获取多边形面积的函数 - 回复 "dacsd" 的问题

问题来源: http://www.cnblogs.com/del/archive/2008/07/09/1237697.html#1250073

我曾经傻呵呵地这样做过: http://www.cnblogs.com/del/archive/2008/07/08/1238238.html#1249117

---

代码文件:

---

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Button2: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
  end;

var
  Form1: TForm1;

implementation

{$R *.dfm}

{获取多边形面积的函数}
function GetPolygonArea(const pt: PPoint; const Count: Integer): Double;
  function LineSqr(const pt1,pt2: TPoint): Double;
  begin
    Result := (pt2.X - pt1.X) * (pt1.Y + pt2.Y) / 2;
  end;
type
  ArrPt = array of TPoint;
var
  i: Integer;
begin
  Result := 0;
  for i := 0 to Count - 2 do
    Result := Result + LineSqr(ArrPt(pt)[i], ArrPt(pt)[i+1]);
  Result := Result + LineSqr(ArrPt(pt)[Count-1], ArrPt(pt)[0]);
end;

{测试1: 把窗体矩形当个多边形测试一下, 使用静态数组}
procedure TForm1.Button1Click(Sender: TObject);
var
  pts: array[0..3]of TPoint;
  Area1: Double;
  Area2: Integer;
begin
  pts[0] := ClientRect.TopLeft;
  pts[1] := Point(0, ClientHeight);
  pts[2] := ClientRect.BottomRight;
  pts[3] := Point(ClientWidth, 0);

  {用上面的函数获取面积}
  Area1 := GetPolygonArea(@pts, Length(pts));
  {用 宽*高 获取面积}
  Area2 := ClientWidth * ClientHeight;
  {对比显示两个结果}
  Text := Format('%f, %d', [Area1, Area2]); {没有问题}
end;

{测试2: 把窗体用对角线分隔的三角形当个多边形测试一下, 使用动态数组}
procedure TForm1.Button2Click(Sender: TObject);
var
  pts: array of TPoint;
  Area1: Double;
  Area2: Integer;
begin
  SetLength(pts, 3);
  pts[0] := ClientRect.TopLeft;
  pts[1] := Point(0, ClientHeight);
  pts[2] := ClientRect.BottomRight;

  {用上面的函数获取面积}
  Area1 := GetPolygonArea(@pts[0], Length(pts));
  {用 宽*高/2 获取面积}
  Area2 := ClientWidth * ClientHeight div 2;
  {对比显示两个结果}
  Text := Format('%f, %d', [Area1, Area2]); {没有问题}
end;

end.

窗体文件:

---

object Form1: TForm1
  Left = 0
  Top = 0
  Caption = 'Form1'
  ClientHeight = 206
  ClientWidth = 339
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -11
  Font.Name = 'Tahoma'
  Font.Style = []
  OldCreateOrder = False
  PixelsPerInch = 96
  TextHeight = 13
  object Button1: TButton
    Left = 136
    Top = 72
    Width = 75
    Height = 25
    Caption = 'Button1'
    TabOrder = 0
    OnClick = Button1Click
  end
  object Button2: TButton
    Left = 136
    Top = 112
    Width = 75
    Height = 25
    Caption = 'Button2'
    TabOrder = 1
    OnClick = Button2Click
  end
end