，小弟我的directdraw视频显示程序被别的窗口覆盖时会一片雪白，里面是代码

求助，我的directdraw视频显示程序被别的窗口覆盖时会一片雪白，里面是代码。
我的代码是照着网上的代码写的，没有干扰的时候显示没问题，一有别的窗口覆盖，甚至聊天软件弹出个窗口我的播放窗口就会一片雪白，但是程序还在跑，因为还在读硬盘里的数据。代码如下。

#define w 352
#define h 288

LPDIRECTDRAW lpDD = NULL;
LPDIRECTDRAWSURFACE lpDDSPrimary = NULL;
LPDIRECTDRAWSURFACE lpDDSOffScr = NULL;
DDSURFACEDESC ddsd;

HWND h_Wnd;
RECT oldRect;
//销毁
int ddraw_uninit()
{
if(lpDD != NULL)
{
if(lpDDSPrimary != NULL)
{
lpDDSPrimary->Release();
lpDDSPrimary = NULL;
}
if(lpDDSOffScr != NULL)
{
lpDDSOffScr->Release();
lpDDSOffScr = NULL;
}
lpDD->Release();
lpDD = NULL;
}
return 0;
}

//初始化
int ddraw_init()
{
ddraw_uninit();
  if (DirectDrawCreate(NULL, &lpDD, NULL) != DD_OK) 
return -1;

  if (lpDD->SetCooperativeLevel(h_Wnd,
DDSCL_NORMAL | DDSCL_NOWINDOWCHANGES) != DD_OK)
  return -1;

ZeroMemory(&ddsd, sizeof(ddsd));
  ddsd.dwSize = sizeof(ddsd);
  ddsd.dwFlags = DDSD_CAPS;
  ddsd.ddsCaps.dwCaps = DDSCAPS_PRIMARYSURFACE;
  if (lpDD->CreateSurface(&ddsd, &lpDDSPrimary, NULL) != DD_OK)
  return -1;

LPDIRECTDRAWCLIPPER pcClipper; 
if( lpDD->CreateClipper( 0, &pcClipper, NULL ) != DD_OK )
return FALSE;
if( pcClipper->SetHWnd( 0, h_Wnd ) != DD_OK )
{
pcClipper->Release();
return FALSE;
}
if( lpDDSPrimary->SetClipper( pcClipper ) != DD_OK )
{
pcClipper->Release();
return FALSE;
}
pcClipper->Release();



ZeroMemory(&ddsd, sizeof(ddsd));
ddsd.dwSize = sizeof(ddsd);
ddsd.ddsCaps.dwCaps = DDSCAPS_OFFSCREENPLAIN | DDSCAPS_VIDEOMEMORY; //DDSCAPS_OVERLAY DDSCAPS_OFFSCREENPLAIN;
ddsd.dwFlags = DDSD_CAPS | DDSD_HEIGHT | DDSD_WIDTH | DDSD_PIXELFORMAT;
ddsd.dwWidth = w;
ddsd.dwHeight = h;
ddsd.ddpfPixelFormat.dwSize = sizeof(DDPIXELFORMAT);
ddsd.ddpfPixelFormat.dwFlags = DDPF_FOURCC | DDPF_YUV ;
ddsd.ddpfPixelFormat.dwFourCC = MAKEFOURCC('Y','V','1','2');
ddsd.ddpfPixelFormat.dwYUVBitCount = 8;
if (lpDD->CreateSurface(&ddsd, &lpDDSOffScr, NULL) != DD_OK)
{
AfxMessageBox("CreateSurface错误！");
return -1;
}
return 0;
}

//播放
int ddraw_play(char *buffer)
{
RECT rctDest;
RECT rctSour;
HRESULT ddRval;

do {
ddRval = lpDDSOffScr->Lock(NULL, &ddsd, DDLOCK_WAIT|DDLOCK_WRITEONLY, NULL);
} while(ddRval == DDERR_WASSTILLDRAWING);
if(ddRval != DD_OK)
{
AfxMessageBox("Lock错误！");
return -1;
}
LPBYTE lpSurf = (LPBYTE)ddsd.lpSurface;
LPBYTE lpY = (LPBYTE)buffer;
LPBYTE lpU = (LPBYTE)buffer + w * h;
LPBYTE lpV = (LPBYTE)buffer + w * h * 5 / 4;
if(lpSurf)
{
int i = 0;
for(i=0; i<ddsd.dwHeight; i++)
{
memcpy(lpSurf, lpY, ddsd.dwWidth);
lpY += w;
lpSurf += ddsd.lPitch;
}
for(i=0; i<ddsd.dwHeight/2; i++)
{
memcpy(lpSurf, lpV, ddsd.dwWidth / 2);
lpV += w / 2;
lpSurf += ddsd.lPitch / 2;
}
for(i=0; i<ddsd.dwHeight/2; i++)
{
memcpy(lpSurf, lpU, ddsd.dwWidth / 2);
lpU += w / 2;
lpSurf += ddsd.lPitch / 2;
}
}
lpDDSOffScr->Unlock(NULL);
/*
rctSour.left = 0;
rctSour.top = 0;
rctSour.right = ddsd.dwWidth;
rctSour.bottom = ddsd.dwHeight;
GetClientRect(h_Wnd,&rctDest);
ClientToScreen(h_Wnd, (LPPOINT)&rctDest.left);
ClientToScreen(h_Wnd, (LPPOINT)&rctDest.right);
*/
do 
{
// ddRval = lpDDSPrimary->Blt(&rctDest, lpDDSOffScr, &rctSour, DDBLT_WAIT, NULL);