Re: HDC

Thanks Scott.

So, just INSIDE OnDraw(), are they same or different?

I asked it because another problem I'm facing.

My app need to deal with large DIB bitmap(256color, cx=4000, cy=4000) with
scale(x8) and mask display function.
I did as follows,

 hdcImage = CreateCompatibleDC(hdcParent);
 hbmImage = Create256DIBitmap(hdcParent, cxImage, cyImage, FALSE );
 SelectObject(hdcImage, hbmImage);

 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;
     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);
}

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);

    //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;

     //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);
}

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.

William