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