dc.LineTo not drawing on OnPaint() unless I move the use the mouse and move the window out of view?

Unless I make "static CPaintDC dc(this);" the line won't draw? But this is not good as it will eventually error, also the graphics wont' keep on the screen.

Not sure what I am doing wrong

Note: I have a Timer that calls to this every 100ms(x and y are incremented) thx

void CGraphicsDlg::OnPaint() 
{
    CString s;
    CPaintDC dc(this);// device context for painting

    if (IsIconic())
    {
        SendMessage(WM_ICONERASEBKGND, (WPARAM) dc.GetSafeHdc(), 0);

        // Center icon in client rectangle
        int cxIcon = GetSystemMetrics(SM_CXICON);
        int cyIcon = GetSystemMetrics(SM_CYICON);
        CRect rect;
        GetClientRect(&rect);
        int x = (rect.Width() - cxIcon + 1) / 2;
        int y = (rect.Height() - cyIcon + 1) / 2;

        // Draw the icon
        dc.DrawIcon(x, y, m_hIcon);
    }
    else if(x==0)
    {
        s.Format("%d", x);
        edXa->SetWindowText(s);

        dc.MoveTo(20,400);
    }
    else if (x>0){
        s.Format("%d", x);
        edXb->SetWindowText(s);

        dc.LineTo(20 + x, 40);  // doesn't draw unless I make "static CPaintDC dc(this);" <- which will error out 
    }
    CDialog::OnPaint();
}

void CGraphicsDlg::OnTimer(UINT nIDEvent)
{
    if(nIDEvent==1){
        srand( (unsigned)time( NULL ) );

        //y = rand() % 100;
        y++;
        x++;

        OnPaint();
    }
}

2012-04-03 21:11
by jdl

LineTo draws a line from one point to another, using the selected pen. You need to use MoveTo to define the start of the line, and you need to select a pen into the DC.

The larger problem is how you're trying to use the DC. It isn't meant to be permanent; you're supposed to acquire it, draw everything to it, then shut it down. When you try to make CPaintDC static, Windows will eventually shut it down and every attempt to use it thereafter will return an error.

The proper way is to save any coordinates that you need for all of the drawing you need to do. Use a combination of MoveTo and LineTo to draw individual line segments, and every time you reenter OnPaint you need to start over.

2012-04-03 21:15
by Mark Ransom

I do that when x==0 condition is met... you can see in the cod - jdl 2012-04-03 21:17

What do you mean "pen into the DC" ? th - jdl 2012-04-03 21:17

@jdl, create a CPen object and use dc.SelectObject to select it. You have code to do a MoveTo, but you need to do both at the same time. The MoveTo will be lost the next time OnPaint is called - Mark Ransom 2012-04-03 21:30

I am trying to draw a plot that constantly updates every 100ms. Unless I make "static CPaintDC dc(this);" it won't do that but just the once. But this is not good to do as the app will error when I try and close out. Also the graphics disappear if a hide the window and display it again. th - jdl 2012-04-03 21:46

From the OnTimer function I called OnPaint(). That was the problem I needed to do: CRect rect; GetClientRect(&rect); this->InvalidateRect(rect, true);

Using the "pens" has helped... thank - jdl 2012-04-03 22:22

I am not answering your question, but have you noticed that CDialog::OnPaint() will be called even if IsIconic() returns TRUE ?

I think you will need to use an extra pair of {} to solve this ;-)

2012-04-19 06:29
by Wartin