/*
** 画角から透視投影変換行列を求める
*/
void cameraMatrix(float fovy, float aspect,
  float zNear, float zFar,
  GLfloat* matrix)
{
  float f = 1.0f / tanf(fovy * 0.5f * 3.141593f / 180.0f);
  float dz = zFar - zNear;

  matrix[0] = f / aspect;
  matrix[5] = f;
  matrix[10] = -(zFar + zNear) / dz;
  matrix[11] = -1.0f;
  matrix[14] = -2.0f * zFar * zNear / dz;
  matrix[1] = matrix[2] = matrix[3] = matrix[4] =
    matrix[6] = matrix[7] = matrix[8] = matrix[9] =
    matrix[12] = matrix[13] = matrix[15] = 0.0f;
}