Problems with display-pixels-per-inch variable

Problems with display-pixels-per-inch variable

[Kim F. Storm]

In the C code, we have this variable, which can be used by the
calculation of strech glyph width and height:

  DEFVAR_LISP ("display-pixels-per-inch",  &Vdisplay_pixels_per_inch,
    doc: /* Pixels per inch on current display.
Value is a number or a cons (WIDTH-DPI . HEIGHT-DPI).  */);
  Vdisplay_pixels_per_inch = make_float (72.0);


There are two problems with this:

1) It is a fixed value, so if it is not correct, the user
   must set it to the proper value  (possible, but not nice).

2) There is only one common value for all displays.


Re. 1, I would imagine that - at least for some setups - it is
possible to get the proper value from the X-Server or OS.

Does anyone know how to do that?


Re. 2, I guess the proper thing would be store this in the
display structure of each display, and use a function to set
the values for a specific display (if the values cannot be
determined automatically).

I can do item 2, but if someone would like to fix item 1, I would
appreciate if you would fix item 2 as well...

-- 
Kim F. Storm <storm@cua.dk> http://www.cua.dk

Re: Problems with display-pixels-per-inch variable

[Jason Rumney]

Kim F. Storm wrote:

>In the C code, we have this variable, which can be used by the
>calculation of strech glyph width and height:
>
>  DEFVAR_LISP ("display-pixels-per-inch",  &Vdisplay_pixels_per_inch,
>    doc: /* Pixels per inch on current display.
>Value is a number or a cons (WIDTH-DPI . HEIGHT-DPI).  */);
>  Vdisplay_pixels_per_inch = make_float (72.0);
>
>  
>
The x_display_info structure has this information already, though it may 
be initialised in a similar arbritrary fashion. On W32, it is 
initialised from system settings. Anyway, if we must define it 
arbritrarily and leave it to the user to fix up later if they are 
motivated enough to do so, 96dpi is far more common than 72dpi these days.

Re: Problems with display-pixels-per-inch variable

[Thien-Thi Nguyen]

storm@cua.dk (Kim F. Storm) writes:

> Does anyone know how to do that?

that info can be found in 

 FRAME_X_DISPLAY_INFO (frame)->resx and 
 FRAME_X_DISPLAY_INFO (frame)->resy

but those are initialized later.  perhaps you can adjust
the var's value right before entering the command-loop.
fwiw, the default is pretty close to what i see locally
(76.2 for both width and height).

thi