(SOLVED) Problem with perspective

[SimonH]

Hi Folks,

Can anyone help me with a maths problem I've come up against?
I'm drawing a perspective view of an image so I'm converting screen coordinates to a perspective point on a plane, like so;

tx=screen_x-screen_width/2
ty=screen_y-screen_height/2
tz=(ty+250)/zScale
plane_x=tx*(d/tz)
plane_y=ty*(d/tz)

Sort of ray-casting. This works fine.
My problem arises when I want to reverse the process; to find the screen coordinates for any given coordinates on the plane.
I've covered sheets of paper with scribblings but I can't seem to get it right!
Can anyone point me in the right direction?

[ra4king]

So to go from plane_x,plane_y to screen_x,screen_y?

Considering 'd' and 'zscale' are also knowns, then:

screen_y = (d*zscale*(screen_height/2) - plane_y * (screen_height/2) + plane_y * 250) / (d*zscale - plane_y)
screen_x = (plane_x * (screen_y - (screen_height/2) + 250))/(zscale * d) + screen_width/2

EDIT: And the work painstakingly "drawn" in Paint.NET:

[SimonH]

I haven't checked this through thoroughly yet but it looks pretty good to me!
Ra4King you are a star! Many thanks!

Edit: Works a treat - hopefully you'll get to see it in action shortly!