求助!关于IDirect3dSurface9和IDirect3DDevice9::StretchRect()

rUIing

用 StretchRect() 时出现错误,不太明白,请高人相助!
                LPDIRECT3DSURFACE9 lpTexSur,lpBackSur;
        LPDIRECT3DTEXTURE9 lpTex;
        RECT rect1={0,0,256,256};
        RECT rect2={100,100,356,356};
        if(FAILED(D3DXCreateTextureFromFile(g_lpD3DDev,"res\\x.bmp",&lpTex)))
                MessageBox(hWnd,"no x.bmp!",NULL,NULL);
               if(FAILED(g_lpD3DDev->GetBackBuffe(      0,0,D3DBACKBUFFER_TYPE_MONO,&lpBackSur)))
                MessageBox(hWnd,"Get Back Buffer Fail!",NULL,NULL);
        if(FAILED(lpTex->GetSurfaceLevel(0,&lpTexSur)))
                MessageBox(hWnd,"Tex Get Surface Level Fail!",NULL,NULL);
        if(FAILED(g_lpD3DDev->StretchRect( lpTexSur,&rect1,lpBackSur,&rect2,D3DTEXF_NONE )))
                MessageBox(hWnd,"Stretch Rect Fail!!!",NULL,NULL);
        else
                MessageBox(hWnd,"SUCCEEDED!!!",NULL,NULL);

运行结果是StretchRect()时失败.看了SDK,只能看懂个大概,是因为TEXSURFACE不能向BACKSURFACE复制?不太清楚.请大大帮忙啊,THANK YOU!