How to get a 32bpp bitmap/image from a GDI Device Context?

Question

I am using code from this Project http://www.codeproject.com/Articles/9064/Yet-Another-Transparent-Static-Control in order to draw transparent button images from my subclassed Button control onto my CDialogEx.

This code is meant for legacy 24bpp GDI functions:

BOOL CTransparentStatic2::OnEraseBkgnd(CDC* pDC)
{
   if (m_Bmp.GetSafeHandle() == NULL)
   {
      CRect Rect;
      GetWindowRect(&Rect);
      CWnd *pParent = GetParent();
      ASSERT(pParent);
      pParent->ScreenToClient(&Rect); //convert our corrdinates to our parents

      //copy what's on the parents at this point
      CDC *pDC = pParent->GetDC();
      CDC MemDC;
      MemDC.CreateCompatibleDC(pDC);
      m_Bmp.CreateCompatibleBitmap(pDC,Rect.Width(),Rect.Height());
      CBitmap *pOldBmp = MemDC.SelectObject(&m_Bmp);

      MemDC.BitBlt(0,0,Rect.Width(),Rect.Height(),pDC,Rect.left,Rect.top,SRCCOPY);

      MemDC.SelectObject(pOldBmp);

      pParent->ReleaseDC(pDC);
   }
   else //copy what we copied off the parent the first time back onto the parent
   {
      CRect Rect;
      GetClientRect(Rect);
      CDC MemDC;
      MemDC.CreateCompatibleDC(pDC);
      CBitmap *pOldBmp = MemDC.SelectObject(&m_Bmp);
      pDC->BitBlt(0,0,Rect.Width(),Rect.Height(),&MemDC,0,0,SRCCOPY);
      MemDC.SelectObject(pOldBmp);
   }

   return TRUE;
}

However the background of my CDialogEx is being drawn with GDI+ 32bpp rendering like this:

BOOL CParentDialogEx::OnEraseBkgnd(CDC* pDC)
{
   // Get GDI+ Graphics for the current Device Context
   Graphics gr(*pDC);

   // Get the client area
   CRect clientRect;
   GetClientRect(&clientRect);

   // Draw the dialog background
   // PLEASE NOTE: m_imgDlgBkgnd is a Gdiplus::Image with PNG format ==> 32bpp Image
   gr.DrawImage(m_imgDlgBkgnd, 0, 0, clientRect.Width(), clientRect.Height());
}

This causes the first code snippet to make a backup of a black rectangle instead of the 32bpp drawn content to it. This again causes my button control to always have a black background.

To make my problem clear, please see the pictures below:

1. Button images are being drawn onto the CDialogEx background (normally):

2. Button images are being drawn with the first code snippet

As you can see GDI 24bpp cannot see the dialog background. It just assumes plain black background. Only GDI+ could see it. However I was not able to find a way to get a bitmap from a Gdiplus::Graphics object.

How can I get a 32bpp background backup in order to draw my transparent images correctly?

Using no backup images at all causes the alpha blending of GDI+ to blur the background more and more on every draw.

Answer 1

After 3 days of figuring around I found something by testing. This can actually be done way easier and with 32bpp rendering!

// Get the client area on the parent
CRect Rect;
GetWindowRect(&Rect);
CWnd *pParent = GetParent();
ASSERT(pParent);
pParent->ScreenToClient(&Rect);

// Get the Parent's DC
CDC *parentDC = pParent->GetDC();

// GDI Code (only 24bpp support)
//CDC MemDC;
//CBitmap bmp;
//MemDC.CreateCompatibleDC(parentDC);
//bmp.CreateCompatibleBitmap(parentDC,Rect.Width(),Rect.Height());
//CBitmap *pOldBmp = MemDC.SelectObject(&bmp);
//MemDC.BitBlt(0,0,Rect.Width(),Rect.Height(),parentDC,Rect.left,Rect.top,SRCCOPY);
//MemDC.SelectObject(pOldBmp);

// GDI+ Code with 32 bpp support (here comes the important change)
Bitmap* bmp = new Bitmap(Rect.Width(), Rect.Height());
Graphics bmpGraphics(bmp);
HDC hBmpDC = bmpGraphics.GetHDC();
BitBlt(hBmpDC, 0, 0, Rect.Width(), Rect.Height(), parentDC->GetSafeHdc(), Rect.left, Rect.top, SRCCOPY); // BitBlt is actually capable of doing 32bpp blts

// Release DCs
bmpGraphics.ReleaseDC(hBmpDC);
pParent->ReleaseDC(parentDC);

Here you go! 4 lines of code and you get a 32bpp Gdiplus::Bitmap! Which you can draw later on in OnEraseBkgnd:

// Get client Area
CRect rect;
GetClientRect(&rect);

// Draw copied background back onto the parent
Graphics gr(*pDC);
gr.DrawImage(bmp, 0, 0, rect.Width(), rect.Height());

Please note that the Bitmap should be a member variable for performance increase. It also has to be freed on the controls destruction.

Answer 2

Instead of Gdiplus::Image, use Gdiplus::Bitmap which has GetHBITMAP

In this example the background is set to red for png image with transparent background:

Gdiplus::Bitmap gdi_bitmap(L"c:\\test\\filename.png");
HBITMAP hbitmap = NULL;
gdi_bitmap.GetHBITMAP(Gdiplus::Color(128, 0, 0), &hbitmap);
CBitmap bmp;
bmp.Attach(hbitmap);
CDC memdc;
memdc.CreateCompatibleDC(pDC);
memdc.SelectObject(&bmp);
pDC->StretchBlt(0, 0, clientRect.Width(), clientRect.Height(), &memdc, 0, 0, 
        gdi_bitmap.GetWidth(), gdi_bitmap.GetHeight(), SRCCOPY);

Alternatively, you can use a brush to handle the painting:

CBrush m_Brush;
BOOL CMyDialog::OnInitDialog()
{
    CDialogEx::OnInitDialog();
    Gdiplus::Bitmap bitmap(L"c:\\test\\filename.png");
    HBITMAP hbitmap = NULL;
    bitmap.GetHBITMAP(Gdiplus::Color(128, 0, 0), &hbitmap);
    CBitmap bmp;
    bmp.Attach(hbitmap);
    m_Brush.CreatePatternBrush(&bmp);
    return 1;
}

HBRUSH CMyDialog::OnCtlColor(CDC* pDC, CWnd* wnd, UINT nCtlColor)
{
    //uncomment these two lines if you only want to change dialog background
    //if (wnd != this)
    //    return CDialogEx::OnCtlColor(pDC, wnd, nCtlColor);
    if (nCtlColor == CTLCOLOR_STATIC)
    {
        pDC->SetTextColor(RGB(0, 128, 128));
        pDC->SetBkMode(TRANSPARENT);
    }
    CPoint pt(0, 0);
    if (wnd != this)
        MapWindowPoints(wnd, &pt, 1);
    pDC->SetBrushOrg(pt);
    return m_Brush;
}

BOOL CMyDialog::OnEraseBkgnd(CDC* pDC)
{//don't do anything
    return CDialogEx::OnEraseBkgnd(pDC);
}