Bitmap/Bresenham's line algorithm: Difference between revisions

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@@ Line 1,640: / Line 1,640: @@
 	send(Win,open).
-draw_recursive_line(_Pict,X,X,_DX,_DY,_Y,_D).%Don't iterate if X and X2 are the same number
+draw_recursive_line(_Pict,X,X,_DX,_DY,Y,Y,_D,_Sx,_Sy).%Don't iterate if X and X2 are the same number
-draw_recursive_line(Pict,X,X2,DX,DY,Y,C):-
+draw_recursive_line(Pict,X,X2,DX,DY,Y,Y2,C,Sx,Sy):-
 	(   C>0->%If the difference is greater than one, add Y one to Y.
-	Y1 is Y+1,
+	Y1 is Y+Sy,
 	    send(Pict,pixel(X,Y1,colour(black))),
 	    C2 is C+(2*DY-2*DX);
@@ Line 1,649: / Line 1,649: @@
 	send(Pict,pixel(X,Y,colour(black))),
 	    C2 is C+(2*DY)),
-	X0 is X+1,%The next iteration
+	X0 is X+Sx,%The next iteration
-	draw_recursive_line(Pict,X0,X2,DX,DY,Y1,C2).
+	draw_recursive_line(Pict,X0,X2,DX,DY,Y1,Y2,C2,Sx,Sy).
+isneg(X,O):-
+	(   X<0->
+	O is -1;
+	(   X\==0->
+	O is 1;
+	O is 0)).
 draw_line(Pict,X1,Y1,X2,Y2):-
-	DY is Y2-Y1,
+	DY is abs(Y2-Y1),
-	DX is X2-X1,
+	DX is abs(X2-X1),
+	isneg(DX,Sx),
+	isneg(DY,Sy),
 	D = 2*DY-DX,%The slope of the line
-	draw_recursive_line(Pict,X1,X2,DX,DY,Y1,D).
+	draw_recursive_line(Pict,X1,X2,DX,DY,Y1,Y2,D,Sx,Sy).
 </lang>