CTransBmp::CreateMask(CDC* pDC)

gistfile1.txt

void CTransBmp::CreateMask(CDC* pDC)
{
	m_hbmMask = new CBitmap;    
// Nuke any existing mask
    if (m_hbmMask) m_hbmMask->DeleteObject();

// Create memory DCs to work with
	CDC* hdcMask = new CDC;
	CDC* hdcImage = new CDC;

    hdcMask->CreateCompatibleDC(pDC);
    hdcImage->CreateCompatibleDC(pDC);

// Create a monochrome bitmap for the mask
    m_hbmMask->CreateBitmap(GetWidth(),
                               GetHeight(),
                               1,
                               1,
                               NULL);
	CBitmap* pTempBmp;
// Select the mono bitmap into its DC
    CBitmap* hbmOldMask = hdcMask->SelectObject(m_hbmMask);
// Select the image bitmap into its DC
    CBitmap* hbmOldImage = hdcImage->SelectObject(pTempBmp->FromHandle((HBITMAP)m_hObject));

// Set the transparency color to be the top-left pixel
    hdcImage->SetBkColor(hdcImage->GetPixel(0, 0));
// Make the mask
    hdcMask->BitBlt(0, 0,
             GetWidth(), GetHeight(),
             hdcImage,
             0, 0,
             SRCCOPY);
// clean up
    hdcMask->SelectObject(hbmOldMask);
    hdcImage->SelectObject(hbmOldImage);
    delete hdcMask;
    delete hdcImage;
}