將Windows BITMAP轉換為PIX（無符號字符緩沖區）

Question

我正在拍攝一個窗口的屏幕截圖，以便使用Leptonica然后再使用Tesseract進行一些OCR

問題是，出於性能考慮，我想避免將BMP寫入和讀取到光盤，而只是在內存中工作。 這就是我制作屏幕截圖的方式：

int width, height = 0;

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);

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);


// 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"pics/UI.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);

width = rcClient.right - rcClient.left;
height = rcClient.bottom - rcClient.top;

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

這是我的閱讀方式：

PIX* pixUI = pixRead("pics/UI.bmp");

因此，我已經看到該庫具有一個PIX * pixReadMemBmp ( const l_uint8 *cdata, size_t size ) 方法 ，該方法采用一個l_uint8 ，這是一個unsigned char緩沖區

問題是，我不明白如何從HBITMAP或BITMAP對象中獲得這樣的緩沖區。

Answer 1

首先將位圖復制到緩沖區中，然后將該緩沖區移至pixReadMemBmp() 。 復制是必須完成的，因為我假設pixReadMemBmp()函數需要位圖數據前面的兩個位pixReadMemBmp()頭，就像在文件中一樣。 偽代碼：

std::vector<unsigned char> buffer(sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER) + dwBmpSize);
std::copy(reinterpret_cast<unsigned char*>(&bmfHeader), reinterpret_cast<unsigned char*>(&bmfHeader) + sizeof(BITMAPFILEHEADER), buffer.begin());
std::copy(reinterpret_cast<unsigned char*>(&bi), reinterpret_cast<unsigned char*>(&bi) + sizeof(BITMAPINFOHEADER), buffer.begin() + sizeof(BITMAPFILEHEADER));
std::copy(lpbitmap, lpbitmap + dwBmpSize, buffer.begin() + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER));
pixReadMemBmp(&buffer[0], buffer.size());