perspective projection

I always see these complicated and poorly explained formulas for
 perspective projection, and I'm not sure if I'm doing it right.

Let's say I have the 3d point a.

Now let's say the new 2d screen coordinate is a2d.

Is this code correct for converting the 3d point to 2d?


	Vertex2d a2d;

	a2d.x = a.x / a.z * SCREENWIDTH;
	a2d.y =  -a.y / a.z * SCREENHEIGHT;

	a2d.x = a2d.x/2.0f + SCREENWIDTH/2.0;
	a2d.y = a2d.y/2.0f + SCREENHEIGHT/2.0;



If not, what's wrong with it?

Thank you.