本例效果图:

unit Unit1;interfaceusesWindows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,Dialogs;typeTForm1 = class(TForm)procedure FormPaint(Sender: TObject);procedure FormCreate(Sender: TObject);procedure FormDestroy(Sender: TObject);procedure FormMouseDown(Sender: TObject; Button: TMouseButton;Shift: TShiftState; X, Y: Integer);procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);procedure FormMouseUp(Sender: TObject; Button: TMouseButton;Shift: TShiftState; X, Y: Integer);end;varForm1: TForm1;implementation{$R *.dfm}uses GDIPOBJ, GDIPAPI;varimg: TGPImage;pts: array[0..2] of TGPPoint;flag: Integer = -1;procedure TForm1.FormCreate(Sender: TObject);
varw,h: Integer;
beginimg := TGPImage.Create('C:\temp\test.jpg');w := img.GetWidth;h := img.GetHeight;pts[0] := MakePoint((ClientWidth - w) div 2 + w, (ClientHeight - h) div 2);pts[1] := MakePoint((ClientWidth - w) div 2, (ClientHeight - h) div 2);pts[2] := MakePoint((ClientWidth - w) div 2 + w, (ClientHeight - h) div 2 + h);DoubleBuffered := True;
end;procedure TForm1.FormDestroy(Sender: TObject);
beginimg.Free;
end;procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;Shift: TShiftState; X, Y: Integer);
vari: Integer;
beginfor i := 0 to Length(pts) doif PtInRect(Bounds(pts[i].X-3, pts[i].Y-3, 6, 6), Point(X, Y)) thenbeginflag := i;Exit;end;
end;procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,Y: Integer);
beginif flag = -1 then Exit;pts[flag] := MakePoint(X, Y);Repaint;
end;procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;Shift: TShiftState; X, Y: Integer);
beginflag := -1;
end;procedure TForm1.FormPaint(Sender: TObject);
varg: TGPGraphics;p: TGPPen;i: Integer;
beging := TGPGraphics.Create(Self.Canvas.Handle);p := TGPPen.Create(aclRed);{第一点: 右上角; 第二点: 左上角; 第三点: 右下角; 第四点将根据平行四边形计算; 只能是三个点.}g.DrawImage(img, PGPPoint(@pts), Length(pts));for i := 0 to Length(pts)-1 do g.DrawRectangle(p, pts[i].X-3, pts[i].Y-3, 6, 6);p.Free;g.Free;
end;end.

窗体文件:

object Form1: TForm1Left = 0Top = 0Caption = 'Form1'ClientHeight = 206ClientWidth = 307Color = clBtnFaceFont.Charset = DEFAULT_CHARSETFont.Color = clWindowTextFont.Height = -11Font.Name = 'Tahoma'Font.Style = []OldCreateOrder = FalsePosition = poDesktopCenterOnCreate = FormCreateOnDestroy = FormDestroyOnMouseDown = FormMouseDownOnMouseMove = FormMouseMoveOnMouseUp = FormMouseUpOnPaint = FormPaintPixelsPerInch = 96TextHeight = 13
end