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.

1 files changed, 18 insertions, 1 deletions

diff --git a/video/out/x11_common.c b/video/out/x11_common.c
index d42eb70dde..6f3cc41637 100644
--- a/video/out/x11_common.c
+++ b/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;