bug#4547: a bug: emacs 23.1: very large font

[Stefan Monnier <monnier@iro.umontreal.ca>]

> While not technically a bug, it is unfortunate that Emacs uses a DPI value
> other than the rest of the applications.

The main problem I see is that Emacs tries to choose a default font such
that it will always have the same physical screen size.

This seemed like a good idea at some point in time, but experience shows
that with higher DPIs, the smoother curves of letters makes the text
more legible at the same physical display size, so people are happy to
use slightly smaller fonts (in other words, small fonts used to be
illegible not just because they were physically small but also because
there weren't enough pixels to make them legible).

So choosing a fixed physical size is not really a good idea.

Another reason why this is, is that what "really" matters is not the
dimension of text on the physical screen, but the "apparent size" seen
from the user, which is approximately "screen-size /
distance-from-the-eye".  And it turns out that the distance between the
eye and the screen depends on the device being used.

For all these reasons, the real physical DPI is not an ideal basis to
choose the default font size.  So using some kind of user-controlled
"perceived DPI" would be a much better choice.  IIUC the DPI provided by
Xsettings is a good choice for that.


        Stefan