Re: display-mm-width return value off on Windows

[Jason Rumney <jasonr@gnu.org>]

David Kastrup wrote:
> And for that it is necessary that Emacs and/or Windows convert the
> points to pixels in a predictable way, and the display geometry is the
> only pointer we have for that.
>   

 Windows does not use the display geometry to convert points to pixels 
when it displays fonts. It uses the display geometry to decide whether 
to default to 96dpi or 120dpi (where i is a "logical inch" in Microsoft 
terminology, not necessarily a physical inch), then uses the dpi setting 
(which can be further customized by the user) to do its conversion 
between points and pixels, while continuing to report the physical size 
of the monitor in the functions that manage that.

I guess display-pixels-per-inch should report the dpi setting from the 
OS. Currently it seems to be hardcoded as 72.0, and the user needs to 
manually change it to match reality. At least on Windows, it is possible 
to read the system settings to get a more accurate value, but even if 
this is not possible on other window systems, 96.0 might be a better 
default these days.