x11: pseudo HiDPI scaling

Scale the window by the assumed DPI scaling factor, using 96 DPI as
base. For example, a screen that reports 192 DPI is assumed to have a
DPI scale factor 2. The window will then be created with twice the size.

For robustness reasons, we accept only integer DPI scales between 1 and
9. We also error out if the X and Y scales are very different, as this
most likely indicates a multiscreen system with botched size reporting.

I'm not sure if reading the X server's DPI is such a good idea - maybe
the Xrdb "Xft.dpi" value should be used instead. The current method
follows what xdpyinfo does.

This can be disabled with --hidpi-window-scale=no.

DOCS/man/options.rst

@@ -2343,7 +2343,7 @@ Window
         - ``--monitoraspect=16:9`` or ``--monitoraspect=1.7777``
 
 ``--hidpi-window-scale``, ``--no-hidpi-window-scale``
-    (OS X only)
+    (OS X and X11 only)
     Scale the window size according to the backing scale factor (default: yes).
     On regular HiDPI resolutions the window opens with double the size but appears
     as having the same size as on none-HiDPI resolutions. This is the default OS X

video/out/win_state.c

@@ -67,13 +67,17 @@ static void apply_autofit(int *w, int *h, int scr_w, int scr_h,
 // Compute the "suggested" window size and position and return it in *out_geo.
 // screen is the bounding box of the current screen within the virtual desktop.
 // Does not change *vo.
+//  screen: position of the screen on virtual desktop on which the window
+//          should be placed
+//  dpi_scale: the DPI multiplier to get from virtual to real coordinates
+//             (>1 for "hidpi")
 // Use vo_apply_window_geometry() to copy the result into the vo.
 // NOTE: currently, all windowing backends do their own handling of window
 //       geometry additional to this code. This is to deal with initial window
 //       placement, fullscreen handling, avoiding resize on reconfig() with no
 //       size change, multi-monitor stuff, and possibly more.
-void vo_calc_window_geometry(struct vo *vo, const struct mp_rect *screen,
-                             struct vo_win_geometry *out_geo)
+void vo_calc_window_geometry2(struct vo *vo, const struct mp_rect *screen,
+                              double dpi_scale, struct vo_win_geometry *out_geo)
 {
     struct mp_vo_opts *opts = vo->opts;
 
@@ -86,12 +90,15 @@ void vo_calc_window_geometry(struct vo *vo, const struct mp_rect *screen,
     if (vo->params)
         params = *vo->params;
 
+    if (!opts->hidpi_window_scale)
+        dpi_scale = 1;
+
     int d_w, d_h;
     mp_image_params_get_dsize(&params, &d_w, &d_h);
     if ((vo->driver->caps & VO_CAP_ROTATE90) && params.rotate % 180 == 90)
         MPSWAP(int, d_w, d_h);
-    d_w = MPCLAMP(d_w * opts->window_scale, 1, 16000);
-    d_h = MPCLAMP(d_h * opts->window_scale, 1, 16000);
+    d_w = MPCLAMP(d_w * opts->window_scale * dpi_scale, 1, 16000);
+    d_h = MPCLAMP(d_h * opts->window_scale * dpi_scale, 1, 16000);
 
     int scr_w = screen->x1 - screen->x0;
     int scr_h = screen->y1 - screen->y0;
@@ -118,6 +125,12 @@ void vo_calc_window_geometry(struct vo *vo, const struct mp_rect *screen,
         out_geo->flags |= VO_WIN_FORCE_POS;
 }
 
+void vo_calc_window_geometry(struct vo *vo, const struct mp_rect *screen,
+                             struct vo_win_geometry *out_geo)
+{
+    vo_calc_window_geometry2(vo, screen, 1.0, out_geo);
+}
+
 // Copy the parameters in *geo to the vo fields.
 // (Doesn't do anything else - windowing backends should trigger VO_EVENT_RESIZE
 //  to ensure that the VO reinitializes rendering properly.)

video/out/win_state.h

@@ -25,6 +25,8 @@ struct vo_win_geometry {
 
 void vo_calc_window_geometry(struct vo *vo, const struct mp_rect *screen,
                              struct vo_win_geometry *out_geo);
+void vo_calc_window_geometry2(struct vo *vo, const struct mp_rect *screen,
+                              double dpi_scale, struct vo_win_geometry *out_geo);
 void vo_apply_window_geometry(struct vo *vo, const struct vo_win_geometry *geo);
 
 #endif

video/out/x11_common.c

@@ -544,6 +544,7 @@ int vo_x11_init(struct vo *vo)
         .screensaver_enabled = true,
         .xrandr_event = -1,
         .wakeup_pipe = {-1, -1},
+        .dpi_scale = 1,
     };
     vo->x11 = x11;
 
@@ -595,6 +596,22 @@ int vo_x11_init(struct vo *vo)
                x11->ws_width, x11->ws_height, dispName,
                x11->display_is_local ? "local" : "remote");
 
+    int w_mm = DisplayWidthMM(x11->display, x11->screen);
+    int h_mm = DisplayHeightMM(x11->display, x11->screen);
+    double dpi_x = x11->ws_width * 25.4 / w_mm;
+    double dpi_y = x11->ws_height * 25.4 / h_mm;
+    double base_dpi = 96;
+    if (isfinite(dpi_x) && isfinite(dpi_y)) {
+        int s_x = lrint(MPCLAMP(dpi_x / base_dpi, 0, 10));
+        int s_y = lrint(MPCLAMP(dpi_y / base_dpi, 0, 10));
+        if (s_x == s_y && s_x > 1 && s_x < 10) {
+            x11->dpi_scale = s_x;
+            MP_VERBOSE(x11, "Assuming DPI scale %d for prescaling. This can "
+                       "be disabled with --hidpi-window-scale=no.\n",
+                       x11->dpi_scale);
+        }
+    }
+
     x11->wm_type = vo_wm_detect(vo);
 
     x11->event_fd = ConnectionNumber(x11->display);
@@ -1604,7 +1621,7 @@ void vo_x11_config_vo_window(struct vo *vo)
     vo_x11_update_screeninfo(vo);
 
     struct vo_win_geometry geo;
-    vo_calc_window_geometry(vo, &x11->screenrc, &geo);
+    vo_calc_window_geometry2(vo, &x11->screenrc, x11->dpi_scale, &geo);
     vo_apply_window_geometry(vo, &geo);
 
     struct mp_rect rc = geo.win;

video/out/x11_common.h

@@ -54,6 +54,7 @@ struct vo_x11_state {
     int display_is_local;
     int ws_width;
     int ws_height;
+    int dpi_scale;
     struct mp_rect screenrc;
     char *window_title;