Screen flicker problem? InvalidateRect()

I have some problem with screen flicker, I copy one image from hard disk to view, and draw one circle on the screen, when I move the circle using direction arrow of keyboard, there is flicker problem because redrawing the whole screen. So I called the function InvalidateRect() to redraw specific rectangular, but it seems no action. the following is my code, can you please give some suggestion?

Thanks in advance!

void MyView::OnDraw(CDC* pDC)
{
CString szFilename("c:\\image.bmp");
HBITMAP hBmp = (HBITMAP)::LoadImage(NULL,szFilename,
IMAGE_BITMAP,0,0,
LR_LOADFROMFILE|LR_CREATEDIBSECTION);

CBitmap bmp;
bmp.Attach(hBmp);

CClientDC dc(this);
CDC bmDC;
bmDC.CreateCompatibleDC(&dc);
CBitmap *pOldbmp = bmDC.SelectObject(&bmp);

BITMAP bi;
bmp.GetBitmap(&bi);

dc.BitBlt(0,0,bi.bmWidth,bi.bmHeight,&bmDC,0,0,SRCCOPY);
dc.SelectObject(pOldbmp);

DrawCircle(pDC);
}

void MyView::DrawCircle(CDC *pDC)
{
CPen *penold, pennew;
pennew.CreatePen(PS_DOT,1,RGB(255,255,255));
penold=pDC->SelectObject(&pennew);

CBrush *pBrhold, Brhnew;
Brhnew.CreateStockObject(NULL_BRUSH);
pBrhold=pDC->SelectObject(&Brhnew);

m_tracker.AdjustRect(false,&m_tracker.m_rect);
m_tracker.m_rect.bottom=m_tracker.m_rect.top+m_tracker.m_rect.right -m_tracker.m_rect.left;
m_tracker.AdjustRect(true,&m_tracker.m_rect);

pDC->Ellipse(m_tracker.m_rect);

pDC->SelectObject(penold);
pDC->SelectObject(pBrhold);
pennew.DeleteObject();
}

void MyDlg::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags)
{
// TODO: Add your message handler code here and/or call default

CRect rcClient;
p_view->GetClientRect(&rcClient);
rcClient.top=rcClient.top/2;

if(nChar==VK_LEFT)
{
p_view->m_tracker.m_rect.left-=1;
p_view->m_tracker.m_rect.right-=1;

InvalidateRect(rcClient);
//Invalidate();
}
}

Quote:

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Originally Posted by berrywong

I have some problem with screen flicker, I copy one image from hard disk to view, and draw one circle on the screen, when I move the circle using direction arrow of keyboard, there is flicker problem because redrawing the whole screen. So I called the function InvalidateRect() to redraw specific rectangular, but it seems no action. the following is my code, can you please give some suggestion?

---

A few things:

• Calling InvalidateRect() instead of Invalidate() makes no difference if you pass the entire client rect - actually, that's exactly what Invalidate does (calling InvalidateRect with the entire client rect). It only makes sense if you call it for a smaller rectangle (e.g. your circle's bounds, but note that you would have to call that twice: Before the move, and after the move).
• Why are you creating a CClientDC in OnDraw()? You should use the readily prepared CPaintDC you are getting as the pDC parameter.
• To completely avoid flickering, I would suggest to handle WM_ERASEBKGND, and return TRUE. You could also pass FALSE as the second parameter to InvalidateRect(), preventing the background from being drawn.

Hi,

Quote:

---

Originally Posted by berrywong

void MyDlg::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags)
{
// TODO: Add your message handler code here and/or call default

CRect rcClient;
p_view->GetClientRect(&rcClient);
rcClient.top=rcClient.top/2;

if(nChar==VK_LEFT)
{
p_view->m_tracker.m_rect.left-=1;
p_view->m_tracker.m_rect.right-=1;

InvalidateRect(rcClient);
//Invalidate();
}
}

---

rcClient.top = rcClient.top / 2 could be a problem. The top of the client is 0.
Another thing is that InvalidateRect() is being called for the dialog and not the view.

Jay

Thank you so much for your suggestions, gstercken! I have changed code as below, but flicker in specific rectangular area (rcClient) is still visible . Is there effective method to eliminate flicker completely?

void MyDlg::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags)
{
CRect rcClient(0,250,600,350);

if(nChar==VK_LEFT)
{
p_view->m_tracker.m_rect.left-=1;
p_view->m_tracker.m_rect.right-=1;

InvalidateRect(rcClient, false);
//Invalidate();
}
}

yes, it should be corrected as rcClient.top=rcClient.right/2!

I inserted the following, it still flicker!

BOOL BIPTTestView::OnEraseBkgnd(CDC* pDC)
{
// TODO: Add your message handler code here and/or call default

return true;//CView::OnEraseBkgnd(pDC);
}

BOOL BIPTTestView::PreCreateWindow(CREATESTRUCT& cs)
{
// TODO: Add your specialized code here and/or call the base class
cs.lpszClass = AfxRegisterWndClass(0);
return CView::PreCreateWindow(cs);
}

Handle WM_ERASEBKGND as gstercken suggested.

Consider to use "double buffering". Should be easy in your case:

Code:

---

CClientDC dc(this);
CDC bmDC;
bmDC.CreateCompatibleDC(&dc);
CBitmap *pOldbmp = bmDC.SelectObject(&bmp);

// Draw the circle on the bitmap.
DrawCircle(&bmDC);

BITMAP bi;
bmp.GetBitmap(&bi);

// Blt the whole bitmap including circle to the output dc.
dc.BitBlt(0,0,bi.bmWidth,bi.bmHeight,&bmDC,0,0,SRCCOPY);
dc.SelectObject(pOldbmp);

---

Also, consider to buffer the image to avoid reading it on every paint.