Why EDID is not trustworthy for DPI

Bob Arendt rda at rincon.com
Wed Oct 5 19:49:40 UTC 2011

On 10/05/11 12:31, Adam Williamson wrote:
> Like I replied to ajax, I suspect when the problem of assuming
> everything's 96dpi becomes simply too acute, instead of fixing
> everything really properly so that all displays correct report their
> size and all desktops actually do resolution independence perfectly so
> it doesn't_matter_  if one of your displays is 98dpi and the other is
> 215dpi, everything still looks perfect, the industry will just wind up
> with a slightly more sophisticated bodge where we have a few 'standard'
> resolutions and just figure out which one your displays are closest to.
> But that's still going to require some kind of sensible handling of the
> case where one monitor is roughly 100dpi and the other is roughly
> 200dpi, unless we simply say 'you can't do that, all your displays have
> to be in the same DPI Category'.

Good point Adam.  Even if the Xserver correctly intuits the resolution
of each display, application behavior is going to be unacceptable.
Consider dragging a window from a 200 dpi display to a 100 dpi display.
Does the application detect this and correctly re-scale it's window
and interior widgets?  If the Xserver re-scales the font, how does
the app detect the change in bounding box pixel geometry?
How's the app supposed to behave if the window straddles monitors?

Unless all the graphic toolkits are significantly redesigned, there's
no nice way to operate in Xinerama with mis-matched montiors.

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