/*
** 画角から透視投影変換行列を求める
*/
void cameraMatrix(float fovy, float aspect, float near, float far,
                  GLfloat *matrix)
{
  float f = 1.0f / tanf(fovy * 0.5f * 3.141593f / 180.0f);
  float dz = far - near;
  
  matrix[ 0] = f / aspect;
  matrix[ 5] = f;
  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[ 8] = matrix[ 9] =
  matrix[12] = matrix[13] = matrix[15] = 0.0f;
}