Different behaviors of GLWidget,Widget,Canvas,Scene when obscured by other windows

[yektaayduk]

Hello,

I tried different scenarios and here are the observations.

case-1: QT3.3.. Application has a Widget which has an openGLWidget , LineEdits and Buttons in its layout.
Part of the drawing is changing every 150 msec.

case-2: QT3.3.. Another Widget is created for zoomed view (but not added to the layout of the MainWidget).
I draw on this Widget in this way: Timer=>repaint=>paintEvent

case-3:QT3.3 ... Another Widget is created for zoomed view which has Canvas and Canvasview.
OCanvasPolygonItem is added to the Canvas.
I draw on this widget in this way Timer->canvas_update()=>update()=>drawShape of Item

case-4:QT 4.23..Another Widget is created for zoomed view which has Scene and Sceneview .
QGraphicItem is added to the Scene.
I draw on this widget in this way Timer->scene_update()=>update()=>paint of Item

When the openGLWidget in case-1 obscured by another window and uncovered again without any erased area.
The erased area repaired somehow without acccessing my drawing functions.

When the ZoomWidget in case-2 obscured by another window and uncovered again erased area appears.
System calls my PaintEvent function but I have nothing for recovery.

When the CanvasWidget in case-3 obscured by another window and uncovered again erased area appears.

When the SceneWidget in case-4 obscured by another window and uncovered again ,
erased area appears and don't call my paint function.



Has someone an explanation for these behaviors? Can You suggest a better way?

Thanks.

[wysota]

but what exactly is the problem?

[yektaayduk]

The problem is: I cant update the Zoom window entirely at once ,because it takes too long ,
other threads which requesting data from an signal processing board(actually from the kernel module of the same board kernel buffer overflows before I read it) and forwarding data via ethernet loosing data .
That's way I update the zoom window in time ,in small parts.Refreshing an erased part of the window obscured by another window, by my functions will take again too long.I need a solution without my effort,like i observed in opengl case (it recovers without calling my code ).Is this unique to opengl?

[wysota]

This is an X11 issue. I think that using extensions such as Damage, you could avoid redraws, but I'm not sure of that. I suggest you cache the result of the last draw in a pixmap and simply redraw the pixmap when the paint event comes in. Just make sure to regenerate the pixmap when the contents of the widget change.

[yektaayduk]

Thank You wysota

I will try .