- so that is what 3d is eh?

- we got our coordinates x and y increasing towards the right on the screen and bottom respectively

- but we imagine 3d items as having a 0,0,0 at the center of the screen where x increases towards right, y increases towards top and z increases if you smashed your head inside the screen

- z is 0 at your eyes? and negative behind your head?

- then you apply a transform for this stated effect

- and you wanna calculate the projection of this fictional 3d object on 2d in your screen?