QWidget inheriter in QScrollArea : problem of scroll clipping

Mika-L

Dear All,
I am quite new to QT5 / QT5 for Python (here on Linux). I created in inheriter of QWidget which is placed inside a QScrollArea. In ma QWidget, I override the paintEvent an I draw stuff inside. It works well, and the things I paint follow the scrollbars. Good.
Now I want to draw an "overlay" element, which position does not follow the scrollbars (it is "fixed" even when I move the scrollbars). About the coordinates system, I played successfully I guess withmapFromParent to get the good coordinates. And it works but I need to resize the window (for example) to obtain the good result. If I only play with the scrollbars, I have only a part of the element that is painted, and sometimes it disappears too.

I think I have a problem with the clipping, but I did not understand what I should do to put the full viewport area as clipping area (or perhaps is there a better way to do this ?).

Can someone help ?

Thanks a lot,
Mike

SGaist

Hi,

Without any code, it's hard to tell. Would it be possible to see it ?

Depending on what you want to do, you might want to check the Graphics View Framework.

Mika-L

Sorry for the delay I was not notified about your kind message.
I wrote a simplified code that exhibits the same problem. First I made a global UI design in "designer". The part that intersts us is pictured here :

Then I wrote a "MImage.py" file that contains the following code (simplified a lot but working example) :

from PyQt5 import QtWidgets, QtCore, uic, QtGui

class MImage(QtWidgets.QWidget):
    def __init__(self, *arg):
        super().__init__()
        self.image = QtGui.QImage("path_to_some_picture_here..jpg")	
        self.MRedBrush = QtGui.QBrush(QtGui.QColor(200,0,0))    
 
    def paintEvent(self, event):
        self.resize(self.image.width(), self.image.height())  
        # below : not sure to understand why I need the parent of the parent widget, it is not similar as in Designer.
        self.parentWidget().parentWidget().setHorizontalScrollBarPolicy( QtCore.Qt.ScrollBarAsNeeded )
        self.parentWidget().parentWidget().setVerticalScrollBarPolicy( QtCore.Qt.ScrollBarAsNeeded )

        painter = QtGui.QPainter(self)
        # draw some scrollable stuff    	
        pixMap = QtGui.QPixmap.fromImage(self.appData.image)
        painter.drawPixmap(QtCore.QRect(0, 0, self.image.width(), self.image.height()), pixMap)

        # draw some non scrollable stuff  << == here is the problem
        topleft=self.mapFromParent(QtCore.QPoint(20,20))
        painter.fillRect(topleft.x(),topleft.y(),500,20,self.MRedBrush)

When I do this, I get this :
---> At program startup :

---> After a horizontal scroll :

---> After resizing a bit the window to force an repaint :

So it seems my code is correct regarding the drawing coordinates. However, during the scroll, it seems a full repaint is not performed. I guess it is due to clipping rectangle (I guess Qt uses hardware acceleration to bitmap copy the contents of a part of the view). So my question is : is there a way to enforce the clipping rectangle to the whole view for example (or to exclude the part that interests me from the clipping region), or is there a better design for this (perhaps superimposing two views, one for scroll, another one for static display).

Thanks a lot,
Mikhaël

SGaist

Why are you calling resize in your paintEvent ?

Accessing the parent of the parent is a bad idea and even more in the paintEvent. Accessing parent is already a bad idea.

Since that rectangle shall not move, why not put it in its own widget that you manually place at that fixed position ?

Mika-L

I am not a skilled user and I don't know how to add a widget superimposed to another one. Can you give some advice about it ?

I however found a solution. My interpretation was not correct, clipping does not seems to come from system areas to update. So what I did was the following : let's call scrollerwidget the scrollarea, I added an update to the event related to scrollers position change :
self.scrollerWidget.horizontalScrollBar().valueChanged.connect(self.update)
self.scrollerWidget.verticalScrollBar().valueChanged.connect(self.update)

It works. But I would like to know, as you suggest, how to add another widget superimposed ?

Thanks a lot,
Mike

mpergand

@Mika-L said in QWidget inheriter in QScrollArea : problem of scroll clipping:

how to add another widget superimposed ?

To add a fixed widget above the scroll area, create a new widget with the scroll area (image_area_scroller) as parent.
It will be at fixed position on the left top corner of the scroll area, regardless the scroll bars.

Mika-L

ok great ! I will try this. Thanks a lot !