Question

When I set up and create a 24-bit bitmap like this:

   //fileheader
    BITMAPFILEHEADER* bf = new BITMAPFILEHEADER;
    bf->bfType = 0x4d42;
    bf->bfSize = 6054400 + 54;
    bf->bfOffBits = 54;

    //infoheader
    BITMAPINFOHEADER* bi = new BITMAPINFOHEADER;
    bi->biSize = 40;
    bi->biWidth = 2752;
    bi->biHeight = -733;
    bi->biPlanes = 1;
    bi->biBitCount = 24;
    bi->biCompression = 0;
    //bi->biSizeImage = 6054400;
    bi->biXPelsPerMeter = 2835;
    bi->biYPelsPerMeter = 2835;
    bi->biClrUsed = 0;
    bi->biClrImportant = 0;

    pFrame->GetImage(m_imageData);

    //
    //create bitmap...
    //(hbit is a global variable)

    BITMAPINFO* bmi;
    bmi = (BITMAPINFO*)bi; 
    HDC hdc = ::GetDC(NULL);

    hbit = CreateDIBitmap(hdc, bi, CBM_INIT, m_imageData, bmi, DIB_RGB_COLORS);

I get an output image like this:

But when I change bitcount from 24 to 8 (which also allows for 3x image size, allowing me to go from 733 width to the image’s natural width of 2200), I get an image like this (along with a lot of instability):

My output looks like this:

    BITMAP* bi = new BITMAP;
    CBitmap bmp;
    bmp.Attach(hbit);
    CClientDC dc(pWnd);
    CDC bmDC;
    bmDC.CreateCompatibleDC(&dc);
    CBitmap *pOldbmp = bmDC.SelectObject(&bmp);
    bmp.GetBitmap(bi);
    dc.BitBlt(384,26,bi->bmWidth/3,bi->bmHeight,&bmDC,0,0,SRCCOPY);
    //note: if bitcount is 8, height and width need to be /3, 
          //if 24, only width gets /3 
    bmDC.SelectObject(pOldbmp);

    //explicitly delete everything just to be safe
    delete bi;
    DeleteObject(bmp);
    DeleteObject(dc);
    DeleteObject(pOldbmp);
    DeleteObject(bmDC);

So my questions are:

• Why is this happening when I switch from 24 to 8?
• Is there an easy way to output the image as monochrome rather than color?

One last thing:

My coworker wrote this function a long time ago for a similar issue, but he said I may be able to use it. I can’t get it to work, unfortunately:

void CopyMono8ToBgrx(byte* pDestBlue, byte* pDestGreen, byte *pDestRed, byte* pDestAlpha)
    {
        byte*               pSrc;
        byte*               pSrcEnd;

        pSrc = ( byte* ) m_imageData;
        pSrcEnd = pSrc + ( 2752*2200 );

        while ( pSrc < pSrcEnd )
        {
            byte data = *pSrc;

            *pDestBlue  = data;
            *pDestGreen = data;
            *pDestRed   = data;
            *pDestAlpha = 255;  // alpha is always 255 (fully opaque)

            pSrc++;
            pDestBlue   += 4;
            pDestGreen  += 4;
            pDestRed    += 4;
            pDestAlpha  += 4;
        }
    }

Answer 1

8 bit per pixel images are assuming a color palette following BITMAPINFOHEADER structure (see BITMAPINFO::bmiColors). If you make the palette to be 256 gray shades, the image is going to me 8 bpp grayscale. Now it’s color with random colors on it.

The function CopyMono8ToBgrx you quoted creates full color bitmap, with gray individual pixels (R=G=B).