Raster Algorithms - Computer Graphics and Geometric Modeling - page 33

while x £ XMAX do
if wasIn
then
begin
if Inside (x,y)
then Set (x,y) { was inside and still inside }
else

begin
{ was inside but not anymore, i.e., we
just passed the right edge of a span }
Stack (dadLx,dadRx,lx,x-1,y,dir);
wasIn := false;
end

end

else

begin
if x > rx then Goto 2;
if Inside (x,y) then
begin
{ we weren't inside but are now, i.e.,
we just found the left edge of a new span }
Set (x,y);
wasIn := true;
lx := x;

end
x := x + 1;
end;

2 : if wasIn then

{ we just hit the edge of the viewport while in a span }
Stack (dadLx,dadRx,lx,x-1,y,dir);

end

end;

boolean function StackNotEmpty ()
{ Returns true if shadowStack is empty and false otherwise }

procedure Push ( integer myl, myr, dadl, dadr, y; direction dir)
{ Pushes record onto shadowStack }

procedure Pop ()
{ Pops top of shadowStack into local variables lx, rx, dadLx, dadRx, y, dir }

procedure Stack ( integer dadLx, dadRx, lx, rx, y; direction dir)
{ Pushes an extra shadow onto shadowStack, given a newly discovered span
and its parent. This is where the three types of shadows are differentiated. }
begin

Algorithm 2.4.3. Continued