[wayland HiDPI support, posible regression?]

[microcai]

on Monday 09 March 2015 08:59:35，Jasper St. Pierre wrote：
> You misunderstood what pq said.
> 
> You work in "logical pixels". On a hi-DPI display, with twice the pixel
> density, each "logical pixel" is backed by 4 device pixels. Your actual
> backing surface containing the pixels is in "device pixels".

assume we design for 72DPI screen, then , yes, we are working on logical 
pixels even when we use  12point font, because 1point=1pixel on 72DPI screen.

then the point became the "logical pixel".

Just look at android, we have 4" screen range from 320p to 1080p, they don't 
really do 2x scale. but people still expect the *same* UI size.

if you force only 2x scale, the you are lost in the real world. you simply can 
not run on 1080p screen when you have your app design for 720p screen, even 
then they share the same physical size, and people expect the same UI element 
sizes on the same physical sized screen.

> 
> So, if you have an window that is 800x600 logical pixels, when you drag it
> to a low-DPI display, your backing buffer is 800x600 device pixels. When
> you drag it to a hi-DPI display, your backing buffer is 1600x1200 device
> pixels.
> 
> When you submit your buffer, you tell the compositor what the ratio of
> device pixels to logical pixels is, in the perhaps misnamed
> "set_buffer_scale" request. So, for the low-DPI buffer, you submit "1",
> since 800x600 / 800x600 = 1. When you submit your hi-DPI buffer, you submit
> "2", since 1600x1200 / 800x600 = 2.
> 
> If you submit "1" and you are on a low-DPI display, it all matches and
> things are fine. If you submit "1" and you are on a hi-DPI display, the
> compositor scales up your buffer to match. If you submit "2" and you are on
> a hi-DPI display, it all matches up and things are fine. If you submit "2"
> and you are on a low-DPI display, the compositor scales down your buffer to
> match.
> 
> On Mon, Mar 9, 2015 at 6:19 AM, microcai <microcai at fedoraproject.org> wrote:
> > 在 2015年3月9日 Monday 10:39:20，您写道：
> > 
> > > Hi,
> > > 
> > > On Monday, March 9, 2015, microcai <microcai at fedoraproject.org> wrote:
> > > > so we are forcing two code pathes in APP? or just support non-integer
> > > > scale?
> > > 
> > > Scaling is a specific fallback for legacy/non-DPI-aware clients, to get
> > > them to show anything at all. Other clients will render at the
> > 
> > appropriate
> > 
> > > resolution already.
> > 
> > That's not the story I was told. pg says scale is the *only way* to
> > support
> > HiDPI.
> > 
> > > Any plan that relies on integer scaling means you're wasting pixels on
> > 
> > your
> > 
> > > lovely dense display. Any plan that relies on non-integer scaling makes
> > > your text blurry to the point of unreadable - again, a waste of pixels
> > > on
> > > such lovely displays. Just render at the correct scale to begin with.
> > > 
> > > Cheers,
> > > Daniel
> > 
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