Three Dimensional Line Algorithm

Prints (to debug console) a list of all the points/pixels a line passes through from (x0,y0,z0) to (x1,y1,z1), i.e. Bresenham's in 3D.

Code:

program

uses pure2d, keyset

procedure Swap(byref a :integer, byref b :integer)
var
   c :integer
begin
   c := a
   a := b
   b := c
end

var
   x, x0, x1, delta_x, step_x :integer
   y, y0, y1, delta_y, step_y :integer
   z, z0, z1, delta_z, step_z :integer
   swap_xy, swap_xz :boolean
   drift_xy, drift_xz :integer
   cx, cy, cz :integer

begin

   //start and end points (change these values)
   x0 := 0 x1 := -2
   y0 := 0 y1 := 5
   z0 := 0 z1 := -10

//'steep' xy Line, make longest delta x plane
swap_xy := Abs(y1 - y0) > Abs(x1 - x0)
if swap_xy
Swap(x0, y0)
Swap(x1, y1)
endif

//do same for xz
swap_xz := Abs(z1 - z0) > Abs(x1 - x0)
if swap_xz
Swap(x0, z0)
Swap(x1, z1)
endif

   //delta is Length in each plane
   delta_x := Abs(x1 - x0)
   delta_y := Abs(y1 - y0)
   delta_z := Abs(z1 - z0)

   //drift controls when to step in 'shallow' planes
   //starting value keeps Line centred
   drift_xy := (delta_x / 2)
   drift_xz := (delta_x / 2)

   //direction of line
   step_x := 1; if (x0 > x1) then step_x := -1
   step_y := 1; if (y0 > y1) then step_y := -1
   step_z := 1; if (z0 > z1) then step_z := -1

   //starting point
   y := y0
   z := z0

   //step through longest delta (which we have swapped to x)
   for x = x0 to x1 step step_x

   //copy position
   cx := x; cy := y; cz := z

   //unswap (in reverse)
   if swap_xz then Swap(cx, cz)
   if swap_xy then Swap(cx, cy)

   //passes through this point
   debugmsg(":" + cx + ", " + cy + ", " + cz)

   //update progress in other planes
   drift_xy = drift_xy - delta_y
   drift_xz = drift_xz - delta_z

   //step in y plane
   if drift_xy < 0
   y = y + step_y
   drift_xy = drift_xy + delta_x
   endif

   //same in z
   if drift_xz < 0
   z = z + step_z
   drift_xz = drift_xz + delta_x
   endif
   next

end