Why EDID is not trustworthy for DPI

Adam Williamson awilliam at redhat.com
Wed Oct 5 02:05:38 UTC 2011

On Tue, 2011-10-04 at 13:54 -0400, Adam Jackson wrote:

> I'm going to limit myself to observing that "greatly" is a matter of
> opinion, and that in order to be really useful you'd need some way of
> communicating "I punted" to the desktop.
> Beyond that, sure, pick a heuristic, accept that it's going to be
> insufficient for someone, and then sit back and wait to get
> second-guessed on it over and over.

All this is interesting, but it basically consists of a long list of
reasons why the EDID info isn't always correct.

96dpi, however, is almost *never* correct, is it? So just taking a
hardcoded number that Microsoft happened to pick a decade ago is hardly
improving matters.

It still seems to me that taking the EDID number if it seems reasonably
plausible and falling back to 96dpi otherwise is likely a better option.

Your examples lean a lot on TVs and projectors, but are those really the
key use cases we have to consider? What about laptops and especially
tablets, whose resolutions are gradually moving upwards (in the laptop
case despite the underlying software problems, in the tablet case
because the underlying software doesn't have such a problem)? Is it
really a great idea, for instance, if we put Fedora 17 on a 1024x600, 7"
tablet and it comes up with zonking huge fonts all over the place?

I think it's worth considering that, even though Microsoft's crappiness
with resolution independence has probably hindered the market
artificially for a while, the 96dpi number which comes from the
capabilities of CRT tubes circa 1995 bears increasingly little
resemblance to the capabilities of modern displays, and assuming we can
just keep hardcoding 96dpi and monitor technology will remain
artificially retarded forever is likely not a great thing to do.
Adam Williamson
Fedora QA Community Monkey
IRC: adamw | Twitter: AdamW_Fedora | identi.ca: adamwfedora

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