Cross product for vectors of size 2. Fixes #1037

doc/QuickReference.dox

@@ -367,7 +367,8 @@ vec2 = vec1.normalized();     vec1.normalize(); // inplace \endcode
 <tr class="alt"><td>
 \link MatrixBase::cross() cross product \endlink \matrixworld</td><td>\code
 #include <Eigen/Geometry>
-vec3 = vec1.cross(vec2);\endcode</td></tr>
+v3c = v3a.cross(v3b);    // size-3 vectors
+scalar = v2a.cross(v2b); // size-2 vectors \endcode</td></tr>
 </table>
 
 <a href="#" class="top">top</a>

doc/TutorialMatrixArithmetic.dox

@@ -158,7 +158,7 @@ For dot product and cross product, you need the \link MatrixBase::dot() dot()\en
 \verbinclude tut_arithmetic_dot_cross.out
 </td></tr></table>
 
-Remember that cross product is only for vectors of size 3. Dot product is for vectors of any sizes.
+Cross product is defined in Eigen not only for vectors of size 3 but also for those of size 2, check \link MatrixBase::cross() the doc\endlink for details. Dot product is for vectors of any sizes.
 When using complex numbers, Eigen's dot product is conjugate-linear in the first variable and linear in the
 second variable.
 

doc/examples/tut_arithmetic_dot_cross.cpp

@@ -9,5 +9,10 @@ int main()
   std::cout << "Dot product: " << v.dot(w) << std::endl;
   double dp = v.adjoint()*w; // automatic conversion of the inner product to a scalar
   std::cout << "Dot product via a matrix product: " << dp << std::endl;
+
   std::cout << "Cross product:\n" << v.cross(w) << std::endl;
+  Eigen::Vector2d v2(1,2);
+  Eigen::Vector2d w2(0,1);
+  double cp = v2.cross(w2); // returning a scalar between size-2 vectors
+  std::cout << "Cross product for 2D vectors: " << cp << std::endl;
 }