#ifndef IGL_GRAD_H
#define IGL_GRAD_H
#include "igl_inline.h"

#include <Eigen/Core>

namespace igl {
  // GRAD
  // G = grad(V,F,X)
  //
  // Compute the numerical gradient at every face of a triangle mesh.
  //
  // Inputs:
  //   V  #vertices by 3 list of mesh vertex positions
  //   F  #faces by 3 list of mesh face indices
  //   X  # vertices list of scalar function values
  // Outputs:
  //   G  #faces by 3 list of gradient values
  //
  
  // Gradient of a scalar function defined on piecewise linear elements (mesh)
  // is constant on each triangle i,j,k: grad(Xijk) = (Xj-Xi) * (Vi - Vk)^R90 /
  // 2A + (Xk-Xi) * (Vj - Vi)^R90 / 2A where Xi is the scalar value at vertex
  // i, Vi is the 3D position of vertex i, and A is the area of triangle
  // (i,j,k). ^R90 represent a rotation of 90 degrees
  //
  template <typename T, typename S>
  IGL_INLINE void grad(
    const Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic> &V,
    const Eigen::Matrix<S, Eigen::Dynamic, Eigen::Dynamic> &F,
    const Eigen::Matrix<T, Eigen::Dynamic, 1>&X,
    Eigen::Matrix<T, Eigen::Dynamic, Eigen::Dynamic> &G );
}

#ifdef IGL_HEADER_ONLY
#  include "grad.cpp"
#endif

#endif