为什么 GetClientRect 包括窗口边框和标题栏？

Question

我正在用 C++ 编写一些代码，根据它的窗口句柄截取另一个应用程序的屏幕截图。我使用的方法是使用BitBlt。我的应用程序正在成功截取屏幕截图，并且我有一个函数可以将该图像数据保存到 bmp 文件中。

屏幕截图包含窗口的镶边。即边框和标题栏。根据我的理解，GetClientRect 应该排除窗口的边框和标题栏。我知道GetWindowRect 返回用户桌面内的坐标，GetClientRect 返回相对于应用程序本身的坐标。

我注意到在我的屏幕截图中，标题栏和左边框是可见的，但应用程序的右边框和底部被截断了。所以，我在想，如果我想排除标题和边框，那么我需要对GetWindowRect 和GetClientRect 进行某种组合，并使用有关窗口本身的信息来抵消GetClientRect 尺寸例如，无论窗口标题栏的高度是多少。

这听起来准确吗，还是我下面的代码做错了什么？

#include <Windows.h>
#include "ScreenshotManager.h"

namespace Managers {

    ScreenshotManager::ScreenshotManager(HWND gameHandle) {

        // get a device context for the window
        m_gameContext = GetWindowDC(gameHandle);

        // create a compatible device context for bitblt
        m_bitmapContext = CreateCompatibleDC(m_gameContext);

        // get window client area dimensions
        GetClientRect(gameHandle, &m_gameClientArea);

    }

    bool ScreenshotManager::TakeScreenshot() {

        // create a compatible bitmap for the game screenshots
        m_bitmap = CreateCompatibleBitmap(m_gameContext, m_gameClientArea.right, m_gameClientArea.bottom);

        // select the bitmap into the compatible device context
        SelectObject(m_bitmapContext, m_bitmap);

        // perform bit block transfer
        if (BitBlt(m_bitmapContext, 0, 0, m_gameClientArea.right, m_gameClientArea.bottom, m_gameContext, 0, 0, SRCCOPY) == false)
            return false;

        // get information about the taken screenshot
        GetObject(m_bitmap, sizeof(BITMAP), &m_bitmapInformation);

        return true;

    }

    void ScreenshotManager::SaveScreenshot(LPCWSTR outputPath) {

        BITMAPFILEHEADER   bmfHeader;    
        BITMAPINFOHEADER   bi;

        bi.biSize = sizeof(BITMAPINFOHEADER);
        bi.biWidth = m_bitmapInformation.bmWidth;
        bi.biHeight = m_bitmapInformation.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 = ((m_bitmapInformation.bmWidth * bi.biBitCount + 31) / 32) * 4 * m_bitmapInformation.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(m_gameContext, m_bitmap, 0, (UINT)m_bitmapInformation.bmHeight, lpbitmap, (BITMAPINFO *)&bi, DIB_RGB_COLORS);

        // A file is created, this is where we will save the screen capture.
        HANDLE hFile = CreateFile(outputPath, 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);

    }

}

Answer 1

GetClientRect() 不包括边框和标题栏。它所做的只是告诉您客户区的尺寸。

BitBlt() 将像素的矩形区域从一个设备上下文复制到另一个设备上下文。在本例中，源 DC 是一个窗口 DC，因此原点坐标是相对于该窗口的。

您的代码正在做的是从 window 的原点复制一个 client 大小的矩形。 （这就是缺少右边缘和下边缘的原因。）

您可能对AdjustWindowRectEx() 感兴趣，以帮助确定您要复制的区域的坐标。