擷取影像

您可以使用點陣圖來擷取影像，並將擷取的影像儲存在記憶體中、在應用程式視窗的不同位置顯示，或在另一個視窗中顯示。

在某些情況下，您可能會希望您的應用程式擷取映射，並暫時儲存它們。 例如，當您縮放或縮放繪圖應用程式中建立的圖片時，應用程式必須暫時儲存影像的一般檢視並顯示縮放檢視。 稍後，當使用者選取一般檢視時，應用程式必須將縮放影像取代為暫時儲存的一般檢視複本。

若要暫時儲存映射，您的應用程式必須呼叫 CreateCompatibleDC ，才能建立與目前視窗 DC 相容的 DC。 建立相容的 DC 之後，您可以藉由呼叫 CreateCompatibleBitmap 函式來建立具有適當維度的點陣圖，然後藉由呼叫 SelectObject 函式將其選取到此裝置內容中。

建立相容的裝置內容並已選取適當的點陣圖之後，您可以擷取影像。 BitBlt函式會擷取映射。 此函式會執行位區塊傳輸，也就是將資料從來源點陣圖複製到目的地點陣圖。 不過，此函式的兩個引數不是點陣圖控制碼。 相反 地，BitBlt 會收到識別兩個裝置內容的控制碼，並將點陣圖資料從選取的點陣圖複製到來源 DC，複製到選取目標 DC 的點陣圖。 在此情況下，目標 DC 是相容的 DC，因此當 BitBlt 完成傳輸時，映射已儲存在記憶體中。 若要重新顯示映射，請再次呼叫 BitBlt ，將相容的 DC 指定為來源 DC，並將視窗 (或印表機) DC 指定為目標 DC。

程式碼範例

本節包含一個程式碼範例，可擷取整個桌面的影像、將它縮小到目前的視窗大小，然後將它儲存到檔案 (，以及將其顯示在工作區) 中。

若要試用程式碼範例，請先根據 Windows 傳統型應用程式 專案範本在 Visual Studio 中建立新專案。 請務必將新專案 GDI_CapturingAnImage 命名為 ，因此下列程式代碼清單會編譯 (例如，如果您將其命名為建議) ，它 GDI_CapturingAnImage.h 就會存在於您的新專案中。

開啟新專案中的 GDI_CapturingAnImage.cpp 原始程式碼檔案，並將其內容取代為下列清單。 接著建置和執行。 每次調整視窗大小時，您會看到在工作區中顯示的擷取螢幕擷取畫面。

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

#include "framework.h"
#include "GDI_CapturingAnImage.h"

#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;                                // current instance
WCHAR szTitle[MAX_LOADSTRING];                  // The title bar text
WCHAR 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 wWinMain(_In_ HINSTANCE hInstance,
    _In_opt_ HINSTANCE hPrevInstance,
    _In_ LPWSTR    lpCmdLine,
    _In_ int       nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);

    // TODO: Place code here.

    // Initialize global strings
    LoadStringW(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadStringW(hInstance, IDC_GDICAPTURINGANIMAGE, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);

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

    HACCEL hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_GDICAPTURINGANIMAGE));

    MSG msg;

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

    return (int)msg.wParam;
}

//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEXW 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_GDICAPTURINGANIMAGE));
    wcex.hCursor = LoadCursor(nullptr, IDC_ARROW);
    wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);
    wcex.lpszMenuName = MAKEINTRESOURCEW(IDC_GDICAPTURINGANIMAGE);
    wcex.lpszClassName = szWindowClass;
    wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));

    return RegisterClassExW(&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)
{
    hInst = hInstance; // Store instance handle in our global variable

    HWND hWnd = CreateWindowW(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, nullptr, nullptr, hInstance, nullptr);

    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 function attempts to create a file called captureqwsx.bmp 
//        

int CaptureAnImage(HWND hWnd)
{
    HDC hdcScreen;
    HDC hdcWindow;
    HDC hdcMemDC = NULL;
    HBITMAP hbmScreen = NULL;
    BITMAP bmpScreen;
    DWORD dwBytesWritten = 0;
    DWORD dwSizeofDIB = 0;
    HANDLE hFile = NULL;
    char* lpbitmap = NULL;
    HANDLE hDIB = NULL;
    DWORD dwBmpSize = 0;

    // 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;

    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.
    hDIB = GlobalAlloc(GHND, dwBmpSize);
    lpbitmap = (char*)GlobalLock(hDIB);

    // Gets the "bits" from the bitmap, and copies them into a buffer 
    // that's 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.
    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.
    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.

    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)
{
    switch (message)
    {
    case WM_COMMAND:
    {
        int wmId = LOWORD(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_PAINT:
    {
        PAINTSTRUCT ps;
        HDC 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;
}