opencv - using cvRetrieveFrame get strange image -

i reading avi file, , background subtrcation work. wierd thing when use cvretrieveframe, got strange image, below:

origin:

cvretrieveframe returns:

i don't know what's problem. here code snippet.

cvcapture* readeravi = cvcapturefromavi( filename.c_str() ); if(readeravi == null) {     std::cerr << "could not open avi file." << std::endl; return 0; }  // retrieve information avi file cvqueryframe(readeravi); //....get information, width, height, ....  // grad next frame input video stream if(!cvgrabframe(readeravi)) {              std::cerr << "could not grab avi frame." << std::endl;     return 0; }        frame_data = cvretrieveframe(readeravi);