user3285714 user3285714 - 19 days ago 5
C++ Question

C++: Hbitmap/BITMAP into .bmp file

Ok, whole story is, I am trying to use Leptonica+Tesseract OCR in C++ to take a screenshot, save it to a *.bmp file, then load it back up to OCR with it. I won't need to do this frequently, but as I cannot seem to copy the screenshot data directly into a Leptonica PIX structure, I need to save it to a file first..actually a solution to this would be preferably.

Here's some code I've found online, trying to help me out.

Screen cap:

HBITMAP ScreenCapture(){
int width=100;
int height=100;
// get the device context of the screen
HDC hScreenDC = CreateDC(L"DISPLAY", NULL, NULL, NULL);
// and a device context to put it in
HDC hMemoryDC = CreateCompatibleDC(hScreenDC);

int x = GetDeviceCaps(hScreenDC, HORZRES);
int y = GetDeviceCaps(hScreenDC, VERTRES);

// maybe worth checking these are positive values
HBITMAP hBitmap = CreateCompatibleBitmap(hScreenDC, x, y);

// get a new bitmap
HBITMAP hOldBitmap = (HBITMAP)SelectObject(hMemoryDC, hBitmap);

BitBlt(hMemoryDC, 0, 0, width, height, hScreenDC, 0, 0, SRCCOPY);
hBitmap = (HBITMAP)SelectObject(hMemoryDC, hOldBitmap);

//GlobalAlloc(GPTR, hBitmap)

WriteDIB(L"test.bmp", (HGLOBAL)hBitmap);

// clean up
DeleteDC(hMemoryDC);
DeleteDC(hScreenDC);

return hBitmap;
// now your image is held in hBitmap. You can save it or do whatever with it
}


Attempt to write function:

BOOL WriteDIB( LPTSTR szFile, HANDLE hDIB)
{
cout<<endl<<"Running save function";
/*HANDLE hDIB=GlobalAlloc(GPTR, sizeof(hDIBtochange));//this doesn't work, the result is four. Also the HANDLE parameter's name would be changed to hDIBtochange, so that the rest of the function uses the old 'hDIB' throughout
cout<<endl<<sizeof(hDIBtochange);*/
BITMAPFILEHEADER hdr;
LPBITMAPINFOHEADER lpbi;
if (!hDIB)
return FALSE;
CFile file;
if( !file.Open( szFile, CFile::modeWrite|CFile::modeCreate) )
return FALSE;
lpbi = (LPBITMAPINFOHEADER)hDIB;
int nColors = 1 << lpbi->biBitCount;
// Fill in the fields of the file header
hdr.bfType = ((WORD) ('M' << 8) | 'B'); // is always "BM"
hdr.bfSize = GlobalSize (hDIB) + sizeof( hdr );
hdr.bfReserved1 = 0;
hdr.bfReserved2 = 0;
hdr.bfOffBits = (DWORD) (sizeof( hdr ) + lpbi->biSize + nColors * sizeof(RGBQUAD));
// Write the file header
file.Write( &hdr, sizeof(hdr) );
// Write the DIB header and the bits
file.Write( lpbi, GlobalSize(hDIB) );
return TRUE;
}


Shamelessly copied from people's posts over the years.
Ok! Problem I face is, I cannot seem to understand how to GlobalAlloc the Hbitmap into a globally accessible Handle, that can be converted or use with LPBITMAPINFOHEADER.
Soon as lpbi is created, every single field inside of it is "Unable to read memory" error in Visual Studio 2012 debugging. It's inaccessible, despite being created.

Solutions..
Go straight from screencap to PIX, inside of memory..
Find a way to save to bitmap and create them periodically to read..
Find another way entirely that makes more sense..

Preferring first, but, I'm asking for a solution in this, to the second one..or third.

If you need more info I can try to provide it. This mostly boils down to "I've never done code like this before and it wasn't taught in my classes so I'm trying to learn as I go".

Answer

A much easier way to save an HBITMAP to file is to make use of GDI+. This gives you the advantage of being able to save to any format that windows supports natively, while freeing you from the muck of playing around with or even needing to understand, various image formats.

In the below example, I've just used LoadImage as a quik and dirty way of loading a pre-existing image - you could simply use the HBITMAP you've already captured.

Here's an example that loads a bitmap and saves it again. (I had initially used "image/png" as the output type, along with an appropriate output filename)

#include <windows.h>
#include <gdiplus.h>
using namespace Gdiplus;

int GetEncoderClsid(const WCHAR* format, CLSID* pClsid)
{
   UINT  num = 0;          // number of image encoders
   UINT  size = 0;         // size of the image encoder array in bytes

   ImageCodecInfo* pImageCodecInfo = NULL;

   GetImageEncodersSize(&num, &size);
   if(size == 0)
      return -1;  // Failure

   pImageCodecInfo = (ImageCodecInfo*)(malloc(size));
   if(pImageCodecInfo == NULL)
      return -1;  // Failure

   GetImageEncoders(num, size, pImageCodecInfo);

   for(UINT j = 0; j < num; ++j)
   {
      if( wcscmp(pImageCodecInfo[j].MimeType, format) == 0 )
      {
         *pClsid = pImageCodecInfo[j].Clsid;
         free(pImageCodecInfo);
         return j;  // Success
      }
   }
   free(pImageCodecInfo);
   return -1;  // Failure
}

int main()
{
    GdiplusStartupInput gdiplusStartupInput;
    ULONG_PTR gdiplusToken;
    GdiplusStartup(&gdiplusToken, &gdiplusStartupInput, NULL);

    HBITMAP hBitmap = (HBITMAP)LoadImage(GetModuleHandle(NULL), "babe.bmp", IMAGE_BITMAP, 0,0, LR_LOADFROMFILE);
    Bitmap *image = new Bitmap(hBitmap, NULL);

    CLSID myClsId;
    int retVal = GetEncoderClsid(L"image/bmp", &myClsId);

    image->Save(L"output.bmp", &myClsId, NULL);
    delete image;

    GdiplusShutdown(gdiplusToken);
    return 0;
}