初学directdraw，帮我看看这个问题

DDSURFACEDESC定义如下：
DDSURFACEDESC2 ddsd;
INIT1(ddsd,DDSURFACEDESC2);
ddsd.dwFlags = (DDSD_WIDTH|DDSD_HEIGHT|DDSD_CAPS|DDSD_PIXELFORMAT); 
ddsd.dwWidth=width;
ddsd.dwHeight=height;
ddsd.ddsCaps.dwCaps = (DDSCAPS_OFFSCREENPLAIN | DDSCAPS_VIDEOMEMORY);
ddsd.ddpfPixelFormat.dwSize = sizeof(DDPIXELFORMAT);
ddsd.ddpfPixelFormat.dwFlags = DDPF_RGB;
ddsd.ddpfPixelFormat.dwRGBBitCount = 24;
ddsd.ddpfPixelFormat.dwRBitMask    = 0x00FF0000;
ddsd.ddpfPixelFormat.dwGBitMask    = 0x0000FF00;
ddsd.ddpfPixelFormat.dwBBitMask    = 0x000000FF;//想显示一个m_videoheight*m_videowidth的矩形
unsigned char test = 100;
for (c=0;c<m_videoheight;c++)
{
for (d=0;d<3*m_videowidth;d+=3)
{
*((unsigned char*)SurfaceDesc.lpSurface+c*SurfaceDesc.lPitch+d) = test;   //R
*((unsigned char*)SurfaceDesc.lpSurface+c*SurfaceDesc.lPitch+d+1) = test;  //G
*((unsigned char*)SurfaceDesc.lpSurface+c*SurfaceDesc.lPitch+d+2) = test;  //B
}
}//反转图象
m_hr = IDirectDrawSurface_Unlock(m_pDirectDrawSurface7,NULL);
if (FAILED(m_hr))
 return FALSE;
m_hr = IDirectDrawSurface_Blt(m_pPrimarySurface7,&m_destrect,m_pDirectDrawSurface7,&m_srcrect,DDBLT_WAIT,NULL);
结果屏幕上什么都不能出来，高手帮我看看好吗