -90,13 +94,13 @@ void compute_hull(std::vector<vec>& points, std::vector<vec>& hull) {
   for(size_t i = 2; i < points.size(); ++i) {
     //check whether the last segment creates a wrong turn
     const vec& p = points[i] ;
-    vec v1 = p - *(hull.end() - 1) ;
+    vec v1 = p - *(hull.end() - 2) ;
     vec v2 = *(hull.end() - 1) - *(hull.end() - 2) ;
     while(hull.size() > 1 && orient(v1, v2) < 0) {
       //in case of wrong turn, rease the last point of the hull and iterate
       hull.erase(hull.end() - 1) ;
-      v1 = p - *(hull.end() - 1) ;
-      v2 = p - *(hull.end() - 2) ;
+      v1 = p - *(hull.end() - 2) ;
+      v2 = *(hull.end() - 1) - *(hull.end() - 2) ;
     }
     hull.push_back(p) ;
   }
diff --git a/Presentation/Figures/hull.svg b/Presentation/Figures/hull.svg
index 7a44f379651decc26660176777ea06ba1ac7524b..e177c819ca41e0b0cb03affa18505a198cda56fa 100644
--- a/Presentation/Figures/hull.svg
+++ b/Presentation/Figures/hull.svg
@@ -27,6 +27,92 @@
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+       inkscape:isstock="true">
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+    <marker
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+         id="path3853" />
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+    <marker
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+       orient="auto"
+       refY="0.0"
+       refX="0.0"
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+       style="overflow:visible;"
+       inkscape:isstock="true"
+       inkscape:collect="always">
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diff --git a/Presentation/Figures/orient.pdf b/Presentation/Figures/orient.pdf
new file mode 100644
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diff --git a/Presentation/Figures/orient.svg b/Presentation/Figures/orient.svg
new file mode 100644
index 0000000000000000000000000000000000000000..772fefe4cc34bf1db466d4c0534482e8df92d693
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diff --git a/Presentation/Figures/sort_comparison.pdf b/Presentation/Figures/sort_comparison.pdf
new file mode 100644
index 0000000000000000000000000000000000000000..e29ce993b2fe28701ee649b9f9fb194db850186e
Binary files /dev/null and b/Presentation/Figures/sort_comparison.pdf differ
diff --git a/Presentation/Figures/sweep_comparison.pdf b/Presentation/Figures/sweep_comparison.pdf
new file mode 100644
index 0000000000000000000000000000000000000000..cfae1bbbcab5446616f0834efdecc54806b9dee4
Binary files /dev/null and b/Presentation/Figures/sweep_comparison.pdf differ
diff --git a/Presentation/presentation.tex b/Presentation/presentation.tex
index d70438fd7851f8cb0f6fa224068996a0c819ef45..a943089726b64c2cb7c1f289646e7174bd588e76 100644
--- a/Presentation/presentation.tex
+++ b/Presentation/presentation.tex
@@ -25,29 +25,29 @@
 \frame {
   \frametitle{Enveloppe convexe}
   \centering
-  \includegraphics[width=0.6\li]{Figures/hull.pdf}
+  \includegraphics[width=0.6\li]{Figures/hull}
 }
 
 \frame {
   \frametitle{1 -- trier par angle}
   \centering
-  \includegraphics[width=0.6\li]{Figures/hull_sort.pdf}
+  \includegraphics[width=0.6\li]{Figures/hull_sort}
 }
 
 \frame {
   \frametitle{2 -- balayage}
   \centering
-  \only<1>{\includegraphics[width=0.6\li]{Figures/partial_hull.pdf}}%
-  \only<2>{\includegraphics[width=0.6\li]{Figures/increment_hull_1.pdf}}%
-  \only<3>{\includegraphics[width=0.6\li]{Figures/increment_hull_2.pdf}}%
-  \only<4>{\includegraphics[width=0.6\li]{Figures/increment_hull_3.pdf}}%
-  \only<5>{\includegraphics[width=0.6\li]{Figures/hull.pdf}}%
+  \only<1>{\includegraphics[width=0.6\li]{Figures/partial_hull}}%
+  \only<2>{\includegraphics[width=0.6\li]{Figures/increment_hull_1}}%
+  \only<3>{\includegraphics[width=0.6\li]{Figures/increment_hull_2}}%
+  \only<4>{\includegraphics[width=0.6\li]{Figures/increment_hull_3}}%
+  \only<5>{\includegraphics[width=0.6\li]{Figures/hull}}%
 }
 
 \frame {
   \frametitle{Robustesse : alignements de points}
   \centering
-  \includegraphics[width=0.6\li]{Figures/hull_aligned.pdf}
+  \includegraphics[width=0.6\li]{Figures/hull_aligned}
 }
 
 \frame {
@@ -80,4 +80,149 @@
   \end{ucblalgo}
 }
 
+\section{Perturbation}
+
+\frame{
+  \frametitle{Prédicat : orientation}
+  \centering
+  \includegraphics[width=0.5\li]{Figures/orient}
+
+  \vspace{8mm}
+  \begin{myblock}{0.8\li}
+    \centering
+    $
+      \det{
+        \left(
+          \begin{array}{cc}
+            x_0 & x_1 \\
+            y_0 & y_1 \\
+          \end{array}
+        \right)
+      }
+    $ : sinus de l'angle si vecteurs normalisés.
+    
+    \hl{positif, négatif ou nul}
+  \end{myblock}
+}
+
+\frame{
+  \frametitle{Utilisation pour l'enveloppe convexe}
+  \centering
+  \only<1>{\includegraphics[width=0.6\li]{Figures/sort_comparison}}%
+  \only<2>{\includegraphics[width=0.6\li]{Figures/sweep_comparison}}
+}
+
+\newcommand{\eps}{\ensuremath{\varepsilon}}
+
+\frame{
+  \frametitle{Perturbation}
+
+  \begin{align*}
+    \det{
+      \left(
+        \begin{array}{cc}
+          x_0 + \eps_{0,0} & x_1 + \eps_{1,0} \\
+          y_0 + \eps_{0,1} & y_1 + \eps_{1,1} \\
+        \end{array}
+      \right)
+    }
+    &= 
+      \det{
+        \left(
+          \begin{array}{cc}
+            x_0 & x_1 \\
+            y_0 & y_1 \\
+          \end{array}
+        \right)
+      } \\
+    &
+      + \eps_{0,0} y_1
+      + \eps_{1,1} x_0
+      - \eps_{0,1} x_1
+      - \eps_{1,0} y_0
+      \\
+    &
+      + \eps_{0,0}\eps_{1,1}
+      - \eps_{0,1}\eps_{1,0}
+  \end{align*}
+
+  \vspace{5mm}
+  \begin{myblock}{0.8\li}
+    \begin{enumerate}
+      \item systématiquement plus de zéro ?
+      \item facilité d'évaluation ?
+    \end{enumerate}
+  \end{myblock}
+}
+
+\frame{
+  \frametitle{Rester dans le pétrin}
+
+  Posons que $ \forall(i,j), \eps_{i,j} = \eps > 0$
+
+  \begin{equation*}
+        \eps y_1
+      + \eps x_0
+      - \eps x_1
+      - \eps y_0
+      + \eps\eps
+      - \eps\eps
+  \end{equation*}
+
+  \vspace{5mm}
+  Si $(x_0,y_0) = (0,1)$ et $(x_1,y_1) = (0,2)$
+
+  \begin{equation*}
+        2\eps
+      - \eps
+      + \eps\eps
+      - \eps\eps
+      = \eps
+      \quad \Rightarrow \mbox{positif !}
+  \end{equation*}
+
+  \visible<2>{
+    \vspace{5mm}
+    Si $(x_0,y_0) = (1,1)$ et $(x_1,y_1) = (2,2)$
+
+    \begin{equation*}
+          2\eps
+        + \eps
+        - 2\eps
+        - \eps
+        + \eps\eps
+        - \eps\eps
+        = 0
+        \quad \Rightarrow \mbox{arf.}
+    \end{equation*}
+  }
+}
+
+\frame{
+  \frametitle{Sortir du pétrin}
+
+  Avec la perturbation $ \begin{pmatrix}\eps^1 & \eps^2 \\ \eps^2 & \eps^2\end{pmatrix}$
+
+  \begin{align*}
+      & \eps^1 y_1
+      + \eps^2 x_0
+      - \eps^2 x_1
+      - \eps^2 y_0
+      + \eps^1\eps^2
+      - \eps^2\eps^2 \\
+      &= \eps y_1 + \eps^2(x_0 - x_1 - y_0) + \eps^3 - \eps^4
+  \end{align*}
+
+  \vspace{8mm}
+  \visible<2->{
+    \begin{myblock}{0.8\li}
+      \begin{itemize}
+        \item<1-> si $y_1$ est non nul, renvoyer son signe ;
+        \item<3-> si $x_0 - x_1 - y_0$ est non nul, renvoyer son signe ;
+        \item<4-> sinon renvoyer positif.
+      \end{itemize}
+    \end{myblock}
+  }
+}
+
 \end{document}