[Development] What to expect from QIcon/the icon engine on screen changes

[Elvis Stansvik]

Den sön 7 apr. 2019 kl 18:45 skrev Olivier Goffart <olivier at woboq.com>:
>
> On 06.04.19 10:40, Elvis Stansvik wrote:
> > Hi all,
> >
> > I'm looking for someone who knows the inner workings of QIcon and the
> > icon engines.
> >
> > In our application, we almost exclusively use SVG icons, and we use a
> > single SVG file for each icon (no @2x versions) that we try to design
> > to work reasonably well at all sizes, since we do not have the
> > resources to make custom variations for each target size.
> >
> > We put these SVG icons into an XDG icon theme, that we ship inside the
> > application resources (.qrc) in the expected XDG layout and with an
> > icon theme index file. We can then use
> > QIcon::fromTheme("our-app-some-icon") to reference an icon (either
> > through the .ui file or in code).
> >
> > The problem we've run into is that when the application is launched in
> > a mixed-DPI setup, for example a retina Mac laptop with an external
> > lower-DPI monitor, the icons appear too large and get cropped. In
> > effect, it seems to always use the DPI of the primary screen (the
> > built-in retina screen) when calculating the size of the pixmaps it
> > generates for the icons.
>
>
> Not sure if this is your problem, but it seems that
> QSvgIconEngine::virtual_hook does not handle the QIconEngine::ScaledPixmapHook
> call, which is needed for the QIcon::pixmap(QWindow *window, ...) call.

Thanks Olivier,

I'm not familiar with the code, but it sounds like that could be it.

I'll try to make a minimal test case tomorrow.

Elvis

>
>
>
>
> >
> > To work around this, we've had to put in special code in all of our
> > widgets that use icons. The code reacts to screen change events (or
> > changes to the underlying QWindow in some cases), and in that code, go
> > through each and every one of our icons and do this monkey dance:
> >
> >          auto pixmap = someButton->icon().pixmap(someButton->iconSize());
> >          pixmap.setDevicePixelRatio(window()->windowHandle()->screen()->devicePixelRatio());
> >          someButton->setIcon(QIcon(pixmap));
> >
> > So essentially taking the pixmap out of the icon, set its DPR to that
> > of the current screen, and then set that pixmap back on the icon.
> >
> > This "works", the icons now look correct on both the retina screen and
> > the external one, and adjust themselves when the application is moved
> > back and forth, or when the external monitor is activated/deactivated.
> >
> > But surely this kludge should not be necessary? We've provided Qt with
> > an SVG, so it should be able to work out on its own that the screen
> > has changed, and regenerate an appropriate pixmap in response to that?
> >
> > Some more details:
> >
> > - We are running with the Qt::AA_UseHighDpiPixmaps application
> > attribute turned on. I'm not sure this is relevant for this issue
> > though, because AFAIK that attribute is only about picking 2x versions
> > of icons (we have a couple of those, where we have PNGs with 2x
> > versions).
> >
> > - We are not running with Qt's built-in scaling activated, but relying
> > on the Mac platform scaling (I'm not even sure Qt's built-in scaling
> > is applicable on Mac). The application runs with
> > NSHighResolutionCapable set to True in its Info.plist (which I also
> > think is the default nowadays).
> >
> > - I have not investigated yet whether this problem also occurs on a
> > mixed-DPI Linux setup, with Qt's high-dpi scaling activated. Nor have
> > I checked if it happens on Windows using it's artificial "screen
> > scaling" (we do not use Qt's built-in scaling on Windows either,
> > trying to follow the advise in the docs to avoid that for new
> > applications). So for now this is only about Mac retina + external
> > monitor.
> >
> > Any advise on this would be highly appreciated, because the code
> > required to re-trigger pixmap generation on screen changes is a real
> > kludge all over our code base, and often it happens that we add
> > buttons et.c. with icons, but forget to update this machinery.
> >
> > I'm not at the office at the moment, but can provide a little test
> > program that mimics what we're doing on Monday.
> >
> > Thanks in advance,
> > Elvis
> > _______________________________________________
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> > Development at qt-project.org
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> >
>
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