问一个API函数（StretchBlt）的使用

Question

为什么我用StretchBlt函数时，当图形大小改变时，它不是按比例拉伸，而是错位了；
代码如下：其中Form1为MDI中的父窗体，Form2为子窗体，并且动态生成；
unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  Menus, ExtCtrls, ComCtrls;type
  TForm1 = class(TForm)
    Image1: TImage;
    MainMenu1: TMainMenu;
    new1: TMenuItem;
    procedure new1Click(Sender: TObject);
    procedure FormCreate(Sender: TObject);
  private
    FClientInstance,FPrevClientProc:TFarProc;
    procedure ClientWndProc(Var Message:TMessage);
    { Private declarations }
  public
    { Public declarations }
  end;var
  Form1: TForm1;implementationuses Unit2;{$R *.DFM}
procedure TForm1.ClientWndProc(var Message: TMessage);
var
  MyDC:hDC;
begin
  if Message.Msg=WM_ERASEBKGND then
  begin
    MyDC:=TWMERASEBKGND(Message).DC;
    StretchBlt(MyDC,0,0,ClientWidth,ClientHeight,Image1.Picture.Bitmap.Canvas.Handle,0,0,Image1.Picture.Width,Image1.Picture.Height,SRCCOPY);
    Message.result:=1;
  end else Message.Result:=CallWindowProc(FPrevClientProc,ClientHandle,Message.Msg,Message.wParam,Message.lParam);
end;procedure TForm1.FormCreate(Sender: TObject);
begin
  FClientInstance := MakeObjectInstance(ClientWndProc);
  FPrevClientProc := Pointer(GetWindowLong(ClientHandle, GWL_WNDPROC));
  SetWindowLong(ClientHandle, GWL_WNDPROC, LongInt(FClientInstance));
end;procedure TForm1.new1Click(Sender: TObject);
begin
  form2:=Tform2.Create(self)
end;end.

Answer 1

procedure TForm1.ClientWndProc(var Message: TMessage);
var
  MyDC:hDC;
begin
  if (Message.Msg=WM_ERASEBKGND)then
  begin
    MyDC:=TWMERASEBKGND(Message).DC;
    StretchBlt(MyDC,0,0,ClientWidth,ClientHeight,Image1.Picture.Bitmap.Canvas.Handle,0,0,Image1.Picture.Width,Image1.Picture.Height,SRCCOPY);
    Message.result:=1;
  end
  else if Message.Msg = WM_SIZE then
  begin
    InvalidateRect(ClientHandle, nil, True);
    Message.Result:=CallWindowProc(FPrevClientProc,ClientHandle,Message.Msg,Message.wParam,Message.lParam);
  end
  else Message.Result:=CallWindowProc(FPrevClientProc,ClientHandle,Message.Msg,Message.wParam,Message.lParam);
end;