c - hwnd 转 ppm 问题

Question

我有一个将 hwnd 保存到 ppm 文件中的函数。这个函数的灵感来自于msdn的一个例子。msdn 示例和我的函数都可以工作，但是...我有一个问题...

但首先，这是函数。

int CaptureAnImage(HWND hWnd)
{
    HDC hdcWindow;
    HDC hdcMemDC = NULL;
    HBITMAP hbmScreen = NULL;
    RECT rc;
    BITMAPINFOHEADER   bi;

    DWORD dwBmpSize;
    HANDLE hDIB;
    char *lpbitmap;
    int w, h;
    FILE *f;

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

    // Create a compatible DC which is used in a BitBlt from the window DC
    hdcMemDC = CreateCompatibleDC(hdcWindow); 
    if(!hdcMemDC) {
        MessageBox(hWnd, "CreateCompatibleDC has failed","Failed", MB_OK);
        goto done;
    }

    // Get the client area for size calculation
    GetClientRect(hWnd, &rc);
    w = rc.right - rc.left;
    h=rc.bottom-rc.top;

    // Create a compatible bitmap from the Window DC
    hbmScreen = CreateCompatibleBitmap(hdcWindow, w, h);
    if(!hbmScreen) {
        MessageBox(hWnd, "CreateCompatibleBitmap Failed","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, 
               w, h, 
               hdcWindow, 
               0,0,
               SRCCOPY)) {
        MessageBox(hWnd, "BitBlt has failed", "Failed", MB_OK);
        goto done;
    }

    bi.biSize = sizeof(BITMAPINFOHEADER);    
    bi.biWidth = w;    
    bi.biHeight = h;  
    bi.biPlanes = 1;    
    bi.biBitCount = 24;    
    bi.biCompression = BI_RGB;    

    bi.biSizeImage = 0;  
    bi.biXPelsPerMeter = 0;    
    bi.biYPelsPerMeter = 0;    
    bi.biClrUsed = 0;    
    bi.biClrImportant = 0;

    dwBmpSize = w*bi.biBitCount*h;

    // 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 
    // which is pointed to by lpbitmap.
    GetDIBits(hdcWindow, hbmScreen, 0,
        (UINT)h,
        lpbitmap,
        (BITMAPINFO *)&bi, DIB_RGB_COLORS);

    f = fopen("./test.ppm", "wb");
    if (!f) {
        fprintf(stderr, "cannot create ppm file\n");
    goto done;
    }
    fprintf(f, "P6\n%d %d\n255\n", w, h);
    fwrite((LPSTR)lpbitmap, dwBmpSize, 1, f);
    fclose(f);

    //Unlock and Free the DIB from the heap
    GlobalUnlock(hDIB);    
    GlobalFree(hDIB);

    //Clean up
done:
    DeleteObject(hbmScreen);
    DeleteObject(hdcMemDC);
    ReleaseDC(hWnd,hdcWindow);

    return 0;
}

所以这是结果图像:

http://imageshack.us/photo/my-images/853/test2ne.jpg/

可以看到，宽度尺寸有问题。也许是因为窗口的边框？如果在代码中，我更改“w = rc.right - rc.left;”改成“w = rc.right - rc.left - 10;”，效果更好。但我不明白为什么我必须输入“-10”并且......图片右侧缺少一些像素(也许是10个像素？)

http://imageshack.us/photo/my-images/207/test3jq.jpg

最后一个问题:有什么方法可以要求 GetDIBits 函数将我的字节按相反的顺序放置吗？我不想逐像素复制，因为这会花费一些CPU时间。 (好吧，你可能会说，既然我将这个文件保存到磁盘，那么我不应该关心CPU时间，但我的目标不是将此图片保存到磁盘。我这样做只是为了调试目的)

提前感谢您的帮助

Answer 1

您的问题是 DIB 中的每一行图像数据必须是 DWORD 对齐的(即按 4 字节的倍数对齐)。

dwBmpSize = w*bi.biBitCount*h;

这实际上应该是:

dwBmpSize = ((w*bi.biBitCount+3)&~3) *h;

您在编写 PPM 文件时必须考虑到这一点。

此外，图像是上下颠倒的，因为默认情况下 DIB 是“自下而上”的(第 0 行位于底部)。要使其“自上而下”，请将 biHeight 字段设置为负值。