How can I get updated system DPI information from X11 in a C program?

Question

I'm trying to create a DPI aware app which responds to user requested DPI change events by resizing the window.

The program in question is created in C and uses SDL2, however to retrieve system DPI information I use xlib directly, as the SDL DPI support in X11 is lacking.

I found two ways to get the correct DPI information on program startup, both involving getting Xft.dpi information from Xresource: one is to use XGetDefault(display, "Xft", "dpi"), while the other is to use XResourceManagerString, XrmGetStringDatabase and XrmGetResource. Both of them return the correct DPI value when the program is created.

The problem is, if the user changes the system scale while the program is running, both XGetDefault abd XrmGetResource still return the old DPI value even though when I run "xrdb -query | grep Xft.dpi" the value has indeed changed.

Does anyone know a way to get the updated Xft.dpi value?

Answer 1

First off it must be noted that the value of the Xft.dpi resource isn't necessarily accurate -- it depends on whether the system and or user login scripts have correctly set it or not.

Also it is important to remember that the Xft.dpi resource is intended to be used by the Xft library, not by arbitrary programs looking for the screen resolution.

The Xft.dpi resource can be set as follows. This example effectively only deals with a display with a single screen, and note that it uses xdpyinfo. This also shows how it might not be exact, but could be rounded. Finally this example shows calculation of both the horizontal and vertical resolution, but Xft really only wants the horizontal resolution:

SCREENDPI=$(xdpyinfo | sed -n 's/^[ ]*resolution:[ ]*\([^ ][^ ]*\) .*$/\1/p;//q')
SCREENDPI_X=$(expr "$SCREENDPI" : '\([0-9]*\)x')
SCREENDPI_Y=$(expr "$SCREENDPI" : '[0-9]*x\([0-9]*\)')
# N.B.:  If true screen resolution is within 10% of 100DPI it makes the most
# sense to claim 100DPI to avoid font-scaling artifacts for bitmap fonts.
if expr \( $SCREENDPI_X / 100 = 1 \) \& \( $SCREENDPI_X % 100 \<= 10 \) >/dev/null; then
    FontXDPI=100
fi
if expr \( $SCREENDPI_Y / 100 = 1 \) \& \( $SCREENDPI_Y % 100 \<= 10 \) >/dev/null; then
    FontYDPI=100
fi
echo "Xft.dpi: ${FontYDPI}" | xrdb -merge

I really wish I knew why Xft didn't at least try to find out the screen's resolution itself instead of relying all of the time on its "dpi" resource being set, but I've found that the current implementation only uses the resource setting, so something like the above is actually always necessary to set the resource properly (and further one must also make sure the X Server itself has been properly configured with the correct physical screen dimensions).

From a C program you want to do just what xdpyinfo itself does and skip all the nonsense about Xft's resources. Here's the xdpyinfo code paraphrased:

Display *dpy;
dpy = XOpenDisplay(displayname);
for (scr = 0; scr < ScreenCount(dpy); scr++) {
    int xres, yres;

    /*
     * there are 2.54 centimeters to an inch; so there are 25.4 millimeters.
     *
     *     dpi = N pixels / (M millimeters / (25.4 millimeters / 1 inch))
     *         = N pixels / (M inch / 25.4)
     *         = N * 25.4 pixels / M inch
     */
    xres = ((((double) DisplayWidth(dpy, scr)) * 25.4) /
        ((double) DisplayWidthMM(dpy, scr))) + 0.5;
    yres = ((((double) DisplayHeight(dpy, scr)) * 25.4) /
        ((double) DisplayHeightMM(dpy, scr))) + 0.5;
}
XCloseDisplay(dpy);

Note also that if you are for some odd reason scaling your whole display (e.g. with xrandr), then you should want the fonts to scale equally with everything else. It's just a horrible bad hack to use whole-screen scaling to scale just the fonts, especially when for most things it's simpler to just tell the application to use properly scaled fonts that will display at a constant on-screen point size (which is exactly what Xft uses the "dpi" resource to do). I'm guessing Ubuntu does something stupid to change the screen resolution, e.g. using xrandr to scale up the apparent size of icons and other on-screen widgets without applications having to know about screen size and resolution, then it has to lie to Xft by rewriting the Xft.dpi resource.

Note that if you avoid whole-screen scaling then applications that don't use Xft can still get proper font scaling by correctly requesting a properly scaled font, i.e. even with bitmap fonts you can get them scaled to the proper physical on-screen size by using the screen's actual resolution in the font-spec. E.g. continuing from the above shell fragment:

# For pre-Xft applications we can specify physical font text sizes IFF we also tell
# it the screen's actual resolution when requesting a font.  Note the use of the
# rounded values here.
#
DecentDeciPt="80"
DecentPt="8"
export DecentDeciPt DecentPt
#
# Best is to arrange one's font-path to get the desired one first, but....
# If you know the name of a font family that you like and you can be sure
# it is installed and in the font-path somewhere....
#
DefaultFontSpec='-*-liberation mono-medium-r-*-*-*-${DecentDeciPt}-${FontXDPI}-${FontYDPI}-m-*-iso10646-1'
export DefaultFontSpec
#
# For Xft we have set the Xft.dpi resource so this allows the physical font size to
# be specified (e.g. with Xterm's "-fs" option) and for a decent scalable font
# to be chosen:
#
DefaultFTFontSpec="-*-*-medium-r-*-*-*-*-0-0-m-*-iso10646-1"
DefaultFTFontSpecL1="-*-*-medium-r-*-*-*-*-0-0-m-*-iso8859-1"
export DefaultFTFontSpec DefaultFTFontSpecL1

# Set a default font that should work for everything
#
eval echo "*font: ${DefaultFontSpec}" | xrdb -merge

Finally here's an example of starting an xterm (that's been compiled to use Xft) with the above settings (i.e. the Xft.dpi resource and the shell variables above) to show text at physical size of 10.0 Points on the screen:

xterm -fs 10 -fa $DefaultFTFontSpec