gcc -Wall -g unity-scale-factor.c `pkg-config --cflags --libs gio-2.0 xrandr x11` -lm -o unity-scale-factor

unity-scale-factor.c

/* -*- coding: utf-8; indent-tabs-mode: nil; tab-width: 4; c-basic-offset: 4; -*- */
/**
 * Copyright (C) 2014 Shih-Yuan Lee (FourDollars) <fourdollars@gmail.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include <stdio.h>
#include <math.h>
#include <gio/gio.h>
#include <X11/extensions/Xrandr.h>

static GVariant*
add_dict_entry (GVariant *dict, const char *key, int value)
{
        GVariantBuilder builder;
        GVariantIter iter;

        const gchar *k;
        guint32 v;

        g_variant_builder_init (&builder, G_VARIANT_TYPE ("a{si}"));
        g_variant_iter_init (&iter, dict);

        while (g_variant_iter_next (&iter, "{&si}", &k, &v))
        {
                if (!g_str_equal (k, key))
                        g_variant_builder_add (&builder, "{si}", k, v);
        }
        g_variant_builder_add (&builder, "{si}", key, value);

        return g_variant_builder_end (&builder);
}

int main(int argc, char* argv[])
{
    int i;
    Display *display = XOpenDisplay (NULL);

    if (!display)
        return 0;

    Window window = RootWindow (display, XDefaultScreen (display));

    XRRScreenResources *resources = XRRGetScreenResourcesCurrent (display, window);

    if (!resources)
        return 0;

    GSettingsSchemaSource *source   = g_settings_schema_source_get_default ();
    GSettingsSchema       *schema   = g_settings_schema_source_lookup (source, "com.ubuntu.user-interface", FALSE);
    GSettings             *settings = g_settings_new ("com.ubuntu.user-interface");
    GVariant              *dict     = g_settings_get_value (settings, "scale-factor");

    for (i = 0; i < resources->noutput; ++i) {
        XRROutputInfo *output = XRRGetOutputInfo (display, resources, resources->outputs[i]);

        if (output->connection == RR_Disconnected) {
            XRRFreeOutputInfo (output);
            continue;
        }

        if (output->crtc) {
            XRRCrtcInfo *crtc        = XRRGetCrtcInfo (display, resources, output->crtc);
            double       diagonal    = sqrt (crtc->width * crtc->width + crtc->height * crtc->height);
            double       diagonal_mm = sqrt (output->mm_width * output->mm_width + output->mm_height * output->mm_height);
            int          ppi         = (int) floor (diagonal * 25.4 / diagonal_mm);
            int          scale       = 8;

            if (ppi >= 192 && crtc->height >= 1200)
                scale = 16;

            GVariant *new_dict = add_dict_entry (dict, output->name, scale);
            g_variant_unref (dict);
            dict = new_dict;

            XRRFreeCrtcInfo (crtc);
        }
        XRRFreeOutputInfo (output);
    }

    g_settings_set_value (settings, "scale-factor", dict);
    g_settings_apply (settings);
    g_settings_sync ();

    g_object_unref (settings);
    g_settings_schema_unref (schema);

    XRRFreeScreenResources (resources);
    XCloseDisplay (display);

    return 0;
}

/* vim:set fileencodings=utf-8 tabstop=4 expandtab shiftwidth=4 softtabstop=4: */