Barry's Screen Capture

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• Download demo project - 12.5 Kb

Introduction

This article demonstrates the methods of capturing different portions of the screen. You have the facility to then save the captured image to a file.

Background

The idea for developing the article came to me when I had downloaded such a utility for capturing screen-shots for use in my articles like this one, on the Internet, but unfortunately the downloaded utility had too many nagging screens and obviously for a no-nags version I had to pay which I cannot afford. So the only way out was to develop one on my own, the MSDN sample WinCap could have served the purpose but the source was not available on the CD. So I started developing the article in MFC using the Win32 sample provided on the MSDN CD (WinCap). And also so as to be able to capture portions of the screen which I had done sometime ago in Visual Basic, since screen capture in Visual Basic is much easier and code for this is available in the Knowledge Base section of Visual Basic.

Features

This article has the following features.

• Capture the Desktop
• Capture a window
• Capture a client area
• Capture a control (e.g. Button)

In addition it has the following, interesting but not so useful (in my view) features:

• Track the X and Y coordinates of the captured image
• Trace the color code of a specific point of the captured image

Option for color and position tracking are on the MainFrame window of the system and can only be used after capturing an image.

How to use the project

Download the project source, unzip the files to some folder, build the project, in case there are no error at build-time, execute the project, a Icon of a Camera will appear in the System Tray (bar at the bottom of the screen), Right-Click on the the Icon and a menu will pop-up, select the options from the menu.

Pop-up Menu

In addition to the pop-up menus the CapIT System has it own main menu which cap be displayed when you select "Show Window" from the pop-up menu.

How it works

When capturing window or client area or control, the system calls the function SetCapture() and waits for the user to click on some window on the Desktop which is to be captured. After clicking on the area to be captured, the system framework call the member function void CCapITView::OnCaptureChanged(CWnd *pWnd) which calls WindowCapture(HWND hwnd) which in turn carries out the job of capturing the image (till the menu are disabled). After completion of capture the system calls ReleaseCapture() to release SetCapture(). But in the case of capturing the Desktop SetCapture() is not called, but directly calls WindowCapture(HWND hwnd) passing to it GetDesktopWindow()->m_hWnd and the system proceeds with capturing the Desktop image immediately, since there is only one Desktop area and its HWND is known to the System.

Note :

The following member functions in the class CCapITView are borrowed from the MSDN sample WinCap. Could I have written them on my own, No Way.

• HBITMAP CopyWindowToBitmap(CWnd* wnd , HWND hWnd, WORD fPrintArea)
• HBITMAP CopyScreenToBitmap(LPRECT lpRect)
• BOOL PaintBitmap(HDC hDC, LPRECT lpDCRect, HBITMAP hDDB, LPRECT lpDDBRect, HPALETTE hPal)
• void DoSize(CWnd* wnd , HBITMAP ghBitmap)
• void SetupScrollBars(CWnd* wnd , WORD cxBitmap, WORD cyBitmap)
• void ReallyGetClientRect(CWnd* wnd , LPRECT lpRect)
• WORD SaveDIB(HDIB hDib, LPSTR lpFileName)
• WORD PaletteSize (VOID FAR * pv)
• WORD DibNumColors (VOID FAR * pv)
• HDIB ChangeBitmapFormat(HBITMAP hBitmap, WORD wBitCount, WORD dwCompression, HPALETTE hPal, HWND hwnd)
• HANDLE AllocRoomForDIB(BITMAPINFOHEADER bi, HBITMAP hBitmap , HWND hwnd)
• HPALETTE GetSystemPalette(HWND hwnd)
• int PalEntriesOnDevice(HDC hDC)

Known problems

Color of the saved image under Windows 98 is not the same as of the captured image, but I think it is nearly the same in Windows 2000.

Points of interest

The project uses a CScrollView instead of CView because if the image being captured is bigger in size (e.g. Desktop) then in the Frame Window you may not be able to capture the entire image since a CView will capture as per the CView Client Area size.

History

• Version 1

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