Dialog Redraw vs Control Redraw - flicker reduction

[Gary Beene]

I'm just beginning to look at controlling flicker when resizing a dialog.

I wrote a simple app with two image controls and put a picture in each.

Then I used %WM_SIZE to trigger resizing the controls. The resizing works fine but the use of Dialog Redraw gives MUCH more noticeable flicker than using a redraw for each of the two controls I placed on the dialog.

          Dialog Redraw hDlg
vs
          Control ReDraw hDlg, 110
          Control ReDraw hDlg, 120

This is the opposite of what Help says - which is that Control Redraw schedules a redraw whereas Dialog Redraw gives immediate results.

Is there a general rule about using redraw commands, or more likely a body of experience on the topic to get flicker free resizing of controls on a dialog.

I did a brief forum search on flicker, but most posts didn't seem to address the question, although I must admit I didn't search long and hard.

Here's the whole example, if it's needed. I used one of my PBR files to get an image for the imagex controls.

   #Compile Exe
   #Include "Win32API.inc"
   #Resource "pb-test.pbr"
   Global hDlg As Dword, hLst As Dword, hTemp As Dword
   Global style&, extstyle&
   Function PBMain() As Long
    Dialog New Pixels, 0, "Graphic Control Test",300,300,450,380, _
                              %WS_OverlappedWindow, 0 To hDlg

    style& = %WS_Visible Or %SS_Notify
    Control Add ImageX, hDlg, 110,"cowgirl", 0,0,200,300, style&
    Control Add ImageX, hDlg, 120,"cowgirl", 210,0,200,300, style&
    Dialog Show Modal hDlg Call DlgProc
   End Function

   CallBack Function DlgProc() As Long
      If Cb.Msg = %WM_Size Then
          Dim w As Long, h As Long   'width/height of dialog
          Dialog Get Client hDlg To w,h

          Control Set Size hDlg, 110, w/2-4, h

          Control Set Size hDlg, 120, w/2-4, h
          Control Set Loc hDlg, 120, w/2+4, 0

'          Dialog ReDraw hDlg
          Control ReDraw hDlg, 110
          Control ReDraw hDlg, 120
      End If
    End Function

[Elias Montoya]

Hello Gary. Try using WS_CLIPCHILDREN in the creation of your dialog, flicker will be gone.

[Eros Olmi]

I think Elias already gave you the right indication: use %WS_CLIPCHILDREN style in your DIALOG NEW ... statement. It is a simple solution to many flickering problems.

Also, I would suggest to avoid to redraw your controls (or dialog) in response to a %WM_SIZE event. Thare is almost always another way to do it.

Ciao
Eros

[Chris Holbrook]

Also, I would suggest to avoid to redraw your controls (or dialog) in response to a %WM_SIZE event.

Why should this be avoided Eros?

[Eros Olmi]

Mainly to avoid flickering.

When you resize a control, a redraw message is already sent to you control. The same when you resize a dialog. So if you fire another redraw you are just redrawing all twice.

I'm not an expert of MS documentation. I just say it on top of my personal experience so I can be wrong.

[Chris Holbrook]

Mainly to avoid flickering.

When you resize a control, a redraw message is already sent to you control. The same when you resize a dialog. So if you fire another redraw you are just redrawing all twice.

Thanks Eros!

[Dave Biggs]

Flicker and the use of WS_CLIPCHILDREN described here - http://www.catch22.net/content/flicker-free-drawing

[Gary Beene]

Elias,

Thanks for the tip. I tried it and it is much better, with both the Dialog and Control redraws. I read the Help entry on it, but since I wasn't using %WS_THICKFRAME - which Help gave as a possible complicating factor, I didn't give it a try.

Eros,

I also saw that warning in the PB Help file, but when I did it anyway nothing bad seemed to happen. When I saw the warning I assumed the problem would have been an unending series of redraw, but didn't see it. Why would it only be 2, as your comment noted? I guess I could test it easily enough with a static counter, unless there are timing issues which make the answer different every time. Especiially with multiple redraws, I can see why flickering would be a risk.

If I'm using the mouse to resize the dialog, and want the image updated realtime, what message other than %WM_SIZE is available that I should use to REDRAW? If I wait for a mouseup, then I don't get a realtime update to the dialog content.

Dave,
Thanks for the link. I did a quick read and it has good information. I'll go back and absorb the details.

[Gary Beene]

Actually, I was using %WS_OVERLAPPEDWINDOW, whose value includes %WS_THICKFRAME. So Help actually gave me a clue to a fix if I had just understood it!