求多边形面积

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;