求多边形的面积

const MaxPointNum = 65535;
type
   PXY = ^TXY;
   TXY = record             // 点的数据结构
      x, y: single;
   end;
   XYArray = array[0..MaxPointNum] of TXY;
   PXYArray = ^XYArray;     //  线/面的数据结构

// 积分方法求多边形/面状图形的面积
// 多边形坐标在 xys 中，点数在 nn 中
function AreaOfPolygon(xys: PXYArray; nn: integer):single;
var ii:integer;
    ss: single;
begin
   ss := 0;
   for ii := 0 to nn-2 do
       ss := ss + (xys^[ii].y+xys^[ii+1].y) * (xys^[ii].x-xys^[ii+1].x) / 2;
   result := abs(ss);
end;

// 判别点(x,y)是否落在多边形内
// 多边形坐标在 xys 中，点数在 nn 中
function isPtInRegion(x, y: single; xys: PXYArray; nn: integer): boolean;
var ii, ncross : integer;
    yt, x0, y0, x1, y1, x2, y2 : single;
begin
   ncross := 0;
   x0 := x;   y0 := y;
   for ii := 0 to nn-2 do begin
      x1 := xys^[ii].X;     y1 := xys^[ii].Y;
      x2 := xys^[ii+1].X;   y2 := xys^[ii+1].Y;
      if((x0>=x1) and (x0=x2) and (x00) and ((ncross mod 2) = 1)) then Result := True
                                            else Result := FALSE;
end;