急!在线等~~C++怎么把图象缩小或放大一倍?

急!在线等~~C++怎么把图象缩小或放大一倍?
HDIB ChangeDIBSize(HDIB hDIB, int nWidth, int nHeight)
{ LPBITMAPINFO lpbmi = NULL;
LPBYTE lpSourceBits, lpTargetBits, lpResult;
HDC hDC = NULL, hSourceDC, hTargetDC;
HBITMAP hSourceBitmap, hTargetBitmap, hOldTargetBitmap, hOldSourceBitmap;
DWORD dwSourceBitsSize, dwTargetBitsSize, dwTargetHeaderSize;
HDIB hNewDIB;
DWORD dwSize;
WaitCursorBegin()
if (! hDIB)
{WaitCursorEnd();
return NULL;}
LPBITMAPINFO lpSrcDIB = (LPBITMAPINFO)GlobalLock(hDIB);
if (! lpSrcDIB)
{WaitCursorEnd();
return NULL;}
dwTargetHeaderSize = sizeof( BITMAPINFOHEADER ) + PaletteSize(lpSrcDIB);
lpbmi = (LPBITMAPINFO)malloc( dwTargetHeaderSize );
memcpy(lpbmi, lpSrcDIB, dwTargetHeaderSize);
lpbmi->bmiHeader.biWidth = nWidth;
lpbmi->bmiHeader.biHeight = nHeight;
hDC = GetDC( NULL );
hTargetBitmap = CreateDIBSection( hDC, lpbmi, DIB_RGB_COLORS, (VOID **)&lpTargetBits, NULL, 0 );
hSourceBitmap = CreateDIBSection( hDC, lpSrcDIB, DIB_RGB_COLORS, (VOID **)&lpSourceBits, NULL, 0 );
hSourceDC = CreateCompatibleDC( hDC );
hTargetDC = CreateCompatibleDC( hDC );
dwSourceBitsSize = lpSrcDIB->bmiHeader.biHeight * BytesPerLine((LPBYTE)&(lpSrcDIB->bmiHeader));
dwTargetBitsSize = lpbmi->bmiHeader.biHeight * BytesPerLine((LPBYTE)&(lpbmi->bmiHeader));
memcpy( lpSourceBits, FindDIBBits((LPBYTE)lpSrcDIB), dwSourceBitsSize );
lpbmi->bmiHeader.biSizeImage = dwTargetBitsSize;
hOldSourceBitmap = (HBITMAP)SelectObject( hSourceDC, hSourceBitmap );
hOldTargetBitmap = (HBITMAP)SelectObject( hTargetDC, hTargetBitmap );
SetStretchBltMode( hTargetDC, COLORONCOLOR );
StretchBlt( hTargetDC, 0, 0, lpbmi->bmiHeader.biWidth, lpbmi->bmiHeader.biHeight, hSourceDC, 0, 0, lpSrcDIB->bmiHeader.biWidth, lpSrcDIB->bmiHeader.biHeight, SRCCOPY );
SelectObject( hSourceDC, hOldSourceBitmap );
SelectObject( hTargetDC, hOldTargetBitmap );
DeleteDC( hSourceDC );
DeleteDC( hTargetDC );
ReleaseDC( NULL, hDC );
GdiFlush();
dwSize = dwTargetHeaderSize + dwTargetBitsSize;
hNewDIB = GlobalAlloc(GHND, dwSize);
lpResult = (LPBYTE)GlobalLock(hNewDIB);//malloc( dwTargetHeaderSize + dwTargetBitsSize );
memcpy( lpResult, lpbmi, dwTargetHeaderSize );
memcpy( FindDIBBits( (LPBYTE)lpResult ), lpTargetBits, dwTargetBitsSize );
DeleteObject( hTargetBitmap );
DeleteObject( hSourceBitmap );
free( lpbmi );
GlobalUnlock(hDIB);
GlobalUnlock(hNewDIB);
WaitCursorEnd();
return hNewDIB;}
以上是图象缩放函数的源代码,我现在需要直接把图象缩小到原来的一半和放大一倍.请问需要改动哪里,具体怎么改?用自己的代码也可以~谢谢啦