/*
** 平行投影変換行列を求める
*/
void orthogonalMatrix(float left, float right,
  float bottom, float top,
  float zNear, float zFar,
  GLfloat* matrix)
{
  float dx = right - left;
  float dy = top - bottom;
  float dz = zFar - zNear;

  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] = -(zFar + zNear) / 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;
}