Delphi - drawing/calculating lines without canvas

Sometimes a line has to be drawn onto a control structure 
which has no canvas or is a copy of a canvas. It assumes 
a field where the pixels are :

 MyField=array[0..width-1,0..height-1]of byte, integer;

and a procedure that sets the pixel or throws the pixels 
onto a stack or .. :

 putpixelproc=procedure(x,y:integer);

Note that the starting point (x1,y1) is not set. The 
linewidth is 1 (one)

procedure drawline(x1,y1,x2,y2:integer;p:putpixelproc);
var x,y,dx,dy:integer;
 slope:single;
 first:boolean;

begin
 first:=true;        // the first point, p1 is not set
 dx:=(x2-x1);
 dy:=(y2-y1);
 if (dy=0) then begin      // horizontal line
   x:=x1;
   while (x < > x2) do begin
     if not first then p(x,y1);
     if (x < x2) then inc(x) else dec(x);
     first:=false;
    end;
  end
 else begin
   if dx=0 then begin       // vertical line
     y:=y1;
     while (y < > y2) do begin
       if not first then p(x1,y);
       if (y < y2) then inc(y) else dec(y);
       first:=false;
      end;
    end
   else begin
     slope:=dy/dx;
     x:=x1;y:=y1;
     if (abs(slope)<1) then begin    //step x
       while (x < > x2) do begin
         if not first then p(x,y);
         if x < x2 then inc(x) else dec(x);
         y:=round(y1+(x-x1)*slope);
         first:=false;
        end;
      end
     else begin                 //step y
       while (y < > y2) do begin
         if not first then p(x,y);
         if y < y2 then inc(y) else dec(y);
         x:=round(x1+(y-y1)/slope);
         first:=false;
        end;
      end;
    end;
  end;
end;