[Interest] High-dpi fixing for Qt 5.5

[Sorvig Morten]

> On 24 Mar 2015, at 14:26, Till Oliver Knoll <till.oliver.knoll at gmail.com> wrote:
> 
> 
> Am 19.03.2015 um 14:17 schrieb Sorvig Morten <Morten.Sorvig at theqtcompany.com>:
> 
>> Hi,
>> 
>> ...
>> 
>> In case there issues that I’ve missed I would be interested in hearing about it. I can’t promise that everything will be fixed, but it will be at least put on the radar.
> 
> Hi Morten,
> 
> Not a bug by itself and hence a bit off-topic, but is there a way to detect "screen resolution changes" in some way? The use case is of course when an application window is moved from a Retina (HiDPI) screen to a non-Retina screen (or vice versa) (in a multi-monitor setup).
> 
> My knowledge of available "HiDPI" APIs is currently based on your (excellent) blog post, which does not seem to mention that use case:
> 
>  http://blog.qt.io/blog/2013/04/25/retina-display-support-for-mac-os-ios-and-x11/
> 
> I could imagine that getting informed e.g. by the QDesktopWidget (or some other related "screen/resolution" class) whenever the application is "switched" (dragged) to another screen, and then querying the resolution of the new screen, could be a solution. However I currently do not see any corresponding signal in QDesktopWidget's API, nor anywhere else…

[I see there has been other replies which covers this, so there’s probably some duplicate information here.]

The current approach is that application windows will be repainted at screen change time; The window's devicePixelRatio() accessor will then return the correct value for the new screen and the app will draw at the correct pixel density. 

> 
> In the end I am not even sure whether I need to get informed about this event, since most drawing - and also the switch between Retina/non-Retina worlds - is handled for me already by Qt/underlying Core Graphics APIs. Even the Apple Developer docs mention that "Listening for NSWindowDidChangeBackingPropertiesNotification is something only a few apps [...] will need to do."
> 
> However my app probably /is/ one of those that "specialize in video or graphics work". I draw on a QGraphicsView(-scene) and have QGraphicsPixmapItems with a QPixmap behind them. I imagine that I have to be able to dynamically switch between the normal and the "@2x" version of those pixmaps, and have the QGraphicsView paint itself whenever such a "resolution change" is detected. Or so I imagine…

Pixmaps caches are indeed a special case: you need to factor in the target devicePixelRatio when creating the caches:

qreal dpr = window->devicePixelRatio(); // or painter->device()->devicePixelRatio()
QPixmap cache(size * dpr);
cache.setDevicePixelRatio(dpr); // iff you want scale any QPainters opened on the pixmap.

(this is a recurring pattern and may deserve its own API at some point)

> 
> So I am basically looking for a "cross-platform" way to get notified in analogy to the Cocoa notification
> 
>  NSWindowDidChangeBackingPropertiesNotification
> 
> See chapter "Handle Dynamic Changes in Window Resolution Only When You Must"
> 
> https://developer.apple.com/library/mac/documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/CapturingScreenContents/CapturingScreenContents.html#//apple_ref/doc/uid/TP40012302-CH10-SW1
> 

Looks like the desktop platforms (XCB, OS X and Windows) emit QWindow::screenChanged now. I think the best place to handle the pixel density change is still the repaint, that way you create new graphics at the correct time.

Morten