How can I display a live screenshot of a piece of another application?

Raymond Chen


Today’s Little Program takes a rectangular portion of another application and continuously replicates it in its own client area. You might want to do this if you want to monitor a portion of an application like a custom progress bar, and the application doesn’t use the Windows 7 taskbar progress indicator feature. (Maybe it’s an old application.)

Take our scratch program and make the following changes:

#define STRICT
#include <windows.h>
#include <windowsx.h>
#include <ole2.h>
#include <commctrl.h>
#include <shlwapi.h>
#include <stdio.h>
#include <dwmapi.h>

HINSTANCE g_hinst; /* This application’s HINSTANCE */ HWND g_hwndChild; /* Optional child window */ HTHUMBNAIL g_hthumb;

BOOL OnCreate(HWND hwnd, LPCREATESTRUCT lpcs) { DWM_THUMBNAIL_PROPERTIES props = {}; HWND hwndTarget; if (sscanf(reinterpret_cast<PCSTR>(lpcs->lpCreateParams), “%p %ld %ld %ld %ld”, &hwndTarget, &props.rcSource.left, &, &props.rcSource.right, &props.rcSource.bottom) == 5) { DwmRegisterThumbnail(hwnd, hwndTarget, &g_hthumb); props.dwFlags = DWM_TNP_VISIBLE | DWM_TNP_RECTSOURCE | DWM_TNP_RECTDESTINATION; props.rcDestination = props.rcSource; OffsetRect(&props.rcSource, -props.rcSource.left,; props.fVisible = TRUE; DwmUpdateThumbnailProperties(g_hthumb, &props); } return TRUE; }

void OnDestroy(HWND hwnd) { if (g_hthumb) DwmUnregisterThumbnail(g_hthumb); PostQuitMessage(0); }

int WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev, LPSTR lpCmdLine, int nShowCmd) { …

hwnd = CreateWindow( “Scratch”, /* Class Name */ “Scratch”, /* Title */ WS_OVERLAPPEDWINDOW, /* Style */ CW_USEDEFAULT, CW_USEDEFAULT, /* Position */ CW_USEDEFAULT, CW_USEDEFAULT, /* Size */ NULL, /* Parent */ NULL, /* No menu */ hinst, /* Instance */ lpCmdLine); … }

Our Little Program passes its command line through to the WM_CREATE message, which parses it as a pointer (for Visual C++, a hex value with no 0x prefix) and four integers representing the left, top, right, and bottom coordinates a rectangle within that window. (For example, to get the upper left 100 pixels of the window, pass 0 0 100 100.) It creates a thumbnail from that window and positions it inside the scratch window.

Use Spy or whatever program to get a window handle and run the progarm with the window handle and four integers (described above). A live slice of the window will appear in the scratch program.

Making it easier to select the target window and a rectangle from it is left as an exercise. This is just a Little Program.


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