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rem Mouse position to 3D position demo
rem Lines go wierd when they are positioned behind camera

rem Data for the grid lines
data 200.0,-200.0,-200.0,-200.0,200.0,-160.0,-200.0,-160.0,200.0,-120.0,-200.0,-120.0,200.0,-80.0,-200.0,-80.0,200.0,-40.0,-200.0,-40.0,200.0,0.0,-200.0,0.0
data 200.0,200.0,-200.0,200.0,200.0,160.0,-200.0,160.0,200.0,120.0,-200.0,120.0,200.0,80.0,-200.0,80.0,200.0,40.0,-200.0,40.0
data 200.0,-200.0,-200.0,-200.0,200.0,-160.0,-200.0,-160.0,200.0,-120.0,-200.0,-120.0,200.0,-80.0,-200.0,-80.0,200.0,-40.0,-200.0,-40.0,200.0,0.0,-200.0,0.0
data 200.0,200.0,-200.0,200.0,200.0,160.0,-200.0,160.0,200.0,120.0,-200.0,120.0,200.0,80.0,-200.0,80.0,200.0,40.0,-200.0,40.0

sync on

dim lines#(2,11,2,2)
dim eye#(1)
dim gridz#(1)
dim gridx#(1)

rem Set camera view width
eye#(1)=201

rem Original grid coordinates
gridz#(1)=150.01
gridx#(1)=0.01

rem Read data for grid lines into arrays
for x=1 to 11
   read lines#(1,x,1,1)
   read lines#(1,x,1,2)
   read lines#(1,x,2,1)
   read lines#(1,x,2,2)
next x
for x=1 to 11
   read lines#(2,x,1,2)
   read lines#(2,x,1,1)
   read lines#(2,x,2,2)
   read lines#(2,x,2,1)
next x

rem Main loop
do
   rem Messages
   ink rgb(200,200,255),0
   text 10,20,"Arrow keys = Move Grid"
   text 10,40,"a = Increase camera lens angle"
   text 10,60,"z = Decrease camera lens angle"

   rem Change grid position using arrow keys
   if upkey()=1 then gridz#(1)=gridz#(1)+3
   if downkey()=1 then gridz#(1)=gridz#(1)-3
   if leftkey()=1 then gridx#(1)=gridx#(1)-3
   if rightkey()=1 then gridx#(1)=gridx#(1)+3
   rem Change lens width using a and z
   if inkey$()="z" then eye#(1)=eye#(1)+3
   if inkey$()="a" then eye#(1)=eye#(1)-3

   rem Draw grid
   ink rgb(255,255,255),0
   for x=1 to 2
      for y=1 to 11
         line p_x(lines#(x,y,1,1),lines#(x,y,1,2)),p_y(-100.0,lines#(x,y,1,2)),p_x(lines#(x,y,2,1),lines#(x,y,2,2)),p_y(-100.0,lines#(x,y,2,2))
      next y
   next x

   rem Store adjusted mouse positions to make stuff draw in centre of screen
      `0.01 is to stop "can't devide by zero" thing
   mx#=mousex()-320
   my#=240-mousey()+0.01

   rem Work out the 3D coordinates of lines using -100 as the y coordinate
   tx#=mx#*(eye#(1)+(((-100/my#)*eye#(1))-eye#(1)))/eye#(1)-gridx#(1)
   tz#=(-100/my#)*eye#(1)-eye#(1)-gridz#(1)

   rem Draw position lines
   ink rgb(255,255,0),0
   line p_x(tx#,-200.0),p_y(-100.0,-200.0),p_x(tx#,200.0),p_y(-100.0,200.0)
   line p_x(-200.0,tz#),p_y(-100.0,tz#),p_x(200.0,tz#),p_y(-100.0,tz#)

   sync
   cls
loop

rem Find 2D screen x position of 3D coordinated
function p_x(x#,z#)
   value=320+(x#+gridx#(1))/((eye#(1)+z#+gridz#(1))/eye#(1))
endfunction value

rem Find 2D screen y position of 3D coordinated
function p_y(y#,z#)
   value=240-y#/((eye#(1)+z#+gridz#(1))/eye#(1))
endfunction value