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Re: Default heuristics for variable-format displays

On Tue, 2009-09-15 at 12:06 -0400, Adam Jackson wrote:
> In attempting to document how displays are expected to work in F12 [1],
> I realized we still don't have a decent heuristic for some cases.
> Broadly, displays are either fixed-format or variable-format.  FF means
> you have some set number of pixels, like an LCD.  VF means you don't,
> like a CRT.  (Projectors are often somewhere in between, we'll pretend
> they don't exist for a moment.)  We get FF displays pretty much right,
> since they tend to describe themselves well enough in EDID to figure out
> what their native size is.  Some VF displays are polite enough to define
> a preferred mode, and for that case we'll default to that.
> But, many VF displays don't define a preferred mode.  How are we to
> choose?  What's currently implemented will pick something along the
> lines of "the largest available mode that matches our guess at the
> physical aspect ratio and that fits in the card's DAC and memory
> bandwidth limits".  Which is awful.  So I'm thinking something like (in
> wretched pseudopython):
> def mode_dpi_cmp(x, y):
> 	return cmp(abs(x.dpi - 96), abs(y.dpi - 96))
> def mode_size_cmp(x, y):
> 	return cmp(x.width * x.height, y.width * y.height)
> def best_mode(modes, dpi_known = True):
> 	l = filter(lambda x: x.refresh >= 72, modes)
> 	if l == []:
> 		l = modes
> 	if dpi_known:
> 		l.sort(cmp=mode_dpi_cmp)
> 	else:
> 		l.sort(cmp=mode_size_cmp)
> 	return l[0]
> Which is _pretty_ good, except you'd kinda like to match aspect ratio if
> you happen to know AR but not DPI.  Which is trivial to add, but starts
> to be hard to read.
> If anyone has ideas I'm all ears, but I'd like to get this implemented
> sometime this week, so speak up.

I know that, in the dinosaur days of CRT, I could 'see' flicker (and get
flicker-generated headaches) at anything under 80Hz, and I know there
are even more sensitive people than that. So 72Hz may be a bit of a low
'safe refresh rate' cutoff. I'd like it to be 80 at least. 72/75 were
better than 65 for me, but definitely not acceptable for long-term work.

(/me remembers the days when you could gain any office worker's
everlasting friendship by changing their refresh rate from 60Hz to
85Hz...ahhh, good times.)

Adam Williamson
Fedora QA Community Monkey
IRC: adamw | Fedora Talk: adamwill AT fedoraproject DOT org

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