Smart resize for monochrome bitmaps

If we make a monochrome bitmap smaller by a simple pixel resampling the resulting image looks lousy (pic.2). The problem here is that we take a stand-alone pixel and disregard its surrounding. We can improve the picture a little bit by calculating the value of a pixel as the average of the pixels we disregard and then rounding this value to the nearest color. However the best result is obtained in a grayscale destination bitmap (pic. 3). This is implemented in the following function.
 
Pic.1 Original image Pic 2. Simple resize   Pic 3. Smart resize

#define WIDTHBYTES(bits)    (((bits) + 31) / 32 * 4) // for padding

///////////////////////////////////////////////////////////////////
// Function name    : ZoomOutBmp
// Description      : creates a new bitmap which is a grayscale 
//                    zoomed out version of the original
// Return type      : HDIB - handle to a new bitmap 
// Argument         : double zoom - number of times to zoom out
// Argument         : HDIB hSrcDIB - handle to a source bitmap
///////////////////////////////////////////////////////////////////
HDIB ZoomOutBmp(double zoom, HDIB hSrcDIB)
{
    if (hSrcDIB == NULL) // nothing to do
        return NULL;
    
    if (zoom < 1) // no zoomin in this function
        return NULL;
    
    LPSTR pSrcDIB = (LPSTR) ::GlobalLock((HGLOBAL) hSrcDIB);
    
    BITMAPINFOHEADER& bmihSrc = *(BITMAPINFOHEADER*)pSrcDIB;
    ASSERT(bmihSrc.biBitCount == 1); // only monochrome bitmaps supported
    LPSTR pSrcBits = (LPSTR) (pSrcDIB + sizeof(BITMAPINFOHEADER) + sizeof(RGBQUAD)*2);
    
    BITMAPINFOHEADER bmihDst = bmihSrc;
    bmihDst.biWidth = (LONG)(bmihDst.biWidth / zoom + 0.5);
    bmihDst.biHeight = (LONG)(bmihDst.biHeight / zoom + 0.5);
    bmihDst.biBitCount = 8; // grayscale in any case
    bmihDst.biClrUsed = 0;
    
    // prepare destination bitmap
    DWORD dwDIBSize = sizeof(bmihDst) + sizeof(RGBQUAD)*256 +
        WIDTHBYTES(bmihDst.biWidth * bmihDst.biBitCount) * bmihDst.biHeight;
    
    bmihDst.biSizeImage = dwDIBSize;
    
    // allocate space for the new bitmap
    HDIB hDstDIB = (HDIB) ::GlobalAlloc(GMEM_MOVEABLE | GMEM_ZEROINIT, dwDIBSize);
    
    if (hDstDIB == 0) {
        ::GlobalUnlock((HGLOBAL) hSrcDIB);
        return NULL;
    }
    
    LPSTR pDstDIB = (LPSTR) ::GlobalLock((HGLOBAL) hDstDIB);
    
    // copy header
    memcpy(pDstDIB, &bmihDst, sizeof(bmihDst));
    
    // prepare grayscale palette
    for (int i=0; i < (1 << bmihDst.biBitCount); i++) {
        
        RGBQUAD& palEntry = *(RGBQUAD*)(pDstDIB + sizeof(bmihDst) + i * sizeof(RGBQUAD));
        palEntry.rgbRed = palEntry.rgbGreen = palEntry.rgbBlue = i;
        
    }
    
    LPSTR pDstBits = (LPSTR) (pDstDIB + sizeof(BITMAPINFOHEADER) + sizeof(RGBQUAD)*256);
    
    // now fill the bits
    LPSTR curSrcLineBits, curDstLineBits;
    int j, k;
    int scale = (int)(zoom + 0.5); // integer zoom out factor, i.e. 1:5
    int hBase, vBase;
    unsigned char value;
    
    // for every _scale_ lines in a source bitmap we will get one line
    // in the destination bitmap. Similarly for _scale_ columns in the
    // source we'll obtain one destination column.
    
    for (int strip=0; strip < bmihDst.biHeight; strip++) { // for every dst line
        
        curDstLineBits = pDstBits + strip * WIDTHBYTES(bmihDst.biWidth * bmihDst.biBitCount);
        vBase = int(strip * zoom + 0.5);
        
        for (i=0; i < scale; i++) {  // accumulate _scale_ rows
            
            curSrcLineBits = pSrcBits + (vBase + i) * WIDTHBYTES(bmihSrc.biWidth * bmihSrc.biBitCount);
            
            // prepare horizontally condensed lines for this strip
            for (j=0; j < bmihDst.biWidth; j++) { // for all bits in line
                
                hBase = int(j * zoom + 0.5); // mapped index on source
                for (k=0; k < scale; k++) { // accumulate _scale_ columns
                    
                    value = (curSrcLineBits[(hBase+k)/8] & (1 << (7 - (hBase+k)%8))) ? 0xff : 0;
                    curDstLineBits[j] += value / scale / scale; // main accumulator
                }
            }
        }
        
    }
    
    // unlock memory
    ::GlobalUnlock((HGLOBAL) hSrcDIB);
    ::GlobalUnlock((HGLOBAL) hDstDIB);
    
    return hDstDIB;
}

This function may be improved in several ways, for example, working with a weighted fraction of a pixel or centering the accumulated source pixels. However, the resulting visual impovement is not significant.

Download demo project - 38 KB



Comments

Leave a Comment
  • Your email address will not be published. All fields are required.

Top White Papers and Webcasts

  • Who can you trust? Learn from the IBM X-Force team in this new quarterly report how the Internet of Things and IP reputation tracking are transforming the security landscape.

  • Today, users, applications, and data exist in more places than ever before, creating an unprecedented challenge for IT. How can IT achieve the flexibility and agility it needs to offer multiple types of applications in multiple locations? To better serve business demands for information everywhere, enterprises must develop new strategies for optimizing multiple kinds of networks. Read this white paper to learn how hybrid networks provide an unprecedented level of network dynamism, enterprise agility, and the …

Most Popular Programming Stories

More for Developers

RSS Feeds

Thanks for your registration, follow us on our social networks to keep up-to-date