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1.数字图片使用类似于动画的方式显示，每次显示的是数字的一部分，当动态显示的时候人眼是可以分辨出具体数字的。但是截图的话就只能截取一部分，参考：
http://cups.cs.cmu.edu/soups/2007/posters/p147_lim.pdf

2.屏蔽系统按键：Print Screen 和 Alt + Print Screen，主要原理是注册热键的方式，参考：
http://www.vckbase.com/document/viewdoc/?id=1566

代码: Handling Hotkeys
MainFrame.h

#include "FolderFrame.h"
#include "resource.h"

////////////////
// Typical MFC Main frame window, override to disable PrintScreen.
//
class CMainFrame : public CFrameWnd {
protected:
...
   afx_msg int  OnCreate(LPCREATESTRUCT lpCreateStruct);

   // disable PrintScreen
   afx_msg void OnActivate(UINT nState, CWnd* pWndOther, BOOL bMinimized);
   afx_msg LRESULT OnHotKey(WPARAM wp, LPARAM lp);
   afx_msg void OnDestroy();

   DECLARE_MESSAGE_MAP()
};

MainFrame.cpp

#include "StdAfx.h"
#include "MainFrm.h"
#include "View.h"

IMPLEMENT_DYNCREATE(CMainFrame, CFrameWnd)
BEGIN_MESSAGE_MAP(CMainFrame, CFrameWnd)
...
   // disable PrintScreen:
   ON_WM_CREATE()
   ON_WM_DESTROY()
   ON_WM_ACTIVATE()
   ON_MESSAGE(WM_HOTKEY, OnHotKey)
END_MESSAGE_MAP()
...
int CMainFrame::OnCreate(LPCREATESTRUCT lpCreateStruct)
{
...
   RegisterHotKey(m_hWnd, IDHOT_SNAPDESKTOP, 0, VK_SNAPSHOT);
   return 0;
}

void CMainFrame::OnDestroy()
{
   UnregisterHotKey(m_hWnd, IDHOT_SNAPDESKTOP);
}

//////////////////
// Handle hotkey: should be PrintScreen or Alt-PrintScreen.
// Do nothing (bypass Windows screen capture)
//
LRESULT CMainFrame::OnHotKey(WPARAM wp, LPARAM)
{
   UNREFERENCED_PARAMETER(wp);
   return 0; // ignore
}

//////////////////
// When window is activated/deactivated, disable/enable Alt-PrintScreen.
// (IDHOT_SNAPWINDOW)
//
void CMainFrame::OnActivate(UINT nState, CWnd* pWndOther,
   BOOL bMinimized)
{
   CFrameWnd::OnActivate(nState, pWndOther, bMinimized);
   if (nState)
      RegisterHotKey(m_hWnd, IDHOT_SNAPWINDOW, MOD_ALT, VK_SNAPSHOT);
   else
      UnregisterHotKey(m_hWnd, IDHOT_SNAPWINDOW);
}

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