/*
** 透視投影変換行列を求める
*/
void perspectiveMatrix(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 * zNear / dx;
  matrix[5] = 2.0f * zNear / dy;
  matrix[8] = (right + left) / dx;
  matrix[9] = (top + bottom) / dy;
  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[12] = matrix[13] = matrix[15] = 0.0f;
}