Achieving Integer Scaling through GPU composition

[Pekka Paalanen]

On Wed, 22 Jan 2020 11:27:00 +0530
uday kiran pichika <udaykiran.pichika at gmail.com> wrote:

> On Tue, Jan 21, 2020 at 7:35 PM Pekka Paalanen <ppaalanen at gmail.com> wrote:
> 

> > Right. Expecting a compositor to show your window unchanged is somewhat
> > reasonable, but adding protocol to control the details of how a
> > compositor should composite your surfaces becomes awkward fast on the
> > usual desktop.
> >
> > I say somewhat reasonable, because things like fractional scaling,
> > referring to HiDPI support with non-integer scaling factors, is a thing
> > and happens mostly behind the client's back.
> >  
> 
> When referrring to fractional scaling for HiDPI screens, i think wayland is
> already providing the support as per the below link. Not very sure whether
> this is implemented in weston compositor.
> https://wiki.archlinux.org/index.php/HiDPI
> 
> can you please elaborate on the below line from your comment "happens
> mostly behind the client's back" ? do you mean that the client does not
> aware how the scaling is happening down the line ?

Wayland does not support fractional scaling. Wayland core protocol only
supports integer output and buffer scale factors.

Fractional scaling is implemented in the compositors that want to do
it, like GNOME (Mutter). Clients are largely unaware that it is
happening, they only see the integer output scale the compositor is
telling them. What actually happens then is that clients draw in
scale=1 or scale=N, and then the compositor automatically scales that
to achieve the fractional scaling factor compared to scale=1. This
necessarily causes some aliasing or blurring, but some people prefer
particular size over quality so they want it. This is common when using
monitors with "middle-sized" pixels-per-inch (a solution is to not buy
such monitors in the first place).

There is an optional unstable protocol extension xdg-output that
delivers some information to reveal fractional scaling, but that is
intended mainly for Xwayland due to Xwayland needing to know about the
compositor global coordinate system so that it can translate to/from X11
coordinates.


Thanks,
pq
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