   Blitting Video - Mem to Sys - Mem ( DDraw7 )
  Hey,I am experiencing a strange problem in DDraw7. Due to the very low performance reading pixel data from video memory, I want to copy some parts of the backbuffer into a offscreen surface ( system memory ), do some alpha blending code and copy it back. The code for that goes like this :m_AlphaBuffer->Blt(&rcDest, m_BackBuffer, &rcDest, DDBLT_WAIT, NULL);// alpha blending codem_BackBuffer->Blt(&rcDest, m_AlphaBuffer, &rcDest, DDBLT_WAIT, NULL);The first blt call returns DD_OK but it copies nothing but cr*p to the system memory buffer. When I define the m_AlphaBuffer in video memory, the blit works correctly. Of course, the performance is very poor.The MSDN says that Blt is able to blit display memory to system memory, but why doesn''t it work then ?I greatly apreciate help of any kind.Thank you 
  Could this be hardware-dependent or even a bug in Blt() ? 