[Development] Bringing Qt's high-dpi support to more platforms

Sorvig Morten Morten.Sorvig at digia.com
Tue May 27 13:02:50 CEST 2014

On 27 May 2014, at 11:47, Daiwei Li <daiweili at gmail.com> wrote:

> Hi Morten,
> My understanding of your platform independent change was that it would let the scale factor be set by the user at runtime with an environment variable. Is it meant to replace all platform plugin scaling (I.e. iOS and OSX) or augment it?

The QtGui scaling is meant to augment platform (plugin) scaling in cases where the OS does not support it in a form we can use. Plugin in paranthesis because the platform plugin does not really need to do that much work if there is OS support.

> It seems to me that Android could indeed leverage the platform independent scaling code by setting QT_HIGHDPI_SCALE_FACTOR. However, it would mean different things on Android and iOS (relative to physical resolution and relative to scaled resolution) and if the Android plugin relied on the env variable and iOS kept its platform plugin scaling. Is being able to use QT_HIGHDPI_SCALE_FACTOR to scale relative to the scaled resolution a supported use case?

The effective scale factor (as accessed by QWindow::devicePixelRatio()) will be the product of the OS scale factor and QT_HIGHDPI_SCALE_FACTOR. Not a terribly common use case, but I’ve used it to test the QtGui scaling on my MacBook.

> I suppose another consideration would be setting the scale based on the monitor you're on. I don't know how developed Android's multi monitor support is, but if you wanted your application to change scale when it moved across different density displays, I imagine the platform plugin would need to do some work.

Exactly, I think we want to add API to make it possible to control the QtGui scaling per-window, in addition to QT_HIGHDPI_SCALE_FACTOR. This would be needed for the KDE use case as well. As it stands today the platform plugin can implement QPlatformWindow::devicePixelRatio(), but then Qt assumes the plugin or the OS handles the scaling. 

All in all I see three use cases:
- the platform plugin reports a scale factor when the OS scales.
- the developer sets QT_HIGHDPI_SCALE_FACTOR asking QtGui to scale (great for testing)
- the platform plugin/desktop environment sets a scale factor and asks QtGui to scale.

Screen changes on OS X works as follows: There is a screen change event. The platform plugin then sends an expose/repaint to Qt. The code handling the repaint will at some point call QWindow::devicePixelRatio(), which will then report the correct value for the new screen.

I’m not completely against adding scaling code to Android, but I would rather see the maintenance effort go to the cross-platofrm code. If it turns you we _do_ need the scaling code in the Android plugin for some reason, then yes let’s do that.


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