trouble with CDC

From:

"Bill" <<don't want more spam>>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 12 Apr 2010 18:11:36 +0800

Message-ID:

<#OWUKii2KHA.556@TK2MSFTNGP04.phx.gbl>

I'm doing a program that needs to capture bitmaps off a window of another
program. I'm doing okay except I have an intermittent failure. I admit I'm
not testing for all the possible MFC error returns at this early stage, but
even I did I'm not sure how I'd deal with this error.

My piece of code looks like this.

CDC m_memoryDc;
CBitmap m_bitmap;

void CMyDlg::GrabRect(CWnd *pWnd, CRect rc)
{
  CDC * pSourceDc = pWnd->GetDC();
  Sleep(50); // !!!!! need this or the crash happens almost right away
  m_bitmap.Detach();
  m_bitmap.CreateCompatibleBitmap(pSourceDc, rc.Width(), rc.Height()); <--
error here
  m_memoryDc.Detach();
  BOOL ret = m_memoryDc.CreateCompatibleDC(pSourceDc);
  m_memoryDc.SelectObject(&m_bitmap); <-- error here
  m_memoryDc.BitBlt(0, 0, rc.Width(), rc.Height(), pSourceDc, rc.left,
rc.top, SRCCOPY);
}

I'm trying to capture a bit of window into m_bitmap. I'll later use the
m_memoryDC to access the pixels of the bitmap.

I need the Sleep function call. If I set to 50 ms, then I will get an error
after a few hundred calls to this function. If I set it to 1 msec, I will
get an error after a few calls only. If I set it longer, like 200 msec, I
can run a longer time, but since it's slower, I don't know if it will run
forever without a crash or just takes longer to crash.

Sometimes the error occurs at CreateCompatibleBitmap. pSourceDC is NULL so
the GetDC call failed I guess. But I don't know if I can check the reason
because there is no error code returned.

Sometimes the error occurs at SelectObject. In this case pSourceDC looks
valid (0x00c0fe70) and the m_hDC and m_hAttribDC both have the same value
(0x70015a85) but ret is NULL and two handles of m_memoryDC are both NULL.

I'm not sure what to try next. Am I not releasing some resource and running
out of memory or soemthing? Is there a need for a delay after the calls for
the object to be valid? Doesn't seem right, but if so, is there a way to
check if they are valid myself so I'm not wasting time when they are already
valid?

Actually, is there any way to manipulate bitmaps without going through the
seemingly extra step of using a CDC?

Thanks...