C / C++ / MFC

#include "stdafx.h"
#include "win_adapter.h" // This file contains all you need


struct DATA { int foo; double bar; };


class Test
{
public:
    // Callback for EnumWindows using a user defined type for the UserData parameter
    // You can pass there anything you want
    virtual BOOL WindowLister( HWND hWnd, const DATA& data )
    {
        printf("  Window: %p\n",  hWnd );
        return true;
    }

    // Some function that needs to enumerate windows
    void DoStuff()
    {
        // Calling EnumWindows with a member function as callback
        DATA someData;
        win::EnumWindows( this, &Test::WindowLister, someData );
     }
};

• Use a borderless popup window displaying the bitmap, move it - should be accelerated on any not-completely-worthless OS+video card. Or,
• Save an image of the portions you are covering to memory, and then use a separate in-memory bitmap to draw part of it covered by your slider each timer interval; then draw the complete image to the screen in one go.