better documentation

Former-commit-id: b5b81721b13dee244fbf6e34baeb39caf77157c5

include/igl/cotmatrix.h

@@ -38,6 +38,11 @@ namespace igl
   //
   // See also: adjacency_matrix
   //
+  // Note: This Laplacian uses the convention that diagonal entries are
+  // **minus** the sum of off-diagonal entries. The diagonal entries are
+  // therefore in general negative and the matrix is **negative** semi-definite
+  // (immediately, -L is **positive** semi-definite)
+  //
   // Known bugs: off by 1e-16 on regular grid. I think its a problem of
   // arithmetic order in cotmatrix_entries.h: C(i,e) = (arithmetic)/dblA/4
   template <typename DerivedV, typename DerivedF, typename Scalar>

include/igl/min_quad_with_fixed.h

@@ -21,9 +21,10 @@ namespace igl
 {
   template <typename T>
   struct min_quad_with_fixed_data;
-  // Known Bugs: rows of Aeq **must** be linearly independent. Should be using
-  // QR decomposition otherwise:
-  //   http://www.okstate.edu/sas/v8/sashtml/ormp/chap5/sect32.htm
+  // Known Bugs: rows of Aeq **should probably** be linearly independent.
+  // During precomputation, the rows of a Aeq are checked via QR. But in case
+  // they're not then resulting probably will no longer be sparse: it will be
+  // slow.
   //
   // MIN_QUAD_WITH_FIXED Minimize quadratic energy 
   //