/*
** 平行投影変換行列を求める
*/
void orthogonalMatrix(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 / dx;
  matrix[ 5] =  2.0f / dy;
  matrix[10] = -2.0f / dz;
  matrix[12] = -(right + left) / dx;
  matrix[13] = -(top + bottom) / dy;
  matrix[14] = -(far + near) / dz;
  matrix[15] =  1.0f;
  matrix[ 1] = matrix[ 2] = matrix[ 3] = matrix[ 4] =
  matrix[ 6] = matrix[ 7] = matrix[ 8] = matrix[ 9] = matrix[11] = 0.0f;
}