mesh.h

#ifndef EXAMPLE_MESH_H_
#define EXAMPLE_MESH_H_

#include <vector>
#include <algorithm>
#include <cmath>
#include <limits>

namespace example {


template<typename T>
inline void lerp(T dst[3], const T v0[3], const T v1[3], const T v2[3], float u, float v) {
  dst[0] = (static_cast<T>(1.0) - u - v) * v0[0] + u * v1[0] + v * v2[0];
  dst[1] = (static_cast<T>(1.0) - u - v) * v0[1] + u * v1[1] + v * v2[1];
  dst[2] = (static_cast<T>(1.0) - u - v) * v0[2] + u * v1[2] + v * v2[2];
}

template <typename T>
inline T vlength(const T v[3]) {
  const T d = v[0] * v[0] + v[1] * v[1] + v[2] * v[2];
  if (std::fabs(d) > std::numeric_limits<T>::epsilon()) {
    return std::sqrt(d);
  } else {
    return static_cast<T>(0.0);
  }
}

template <typename T>
inline void vnormalize(T dst[3], const T v[3]) {
  dst[0] = v[0];
  dst[1] = v[1];
  dst[2] = v[2];
  const T len = vlength(v);
  if (std::fabs(len) > std::numeric_limits<T>::epsilon()) {
    const T inv_len = static_cast<T>(1.0) / len;
    dst[0] *= inv_len;
    dst[1] *= inv_len;
    dst[2] *= inv_len;
  }
}

template <typename T>
inline void vcross(T dst[3], const T a[3], const T b[3]) {
  dst[0] = a[1] * b[2] - a[2] * b[1];
  dst[1] = a[2] * b[0] - a[0] * b[2];
  dst[2] = a[0] * b[1] - a[1] * b[0];
}

template <typename T>
inline void vsub(T dst[3], const T a[3], const T b[3]) {
  dst[0] = a[0] - b[0];
  dst[1] = a[1] - b[1];
  dst[2] = a[2] - b[2];
}

template<typename T>
inline void calculate_normal(T Nn[3], const T v0[3], const T v1[3], const T v2[3]) {
  T v10[3];
  T v20[3];

  vsub(v10, v1, v0);
  vsub(v20, v2, v0);

  T N[3];
  vcross(N, v20, v10);
  vnormalize(Nn, N);
}

template<typename T>
class Mesh {
 public:
	explicit Mesh(const size_t vertex_stride) :
		stride(vertex_stride) {
	}

  std::string name;

  std::vector<T> vertices;               /// stride * num_vertices
  std::vector<T> facevarying_normals;    /// [xyz] * 3(triangle) * num_faces
  std::vector<T> facevarying_tangents;   /// [xyz] * 3(triangle) * num_faces
  std::vector<T> facevarying_binormals;  /// [xyz] * 3(triangle) * num_faces
  std::vector<T> facevarying_uvs;        /// [xy]  * 3(triangle) * num_faces
  std::vector<T>
      facevarying_vertex_colors;           /// [xyz] * 3(triangle) * num_faces
  std::vector<unsigned int> faces;         /// triangle x num_faces
  std::vector<unsigned int> material_ids;  /// index x num_faces

  T pivot_xform[4][4];
	size_t stride;													 /// stride for vertex data.

  // --- Required methods in Scene::Traversal. ---

  const T *GetVertices() const {
    return vertices.data();
  }

  const unsigned int *GetFaces() const {
    return faces.data();
  }

  size_t GetVertexStrideBytes() const {
    return stride;
  }

  ///
  /// Get the geometric normal and the shading normal at `face_idx' th face.
  ///
  void GetNormal(T Ng[3], T Ns[3], const unsigned int face_idx, const T u, const T v) const {
    // Compute geometric normal.
    unsigned int f0, f1, f2;
    T v0[3], v1[3], v2[3];

    f0 = faces[3 * face_idx + 0];
    f1 = faces[3 * face_idx + 1];
    f2 = faces[3 * face_idx + 2];

    v0[0] = vertices[3 * f0 + 0];
    v0[1] = vertices[3 * f0 + 1];
    v0[2] = vertices[3 * f0 + 2];

    v1[0] = vertices[3 * f1 + 0];
    v1[1] = vertices[3 * f1 + 1];
    v1[2] = vertices[3 * f1 + 2];

    v2[0] = vertices[3 * f2 + 0];
    v2[1] = vertices[3 * f2 + 1];
    v2[2] = vertices[3 * f2 + 2];

    calculate_normal(Ng, v0, v1, v2);

    if (facevarying_normals.size() > 0) {

      T n0[3], n1[3], n2[3];

      n0[0] = facevarying_normals[9 * face_idx + 0];
      n0[1] = facevarying_normals[9 * face_idx + 1];
      n0[2] = facevarying_normals[9 * face_idx + 2];

      n1[0] = facevarying_normals[9 * face_idx + 3];
      n1[1] = facevarying_normals[9 * face_idx + 4];
      n1[2] = facevarying_normals[9 * face_idx + 5];

      n2[0] = facevarying_normals[9 * face_idx + 6];
      n2[1] = facevarying_normals[9 * face_idx + 7];
      n2[2] = facevarying_normals[9 * face_idx + 8];

      lerp(Ns, n0, n1, n2, u, v);

    } else {

      // Use geometric normal.
      Ns[0] = Ng[0];
      Ns[1] = Ng[1];
      Ns[2] = Ng[2];

    }

  }

  // --- end of required methods in Scene::Traversal. ---

  ///
  /// Get texture coordinate at `face_idx' th face.
  ///
  void GetTexCoord(T tcoord[3], const unsigned int face_idx, const T u, const T v) {

    if (facevarying_uvs.size() > 0) {

      T t0[3], t1[3], t2[3];

      t0[0] = facevarying_uvs[6 * face_idx + 0];
      t0[1] = facevarying_uvs[6 * face_idx + 1];
      t0[2] = static_cast<T>(0.0);

      t1[0] = facevarying_uvs[6 * face_idx + 2];
      t1[1] = facevarying_uvs[6 * face_idx + 3];
      t1[2] = static_cast<T>(0.0);

      t2[0] = facevarying_uvs[6 * face_idx + 4];
      t2[1] = facevarying_uvs[6 * face_idx + 5];
      t2[2] = static_cast<T>(0.0);

      lerp(tcoord, t0, t1, t2, u, v);

    } else {

      tcoord[0] = static_cast<T>(0.0);
      tcoord[1] = static_cast<T>(0.0);
      tcoord[2] = static_cast<T>(0.0);

    }

  }

};

}  // namespace example

#endif // EXAMPLE_MESH_H_