Проверить, находится ли курсор на линии

{ 
  Check if a Point(X,Y) (e.g a Cursor) is on a Linie (x1,y1) ; (x2,y2) 
  d = line width (min. 1) 
 
}
 
 function CursorOnLinie(X, Y, x1, y1, x2, y2, d: Integer): Boolean;
 var
   sine, cosinus: Double;
   dx, dy, len: Integer;
 begin
   if d = 0 then d := 1;
   asm
     fild(y2)
     fisub(y1) // Y-Difference 
    fild(x2)
     fisub(x1) // X-Difference 
    fpatan    // Angle of the line in st(0) 
    fsincos   // Cosinus in st(0), Sinus in st(1) 
    fstp cosinus
     fstp sine
   end;
   dx  := Round(cosinus * (x - x1) + sine * (y - y1));
   dy  := Round(cosinus * (y - y1) - sine * (x - x1));
   len := Round(cosinus * (x2 - x1) + sine * (y2 - y1)); // length of line 
  if (dy > -d) and (dy < d) and (dx > -d) and (dx < len + d) then Result := True
   else
      Result := False;
 end;
 
 procedure TForm1.FormPaint(Sender: TObject);
 begin
   Canvas.Pen.Width := 1;
   Canvas.MoveTo(0, 0);
   Canvas.LineTo(Width, Weight);
 end;
 
 procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
   Y: Integer);
 var
   p: TPoint;
 begin
   GetCursorPos(p);
   p := ScreenToClient(p);
   if CursorOnLinie(p.x, p.y, 0, 0, Width, Height, 1) then
     Caption := 'Mouse on line.'
   else
     Caption := 'Mouse not on line.'
 end;

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