Why EDID is not trustworthy for DPI

Jon Masters jonathan at jonmasters.org
Thu Oct 6 15:14:56 UTC 2011

On Tue, 2011-10-04 at 13:54 -0400, Adam Jackson wrote:
> On Tue, 2011-10-04 at 11:46 -0400, Kaleb S. KEITHLEY wrote:
> > Grovelling around in the F15 xorg-server sources and reviewing the Xorg 
> > log file on my F15 box, I see, with _modern hardware_ at least, that we 
> > do have the monitor geometry available from DDC or EDIC, and obviously 
> > it is trivial to compute the actual, correct DPI for each screen.
> I am clearly going to have to explain this one more time, forever.
> Let's see if I can't write it authoritatively once and simply answer
> with a URL from here out.  (As always, use of the second person "you"
> herein is plural, not singular.)
> EDID does not reliably give you the size of the display.

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?


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