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Re: bug#45596: 12.2.4; Wrong DPI calculation for mixed-DPI multi-monitor

From: Ikumi Keita
Subject: Re: bug#45596: 12.2.4; Wrong DPI calculation for mixed-DPI multi-monitor setups
Date: Thu, 16 Jun 2022 01:47:10 +0900

Hi Tassilo,

>>>>> Tassilo Horn <> writes:
> Grzegorz Kowzan <> writes:
> Hi Grzegorz,

>>> I guess the problem is that mm-size in the `frame-monitor-attributes'
>>> return value differs from `display-mm-height' and `display-mm-width'.
>> 1) Yes, `display-pixel/mm-width/height` functions are defined in terms
>> of Xlib calls, whereas `frame-monitor-attributes` calls Gdk functions
>> (if Emacs was compiled with Gtk support) and only as a fallback it
>> calls Xlib. This is why we have this inconsistency. The standard DPI
>> for Xorg server is 96 regardless of the actual value, so for a given
>> resolution and assumed DPI Xorg gives fake mm width and
>> height. Essentially, the current calculation of DPI in auctex looks
>> pointless to me...

> Ok, I see.

>> With pure Gtk port we get actual physical pixel width/height and
>> actual mm width/height, so I guess when auctex detects pgtk port, it
>> can either ignore these values and hardcode DPI of 96 or make use of
>> the actual DPI. I suppose there is something to be said for not
>> changing auctex's behaviour and hardcoding the value, but I would
>> prefer to use the actual DPI at least as an option (see below).

> That's what I did.  Essentially I've applied your patch with an
> additional fboundp check for `frame-monitor-attributes'.  If it's
> available (24.4), then your variant is used, if not, the previous
> variant basically always saying 96 DPI is used.

Isn't it a good time to carry out this TODO in ;-)
| (defun preview-get-dpi ()
|   ;; TODO: Remove false-case when required emacs version is bumped to
|   ;; 24.4 or newer as this is the version where
|   ;; `frame-monitor-attributes' has been introduced.
|   (if (fboundp 'frame-monitor-attributes)
|       (let* ((monitor-attrs (frame-monitor-attributes))
|              (mm-dims (cdr (assoc 'mm-size monitor-attrs)))
|              (mm-width (nth 0 mm-dims))
|              (mm-height (nth 1 mm-dims))
|              (pixel-dims (cdddr (assoc 'geometry monitor-attrs)))
|              (pixel-width (nth 0 pixel-dims))
|              (pixel-height (nth 1 pixel-dims)))
|         (cons (/ (* 25.4 pixel-width) mm-width)
|               (/ (* 25.4 pixel-height) mm-height)))
|     (cons (/ (* 25.4 (display-pixel-width))
|              (display-mm-width))
|           (/ (* 25.4 (display-pixel-height))
|              (display-mm-height)))))

And I'd like to comment on two minor aspects.
1. The function `cdddr' used above isn't yet available for emacs 25.1,
   so it should be replaced with `cl-cdddr'. I noticed this thanks to a
   byte compile log which Michael Braun sent me off list. Thank you,
   Michael! (And this means that preview-latex didn't work at all for
   emacs 25.1 for this 1.5 years!)

2. Though bug#20171[1] is still open, I hope it has already been
   resolved as well with this bug#45596. If so, let's close it, too.

Ikumi Keita
#StandWithUkraine #StopWarInUkraine


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