[Development] Qt high-dpi support update

[Morten Johan Sørvig]

A couple of weeks ago (some of) the involved parties discussed the road ahead here at the Oslo office. This mail is a summary of the outcome.

The story so far:

High-dpi support is implemented for Qt on OS X and iOS. This is the “devicePixelRatio” scaling mode. For 5.4 a patch was proposed that would enable Qt to re-use the devicePixelRatio scaling implementation on other platforms. This patch was not merged. Instead, the Xcb and Windows platform plugins implemented the necessary functionality, which can now be enabled by setting the QT_DEVICE_PIXEL_RATIO environment variable.

Meeting summary:

* Qt Quick will operate in device pixels and get one or more “device independent” units (pt, cm, mm, inches, …) 

* We will look at the cross-platform devicePixelRatio patch again, with the intent to merge it for 5.5.

* There are no plans to introduce a new unit to QtWidgets, here devicePixelRatio is the only option.

This is a bit brief. We/I can go onto more details, if there are questions.

Morten