SRCINVERT Combines the colors of the source and destination rectangles by using the Boolean XOR operator.
BItBlt 하기 전에 destination dc 를 흰색으로 채운 다음에 해보세요.

이성제 님이 쓰신 글 :
: SRCCOPY 는 잘 되는데
: SRCINVERT 를 하면 화질? 이 꺠지는데
:
: 이유를 좀 알고 싶습니다.
:
:         BitBlt( Changed_BMP->Canvas->Handle, 0, 0,
:             Screen->Width, Screen->Height, GetDC(0), 0, 0, SRCCOPY );
:        
:         BitBlt( Changed_BMP->Canvas->Handle, 0, 0,
:             Screen->Width, Screen->Height, Before_BMP->Canvas->Handle, 0, 0, SRCINVERT );
:
:         TJPEGImage *JPGImage = new TJPEGImage;
:         JPGImage->Assign( Changed_BMP );
:
:         JPGImage->SaveToStream( Stream );
:         delete JPGImage;
:                                  
:         BitBlt( Before_BMP->Canvas->Handle, 0, 0,
:             Screen->Width, Screen->Height, GetDC(0), 0, 0, SRCCOPY );