Conversion of Enhanced Metafile to Bitmap and viceversa

<font color='#b03030'>Pravarakhya</font>

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Oct 18, 2003

CPOL

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An article on conversion of EMF file to Bitmap file format and BMP to EMF

Sample Image - ConvertEMFToBMP.jpg

Introduction

I have come across loads of libraries which convert images from one format to other, but most of them are confined to conversion between raster formats like BMP, JPG, GIF etc., This article explains a class which encapsulates the functionality of converting a vector file format (EMF) to raster file format (BMP). Once an EMF is converted to BMP raster file format, it can then be converted to any other format using the freely available libraries like CxImage.

Background

I had this problem of converting an EMF to BMP when I was doing my earlier project. I have searched a lot for help but couldn't find any except that many people asking similar question in discussion groups. I thought I should share this solution with others and wrote my first article to Code project. I hope it will be useful to you.

Using the code

Using this code is very simple. Just instantiate the class and call the convert function like this:

    CConvertEMFToBMP conv;
    
    //call the convert function
    BOOL bRet = conv.ConvertEMFToBMP(m_strEMF,m_strBMP,m_chkScale);

ConvertEMFToBMP takes three parameters:

//    m_strEMF - path and file name of EMF file to be converted
//    m_strBMP - path and file name of BMP file 
//    m_chkScale - boolean. If this is false, EMF is converted to 
//            BMP in original size.
//            If this is true, the image is scaled to preset size 
//            in the code (explained below)

Example:

    CConvertEMFToBMP conv;
    
    //call the convert function
    BOOL bRet = conv.ConvertEMFToBMP("D:\temp\Sample.emf",
            "D:\temp\sample.bmp",false);

To Convert a BMP to EMF:

    CConvertEMFToBMP conv;
       //call the convert function
    BOOL bRet = conv.ConvertBMPToEMF(m_strBMP,m_strEMF);

ConvertBMPToEMF takes two paramters:

    m_strBMP - path and file name of BMP file to be converted
    m_strEMF - path and file name of EMF file

Example:

    CConvertEMFToBMP conv;
    
    //call the convert function
    BOOL bRet = conv.ConvertBMPToEMF("D:\temp\Sample.bmp",
             "D:\temp\sample.emf",false);

How Does it Work?

There is no straight forward technique to convert an EMF to BMP. ConvertEMFToBMP converts an EMF to Bitmap by playing the metafile in the MemoryDC and then getting the DIB bits and storing them in the bitmap format. To get the memory DC, an invisible window is created and destroyed at the end of conversion. alternatively, the source code can be modified to take the hwnd to the ConvertEMFToBMP function. The scale image option scales the image to a fixed size #defined in the source code. This can also be modified so as to scale as per the user passed parameter. I have used only API functions so as to make the code light avoiding the MFC DLLs. The following is the step by step explanation of core logic of conversion.

Create a window which is invisible, of any size

    //Create an invisible window
    HWND hWnd = CreateWindow("#32770","NULL",~WS_VISIBLE,0,0,
            100,100,NULL,NULL,NULL,0);

Get the DC of the window

    //Get the DC of the Window
    HDC dc = ::GetDC(hWnd);

Get the handle to the emf file and get the EMF header

    //Get the Handle from the enhanced metafile
    hemf = ::GetEnhMetaFile(strFileName);
        
    // Get the header from the enhanced metafile.
    ZeroMemory( &emh, sizeof(ENHMETAHEADER) );
    emh.nSize = sizeof(ENHMETAHEADER);
    if( GetEnhMetaFileHeader( hemf, 
             sizeof( ENHMETAHEADER ), &emh ) == 0 )
    {
        DeleteEnhMetaFile( hemf );
        return FALSE;
    }

Get the width and height of the EMF

    //Declare variables for calculation of metafile rect
    RECT    rect;
    float    PixelsX, PixelsY, MMX, MMY;
    
    float fAspectRatio;
    long lWidth,lHeight;
    
    // Get the characteristics of the output device.
    PixelsX = (float)GetDeviceCaps( dc, HORZRES );
    PixelsY = (float)GetDeviceCaps( dc, VERTRES );
    MMX = (float)GetDeviceCaps( dc, HORZSIZE );
    MMY = (float)GetDeviceCaps( dc, VERTSIZE );
    
    // Calculate the rect in which to draw the metafile based on the
    // intended size and the current output device resolution.
    // Remember that the intended size is given in 0.01 mm units, so
    // convert those to device units on the target device.
    rect.top = (int)((float)(emh.rclFrame.top) * 
            PixelsY / (MMY*100.0f));
    rect.left = (int)((float)(emh.rclFrame.left) * 
            PixelsX / (MMX*100.0f));
    rect.right = (int)((float)(emh.rclFrame.right) * 
            PixelsX / (MMX*100.0f));
    rect.bottom = (int)((float)(emh.rclFrame.bottom) * 
            PixelsY / (MMY*100.0f));

    //Calculate the Width and Height of the metafile
    lWidth = (long)((float)(abs(rect.left - rect.right)));
    lHeight =(long)((float)(abs(rect.top-rect.bottom )));

If the image needs to be scaled, modify the width and height as per the aspect ratio

    fAspectRatio = (float)lWidth/(float)lHeight;

    
    if(bScaleImage) //If miniature option is ON, change 
                  //the width and height accordingly
    {
        if(fAspectRatio > 1 ) //width is more than height 
        {
            //Make width as constant and calculate Height
            lWidth = X_MINIATUREFRAME;
            lHeight = (long)((float)Y_MINIATUREFRAME / fAspectRatio);

        }
        else //width is less than height(or equal to height)
        {
            //Make Height as constant and calculate Width
            lHeight = Y_MINIATUREFRAME;
            lWidth = (long)((float)X_MINIATUREFRAME * fAspectRatio);
        }
    }

Create a Memory DC compatible to WindowDC
```
    memDC=::CreateCompatibleDC(dc); 
```

Create a bitmap compatible to Window DC

    bitmap = ::CreateCompatibleBitmap(dc,lWidth,lHeight);

Select the bitmap into the Mem DC
```
    ::SelectObject(memDC,bitmap);
```
Paint the background of the DC to White
```
    SetBackColorToWhite(memDC);
```
Now play the enhanced metafile into the memory DC; ignore its return value it may be false even if successful
```
    PlayEnhMetaFile(memDC,hemf,&rect);    
    DWORD dwRet = GetLastError();
```

Create logical palette if device support a palette

    HPALETTE pal;
    if( GetDeviceCaps(dc,RASTERCAPS) & RC_PALETTE )
    {
        UINT nSize = sizeof(LOGPALETTE) + (sizeof(PALETTEENTRY) * 256);
        LOGPALETTE *pLP = (LOGPALETTE *) new BYTE[nSize];
        pLP->palVersion = 0x300;

        pLP->palNumEntries = 
            GetSystemPaletteEntries( dc, 0, 255, pLP->palPalEntry );

        // Create the palette
        pal = ::CreatePalette(pLP );

        delete[] pLP;
    }

Convert the bitmap to a DIB (refer to the download for full source code)
```
    HANDLE hDIB = DDBToDIB( bitmap, BI_RGB, pal );
```
Write DIB to file (refer to the download for full source code)
```
    WriteDIB((char*)strBMPFile, hDIB );
```

Clean up. Destroy the window and return true

    DeleteEnhMetaFile( hemf );
    ::DestroyWindow(hWnd);

Bitmap to EMF

I have added a new functionality of converting the BMP to EMF. Following is the step by step process for converting a BMP file to EMF File. This code is based on the comment submitted by Gernot Frisch

Load the bitmap from file to get the bitmap handle

    hBmp = (HBITMAP*)::LoadImage(NULL,pszBMPFile,IMAGE_BITMAP,0,0, 
            LR_CREATEDIBSECTION|LR_LOADFROMFILE);

Get the BITMAP structure to get the bmp's height and width
```
    GetObject(hBmp, sizeof(BITMAP), &bm );
```

Create MemDC

    hdcMem =::CreateCompatibleDC(GetDC(NULL));

Select the BMP into the DC
```
    SelectObject(hdcMem, hBmp);
```

Create EMF and get the emf DC

    HDC emfdc = CreateEnhMetaFile(hdcMem, pszEMFFile, NULL, 
          "Created by Pravarakhya");

Blit from BMF DC to EMF DC

    BitBlt(emfdc,0, 0, bm.bmWidth, bm.bmHeight, hdcMem, 0, 0, SRCCOPY);

Close the EMF file
```
    ::CloseEnhMetaFile(emfdc);
```

Points of Interest

Converting a vector format to raster format has a lot of advantages, one of them is that the image can be used over the web when it is converted to JPG or GIF. One more advantage is that the vector images can be scaled down without losing the quality. So any huge EMF can be brought down to a miniature thumbnail image which can be used over web. In our earlier project, we wanted to create a thumbnail of huge Visio Diagrams (of around 4000 sq inches!), when we saved the diagram as 100X100 pixels bitmap, we were losing quality and image was not acceptable. We then found this solution of saving the diagram as EMF and then scaling it down to 100X100 pixels bitmap. This miniature is as good as its bigger one without any loss of quality.

History

18 Oct 2003 - Version 1.0 Initial Release
24 Oct 2003 - Version 2.0
- Removed the Window Creation code (Thanks to Paolo Messina)
- Added the BMP to EMF functionality (Thanks to Gernot Frisch )