Capturing an Image

You can use a bitmap to capture an image, and you can store the captured image in memory, display it at a different location in your application's window, or display it in another window.

In some cases, you may want your application to capture images and store them only temporarily. For example, when you scale or zoom a picture created in a drawing application, the application must temporarily save the normal view of the image and display the zoomed view. Later, when the user selects the normal view, the application must replace the zoomed image with a copy of the normal view that it temporarily saved.

To store an image temporarily, your application must call CreateCompatibleDC to create a DC that is compatible with the current window DC. After you create a compatible DC, you create a bitmap with the appropriate dimensions by calling the CreateCompatibleBitmap function and then select it into this device context by calling the SelectObject function.

After the compatible device context is created and the appropriate bitmap has been selected into it, you can capture the image. The BitBlt function captures images. This function performs a bit block transfer that is, it copies data from a source bitmap into a destination bitmap. However, the two arguments to this function are not bitmap handles. Instead, BitBlt receives handles that identify two device contexts and copies the bitmap data from a bitmap selected into the source DC into a bitmap selected into the target DC. In this case, the target DC is the compatible DC, so when BitBlt completes the transfer, the image has been stored in memory. To redisplay the image, call BitBlt a second time, specifying the compatible DC as the source DC and a window (or printer) DC as the target DC.

The following example code is from an application that captures an image of the entire desktop, scales it down to the current window size and then saves it to a file.


// GDI_CapturingAnImage.cpp : Defines the entry point for the application.
//

#include "stdafx.h"
#include "GDI_CapturingAnImage.h"

#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                        // current instance
TCHAR szTitle[MAX_LOADSTRING];          // The title bar text
TCHAR szWindowClass[MAX_LOADSTRING];    // the main window class name

// Forward declarations of functions included in this code module:
ATOM                MyRegisterClass(HINSTANCE hInstance);
BOOL                InitInstance(HINSTANCE, int);
LRESULT CALLBACK    WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK    About(HWND, UINT, WPARAM, LPARAM);

int APIENTRY _tWinMain(HINSTANCE hInstance,
                     HINSTANCE hPrevInstance,
                     LPTSTR    lpCmdLine,
                     int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    MSG msg;
    HACCEL hAccelTable;

    // Initialize global strings
    LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadString(hInstance, IDC_GDI_CAPTURINGANIMAGE, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_GDI_CAPTURINGANIMAGE));

    // Main message loop:
    while (GetMessage(&msg, NULL, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }

    return (int) msg.wParam;
}



//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
//  COMMENTS:
//
//    This function and its usage are only necessary if you want this code
//    to be compatible with Win32 systems prior to the 'RegisterClassEx'
//    function that was added to Windows 95. It is important to call this function
//    so that the application will get 'well formed' small icons associated
//    with it.
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEX wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);

    wcex.style            = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra        = 0;
    wcex.cbWndExtra        = 0;
    wcex.hInstance        = hInstance;
    wcex.hIcon            = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_GDI_CAPTURINGANIMAGE));
    wcex.hCursor        = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground    = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName    = MAKEINTRESOURCE(IDC_GDI_CAPTURINGANIMAGE);
    wcex.lpszClassName    = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    return RegisterClassEx(&wcex);
}

//
//   FUNCTION: InitInstance(HINSTANCE, int)
//
//   PURPOSE: Saves instance handle and creates main window
//
//   COMMENTS:
//
//        In this function, we save the instance handle in a global variable and
//        create and display the main program window.
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
   HWND hWnd;

   hInst = hInstance; // Store instance handle in our global variable

   hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
      CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);

   if (!hWnd)
   {
      return FALSE;
   }

   ShowWindow(hWnd, nCmdShow);
   UpdateWindow(hWnd);

   return TRUE;
}

// THIS CODE AND INFORMATION IS PROVIDED "AS IS" WITHOUT WARRANTY OF
// ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING BUT NOT LIMITED TO
// THE IMPLIED WARRANTIES OF MERCHANTABILITY AND/OR FITNESS FOR A
// PARTICULAR PURPOSE.
//
// Copyright (c) Microsoft Corporation. All rights reserved

//
//   FUNCTION: CaptureAnImage(HWND hWnd)
//
//   PURPOSE: Captures a screenshot into a window and then saves it in a .bmp file.
//
//   COMMENTS: 
//
//      Note: This sample will attempt to create a file called captureqwsx.bmp 
//        



int CaptureAnImage(HWND hWnd)
{
    HDC hdcScreen;
    HDC hdcWindow;
    HDC hdcMemDC = NULL;
    HBITMAP hbmScreen = NULL;
    BITMAP bmpScreen;

    // Retrieve the handle to a display device context for the client 
    // area of the window. 
    hdcScreen = GetDC(NULL);
    hdcWindow = GetDC(hWnd);

    // Create a compatible DC which is used in a BitBlt from the window DC
    hdcMemDC = CreateCompatibleDC(hdcWindow); 

    if(!hdcMemDC)
    {
        MessageBox(hWnd, L"CreateCompatibleDC has failed",L"Failed", MB_OK);
        goto done;
    }

    // Get the client area for size calculation
    RECT rcClient;
    GetClientRect(hWnd, &rcClient);

    //This is the best stretch mode
    SetStretchBltMode(hdcWindow,HALFTONE);

    //The source DC is the entire screen and the destination DC is the current window (HWND)
    if(!StretchBlt(hdcWindow, 
               0,0, 
               rcClient.right, rcClient.bottom, 
               hdcScreen, 
               0,0,
               GetSystemMetrics (SM_CXSCREEN),
               GetSystemMetrics (SM_CYSCREEN),
               SRCCOPY))
    {
        MessageBox(hWnd, L"StretchBlt has failed",L"Failed", MB_OK);
        goto done;
    }
    
    // Create a compatible bitmap from the Window DC
    hbmScreen = CreateCompatibleBitmap(hdcWindow, rcClient.right-rcClient.left, rcClient.bottom-rcClient.top);
    
    if(!hbmScreen)
    {
        MessageBox(hWnd, L"CreateCompatibleBitmap Failed",L"Failed", MB_OK);
        goto done;
    }

    // Select the compatible bitmap into the compatible memory DC.
    SelectObject(hdcMemDC,hbmScreen);
    
    // Bit block transfer into our compatible memory DC.
    if(!BitBlt(hdcMemDC, 
               0,0, 
               rcClient.right-rcClient.left, rcClient.bottom-rcClient.top, 
               hdcWindow, 
               0,0,
               SRCCOPY))
    {
        MessageBox(hWnd, L"BitBlt has failed", L"Failed", MB_OK);
        goto done;
    }

    // Get the BITMAP from the HBITMAP
    GetObject(hbmScreen,sizeof(BITMAP),&bmpScreen);
     
    BITMAPFILEHEADER   bmfHeader;    
    BITMAPINFOHEADER   bi;
     
    bi.biSize = sizeof(BITMAPINFOHEADER);    
    bi.biWidth = bmpScreen.bmWidth;    
    bi.biHeight = bmpScreen.bmHeight;  
    bi.biPlanes = 1;    
    bi.biBitCount = 32;    
    bi.biCompression = BI_RGB;    
    bi.biSizeImage = 0;  
    bi.biXPelsPerMeter = 0;    
    bi.biYPelsPerMeter = 0;    
    bi.biClrUsed = 0;    
    bi.biClrImportant = 0;

    DWORD dwBmpSize = ((bmpScreen.bmWidth * bi.biBitCount + 31) / 32) * 4 * bmpScreen.bmHeight;

    // Starting with 32-bit Windows, GlobalAlloc and LocalAlloc are implemented as wrapper functions that 
    // call HeapAlloc using a handle to the process's default heap. Therefore, GlobalAlloc and LocalAlloc 
    // have greater overhead than HeapAlloc.
    HANDLE hDIB = GlobalAlloc(GHND,dwBmpSize); 
    char *lpbitmap = (char *)GlobalLock(hDIB);    

    // Gets the "bits" from the bitmap and copies them into a buffer 
    // which is pointed to by lpbitmap.
    GetDIBits(hdcWindow, hbmScreen, 0,
        (UINT)bmpScreen.bmHeight,
        lpbitmap,
        (BITMAPINFO *)&bi, DIB_RGB_COLORS);

    // A file is created, this is where we will save the screen capture.
    HANDLE hFile = CreateFile(L"captureqwsx.bmp",
        GENERIC_WRITE,
        0,
        NULL,
        CREATE_ALWAYS,
        FILE_ATTRIBUTE_NORMAL, NULL);   
    
    // Add the size of the headers to the size of the bitmap to get the total file size
    DWORD dwSizeofDIB = dwBmpSize + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);
 
    //Offset to where the actual bitmap bits start.
    bmfHeader.bfOffBits = (DWORD)sizeof(BITMAPFILEHEADER) + (DWORD)sizeof(BITMAPINFOHEADER); 
    
    //Size of the file
    bmfHeader.bfSize = dwSizeofDIB; 
    
    //bfType must always be BM for Bitmaps
    bmfHeader.bfType = 0x4D42; //BM   
 
    DWORD dwBytesWritten = 0;
    WriteFile(hFile, (LPSTR)&bmfHeader, sizeof(BITMAPFILEHEADER), &dwBytesWritten, NULL);
    WriteFile(hFile, (LPSTR)&bi, sizeof(BITMAPINFOHEADER), &dwBytesWritten, NULL);
    WriteFile(hFile, (LPSTR)lpbitmap, dwBmpSize, &dwBytesWritten, NULL);
    
    //Unlock and Free the DIB from the heap
    GlobalUnlock(hDIB);    
    GlobalFree(hDIB);

    //Close the handle for the file that was created
    CloseHandle(hFile);
       
    //Clean up
done:
    DeleteObject(hbmScreen);
    DeleteObject(hdcMemDC);
    ReleaseDC(NULL,hdcScreen);
    ReleaseDC(hWnd,hdcWindow);

    return 0;
}

//
//  FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  PURPOSE:  Processes messages for the main window.
//
//  WM_COMMAND    - process the application menu
//  WM_PAINT    - Paint the main window
//  WM_DESTROY    - post a quit message and return
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int wmId, wmEvent;
    PAINTSTRUCT ps;
    HDC hdc;
   
    switch (message)
    {
    case WM_CREATE:
        {
        break;
        }
    case WM_COMMAND:
        wmId    = LOWORD(wParam);
        wmEvent = HIWORD(wParam);
        // Parse the menu selections:
        switch (wmId)
        {
        case IDM_ABOUT:
            DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
            break;
        case IDM_EXIT:
            DestroyWindow(hWnd);
            break;
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
        }
        break;
        
    case WM_MOVE:
        
    case WM_PAINT:
        hdc = BeginPaint(hWnd, &ps);
        CaptureAnImage(hWnd);
        EndPaint(hWnd, &ps);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
        return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}

// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;

    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
        }
        break;
    }
    return (INT_PTR)FALSE;
}


 

 

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