[PyQt] QLabel with scaled contents pixmap not updated after painting

[Florian Bruhin]

Hey,

On Tue, Nov 26, 2019 at 01:47:40PM +0100, Maurizio Berti wrote:
> I noticed a strange behavior when drawing over an existing QPixmap of a
> QLabel.
> When scaledContents is false (the default), the image is updated correctly:
> 
> [...]
>
> But if I use setScaledContents(True) in the init, the image is not updated
> anymore, not even after calling repaint or hiding/showing the label again.
> So far, the only way I found to show the changes is to set the scaled
> contents to false and back again to true, right after closing the painter.
> 
> I have to specify that I'm using an old version of PyQt (5.7.1) which I
> cannot update, so maybe the issue has been already solved. Also, I'm on
> Linux, using xorg-x11.
> 
> Can anyone confirm this?

I can reproduce with Qt 5.14, but I don't think it necessarily is a Qt bug.

Looking at the code, turning on scaledContents resizes the underlying pixmap
once and then caches it:

https://github.com/qt/qtbase/blob/v5.14.0-beta3/src/widgets/widgets/qlabel.cpp#L1092-L1104

What's your actual usecase? You could probably either modify/redisplay the
pixmap, or perhaps have a widget with a transparent background over the QLabel
instead.

Florian

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