/*
** 透視投影変換行列を求める
*/
void perspectiveMatrix(float left, float right,
                       float bottom, float top,
                       float near, float far,
                       GLfloat *matrix)
{
  float dx = right - left;
  float dy = top - bottom;
  float dz = far - near;
  
  /*
  ** dx, dy, dz は 0 になる可能性があるけど,
  ** ここでエラー処理をするかどうかはご自由に.
  */

  matrix[ 0] =  2.0f * near / dx;
  matrix[ 5] =  2.0f * near / dy;
  matrix[ 8] =  (right + left) / dx;
  matrix[ 9] =  (top + bottom) / dy;
  matrix[10] = -(far + near) / dz;
  matrix[11] = -1.0f;
  matrix[14] = -2.0f * far * near / dz;
  matrix[ 1] = matrix[ 2] = matrix[ 3] = matrix[ 4] =
  matrix[ 6] = matrix[ 7] = matrix[12] = matrix[13] = matrix[15] = 0.0f;
}