Re: HDC

From:

"William" <port@mx15.freecom.ne.jp>

Newsgroups:

microsoft.public.vc.mfc

Date:

Mon, 2 Apr 2007 14:05:55 +0900

Message-ID:

<OdmOcSOdHHA.2068@TK2MSFTNGP06.phx.gbl>

hdcParent comes from

HDC hdcParent = GetDC(m_hWnd);

I use HDC in OnDraw because pDC->m_hDC seems to me not doest work correctly
for bitmap show when scroll bar is moved.

hbmImage is a DIB bitmap created by my Create256DIBitmap() and initialized
by ny bitmap file.

CDC * is not the parameters becasue I have many other functions needing a
HDC as a parameter.

MaskByte is a BYTE variable used to control which bit in hbmImage is
displayed. MaskByte can be from 0x00 to 0xff.

I don't think there are any differents bewteen BitBlt(pDC->m_hDC) and
pDC->BitBlt(). It was not the reason of the problem I guess.

Why the fascination with HDCs? I note that you did not tell us where
hdcParent comes from
or why you are using an HDC in an OnDraw handler.

CDC Image;
Image.CreateCompatibleDC(parentDC); // parentDC is a CDC *

I have no idea what Create256DIBitmap is, but presumably it is
somethingyou wrote. Why
not use a CDC * for the parameters? What is hbmImage; I note you did not
show the
declaration.

HBITMAP bmp = Create256DIBitmap(&Image, cxImage, cyImage, FALSE);

Image.SelectObject(bmp);
*****

hdcMask = CreateCompatibleDC(hdcImage);
hbmMask = Create256DIBitmap(hdcParent, cxImage, cyImage, TRUE);
SelectObject(hdcMask, hbmMask);

hdcShow = CreateCompatibleDC(hdcImage);
hbmShow = CreateCompatibleBitmap(hdcImage, cxScale, cyScale);
SelectObject(hdcShow, hbmShow);

To show the image, I call the following function from CMyView::OnDraw(CDC*
pDC)
void CMyView::OnDrawScaleImage(CDC* pDC)
{
BYTE MaskByte = 0x00~0xFF;

****
~ is not an infix operator; what did you intend here?
****

    memset(pvBitsMask, MaskByte, nSizeOfColorBitMask);
   BitBlt( hdcMask, 0, 0, cxImage, cyImage, hdcImage, 0, 0, SRCAND);

   StretchBlt( hdcShow, 0, 0, cxScale, cyScale, hdcMask, 0, 0, cxImage,
cyImage, SRCCOPY);

   // Copy the bits to the screen.
   BitBlt(pDC->m_hDC, 0, 0, cxScale, cyScale, hdcShow, 0, 0, SRCCOPY);

****
Still not sure why the horrid fascination with raw HDCs. and why not
just do
pDC->BitBlt(0,0, cxScale, cyScale, &show, 0, 0, SRCCOPY);
where show is
CDC show;
?
Why program in low-level graphics primitives when you can use MFC? Also,
when using APIs,
it is good practice to prefix them with ::.
****

}

I find that StretchBlt() will use quite a lot of system memory when
cxScale=cxImage*8, cyScale=cyImage*8.

On the other hand, if I edit the same bitmap(256color, cx=cy=1000) under
MSPaint.exe, I can't find obvious increase of memory usage when I change
the
scale from x1 to x8.

To solve the problem, I am trying to StretchBlt just Window size bitmap to
pDC->m_hDC directly(not using hdcShow) as below.

void CMyView::OnDraw(CDC* pDC)
{
   CRect rcClient;
   GetClientRect(&rcClient);

   HDC hDC1 = pDC->m_hDC;
   HDC hDC2 = ::GetDC(m_hWnd);

****
Lose the hDC2. It does not appear to serve a useful purpose
****

   //get scroll info
    SCROLLINFO si;
    si.cbSize = sizeof(SCROLLINFO);
    si.fMask = SIF_POS;
    GetScrollInfo(SB_HORZ, &si);
    m_ptShiftZ.x = -si.nPos;
    GetScrollInfo(SB_VERT, &si);
    m_ptShiftZ.y = -si.nPos;

*****
Why isn't this in OnPrepareDC?
*****

    //move rcClient
   rcClient.OffsetRect(-1 * m_ptShiftZ.x, -1 * m_ptShiftZ.y );

   memset(pvBitsMask, BCtrlByte, nSizeOfColorBitMask);
   BitBlt( hdcMask, 0, 0, cxImage, cyImage, hdcImage, 0, 0, SRCAND);

    StretchBlt( hDC1, 0, 0, rcClient.Width(), rcClient.Height(),
   hdcMask, (rcClient.left) / iZoom, rcClient.top / iZoom,
   rcClient.right / iZoom, rcClient.bottom / iZoom, SRCCOPY);

****
And where, exactly, did you free up hDC2? Why do you use raw HDCs when
there are already
MFC methods of the CDC class.
pDC->StretchBlt(0,0, rcClient.Width(), rcClient.Height(), ...etc...)
would do the job perfectly well.
****

}

But a new problem appears. That is the bitmap on the right side of window
is
not moved in when we click scrollbar button to try to shift the bitmap to
the left. There is no such problem, however, if I use hDC2 instead of
hDC1.

*****
This is because HDC1 has a clipping region. So you are failing to
invalidate the proper
rectangle. For that matter, why isn't your scrollbar logic using
ScrollWindow to do the
scrolling?

If you invalidate the proper area, you will not have the problem.
joe
*****

William

The DC passed to OnDraw is special in several ways. It is normally a
CPaintDC, which is specialized to support the Windows scheme for sharing
the screen with multiple programs. This DC has a clipping region that
corresponds to the part of your window that needs to be redrawn, and the
destruction of this DC is what informs Windows that your window painting
is now valid. It is also used when drawing a print preview window.

See the BeginPaint/EndPaint APIs for additional info. CPaintDC calls
BeginPaint and EndPaint for you.
Scott McPhillips [VC++ MVP]

Joseph M. Newcomer [MVP]
email: newcomer@flounder.com
Web: http://www.flounder.com
MVP Tips: http://www.flounder.com/mvp_tips.htm