如何通过HOOK GDI函数实现后台截图？

#define _WIN32_WINNT 0x0501 //仅XP或以上系统有效
#include
int main()
{
RECT rc;
HWND hwnd = FindWindow(TEXT("Notepad"), NULL); //注意窗口不能最小化
if (hwnd == NULL)
{
cout << "找不到记事本窗口" << endl;
return 0;
}
GetClientRect(hwnd, &rc);

//创建
HDC hdcScreen = GetDC(NULL);
HDC hdc = CreateCompatibleDC(hdcScreen);
HBITMAP hbmp = CreateCompatibleBitmap(hdcScreen, rc.right - rc.left, rc.bottom - rc.top);
SelectObject(hdc, hbmp);

//复制
PrintWindow(hwnd, hdc, PW_CLIENTONLY);
//PW_CLIENTONLY：Only the client area of the window is copied to hdcBlt.
//By default, the entire window is copied.
//PW_CLIENTONLY表示仅仅拷贝窗口的客户区域，而默认情况下，执行printwindow会拷贝整个窗口

//复制到粘贴板
OpenClipboard(NULL);
EmptyClipboard();
SetClipboardData(CF_BITMAP, hbmp);
CloseClipboard();

//释放
DeleteDC(hdc);
DeleteObject(hbmp);
ReleaseDC(NULL, hdcScreen);

cout << "成功把记事本窗口复制到粘贴板,请粘贴到Windows画图工具" << endl;

return 0;
}