Screen flicker problem? InvalidateRect()

[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?

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:rawCircle(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();
}
}

[gstercken]

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.

[Jay Tautges]

Hi,

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

[berrywong]

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();
}
}

[berrywong]

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

[berrywong]

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);
}

[Oliver M.]

Handle WM_ERASEBKGND as gstercken suggested.

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

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.