[Development] Respecting QGuiApplication::highDpiScaleFactorRoundingPolicy on Wayland

[Ilya Fedin]

On Wed, 2 Oct 2024 12:50:11 +0000
Morten Sørvig via Development <development at qt-project.org> wrote:

> Yep, so it depends on the point of view: is the rounding API for
> controlling a particular part of the Qt implementation, or for
> declaring application intent with regards to support for fractional
> scale factors. Now made relevant by recent changes to the Wayland
> platform.
> 
> I think we need the following:
> 
> 1) a proposal for getting to the current state to the new state (a
> QTBUG Task, or similar)
> 
> 2) a consensus that we do want to expand the usage of the
> RoundingPolicy etc APIs
> 
> As you can see from my previous reply and replies on gerrit I’m not
> completely convinced myself, but if there is a consensus in the Qt
> maintainer group that this is something we want to do then let’s go
> ahead.

Oof... I haven't ever created a QTBUG Task... Should it have something
special or just describe the problem as usual (as in bug report/feature
request)?

> I think applications which have their own scaling engine wold be
> quite small subset of all Qt apps? That use case could be supported
> but we should keep in mind that the majority of apps will rely on
> functionality provided by Qt and the native platform.

I believe it's quite a large subset of applications setting rounding
policy. It's not a rocket science after all: divide screen's
logicalDotsPerInch by 96 or 72 and multiply all the sizes/drawing by
the resulting factor. Qt had hidpi support opt-in and of quite a bad
quality for all Qt 5 time while the need to support hidpi was present
all that time so people were doing it the way they can.

GitHub shows quite a lot of results for logicalDotsPerInch as I was
mentioning earlier.