Why EDID is not trustworthy for DPI

Simo Sorce simo at redhat.com
Thu Oct 6 16:00:36 UTC 2011

On Thu, 2011-10-06 at 16:46 +0100, Matthew Garrett wrote:
> On Thu, Oct 06, 2011 at 11:39:16AM -0400, Jon Masters wrote:
> > On Thu, 2011-10-06 at 16:20 +0100, Matthew Garrett wrote:
> > > On Thu, Oct 06, 2011 at 11:14:56AM -0400, Jon Masters wrote:
> > > 
> > > > How about "EDID as it exists today". Since you're able to so beautifully
> > > > explain what the pitfalls are, I'd assume you've raised this with the
> > > > VESA and asked that they revisit this in the future to accurately
> > > > provide DPI information that Operating Systems can rely on?
> > > 
> > > The specification provides everything needed to express this data 
> > > accurately, and proves the worth of specifications by virtue of 
> > > approximately nobody actually implementing it correctly.
> > 
> > How about an actual DPI value?
> The DPI depends on the mode. Not all the world is an LCD, and even there 
> it depends on whether you're native, scaling or scaling with constrained 
> aspect ratio.

My main use case here is video projectors, and in that case there is no
way on earth you'll ever know the DPI as it depends on the distance from
the wall, and again even if you knew the distance from the wall you'd
know nothing because the optimal DPI will also depend on the distance of
the crowd from the wall.

So in that case you really should just give an option to the user to
easily change DPI (no need to call the option 'DPIs', it can be a slider
with no mention of DPI if you prefer) *if* it is needed.
Chances are that a much wider font resulting from high density primary
display derived DPI number combined with low resolution video projector
screen will show big fonts and that will happen to be *exactly* what you
want so the guy back there at the end of the room has a chance to
actually read something :)

Then there will be guy X that hates the big fonts or has a ridiculously
low DPI primary screen and he'll not be happy. You cannot please
everyone with a default, but you can with an easy to discover option.

So whatever is the situation the slider should be right there in the
tool you use to manage the additional monitors or people will be forced
to search and find the menu where he can change "something" to try to
get a better font size and all resulting in poor experience as that menu
is normally well hidden as it is a rarely used option normally.


Simo Sorce * Red Hat, Inc * New York

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