vo: add --video-recenter

This resets ``--video-align-x`` and ``--video-align-y`` to 0 when the
video becomes smaller than the OSD in the respective direction, e.g. by
zooming out.

Unlike doing this by observing osd-dimensions in a script, this is done
before rerendering, so you don't see the image being rendered not
centered for an instant after zooming out before being rerendered
centered.

Defaults to yes.

DOCS/interface-changes/video-recenter.txt

@@ -0,0 +1 @@
+add `--video-recenter` option

DOCS/man/options.rst

@@ -1652,17 +1652,31 @@ Video
     ``--keepaspect=no`` is used.
 
 ``--video-align-x=<-1-1>``, ``--video-align-y=<-1-1>``
-    Moves the video rectangle within the black borders, which are usually added
-    to pad the video to screen if video and screen aspect ratios are different.
-    ``--video-align-y=-1`` would move the video to the top of the screen
-    (leaving a border only on the bottom), a value of ``0`` centers it
-    (default), and a value of ``1`` would put the video at the bottom of the
-    screen.
+    When the video is bigger than the OSD, these move the displayed rectangle to
+    show different parts of the video. ``--video-align-y=-1`` would display the
+    top of the video, ``0`` would display around the center (default), and ``1``
+    would display the bottom.
+
+    When the video is smaller than the OSD and ``--video-recenter`` is disabled,
+    these move the video rectangle within the black borders, which are usually
+    added to pad the video to screen if video and screen aspect ratios are
+    different. ``--video-align-y=-1`` would move the video to the top of the
+    screen (leaving a border only on the bottom), ``0`` would center it, and
+    ``1`` would put the video at the bottom of the screen.
 
     If video and screen aspect match perfectly, these options do nothing.
 
+    Unlike ``--video-pan-x`` and ``--video-pan-y``, these don't go beyond the
+    video's boundaries or make the displayed rectangle drift off after zooming.
+
     This option is disabled if ``--keepaspect=no`` is used.
 
+``--video-recenter=<yes|no>``
+    Whether to reset ``--video-align-x`` and ``--video-align-y`` to 0 when the
+    video becomes smaller than the OSD in the respective direction
+
+    Default: yes.
+
 ``--video-margin-ratio-left=<val>``, ``--video-margin-ratio-right=<val>``, ``--video-margin-ratio-top=<val>``, ``--video-margin-ratio-bottom=<val>``
     Set extra video margins on each border (default: 0). Each value is a ratio
     of the window size, using a range 0.0-1.0. For example, setting the option

options/options.c

@@ -160,6 +160,7 @@ static const m_option_t mp_vo_opt_list[] = {
     {"video-crop", OPT_RECT(video_crop), .flags = UPDATE_IMGPAR},
     {"video-unscaled", OPT_CHOICE(unscaled,
         {"no", 0}, {"yes", 1}, {"downscale-big", 2})},
+    {"video-recenter", OPT_BOOL(recenter)},
     {"wid", OPT_INT64(WinID)},
     {"screen", OPT_CHOICE(screen_id, {"default", -1}), M_RANGE(0, 32)},
     {"screen-name", OPT_STRING(screen_name)},
@@ -250,6 +251,7 @@ const struct m_sub_options vo_sub_opts = {
         .scale_x = 1.0f,
         .scale_y = 1.0f,
         .auto_window_resize = true,
+        .recenter = true,
         .keepaspect = true,
         .keepaspect_window = true,
         .native_fs = true,

options/options.h

@@ -58,6 +58,7 @@ typedef struct mp_vo_opts {
     double window_scale;
 
     bool auto_window_resize;
+    bool recenter;
     bool keepaspect;
     bool keepaspect_window;
     bool hidpi_window_scale;

video/out/vo.c

@@ -230,12 +230,34 @@ static void read_opts(struct vo *vo)
     mp_mutex_unlock(&in->lock);
 }
 
+static void recenter(struct vo* vo)
+{
+    if (!vo->params || !vo->opts->recenter)
+        return;
+
+    struct mp_rect src, dst;
+    struct mp_osd_res dims = osd_get_vo_res(vo->osd);
+    mp_get_src_dst_rects(vo->log, vo->opts, vo->driver->caps, vo->params,
+                         dims.w, dims.h, vo->monitor_par, &src, &dst,
+                         &dims);
+
+    if (dims.ml + dims.mr > 0) {
+        vo->opts->align_x = 0;
+        m_config_cache_write_opt(vo->opts_cache, &vo->opts->align_x);
+    }
+    if (dims.mt + dims.mb > 0) {
+        vo->opts->align_y = 0;
+        m_config_cache_write_opt(vo->opts_cache, &vo->opts->align_y);
+    }
+}
+
 static void update_opts(void *p)
 {
     struct vo *vo = p;
 
     if (m_config_cache_update(vo->opts_cache)) {
         read_opts(vo);
+        recenter(vo);
         if (vo->driver->control) {
             vo->driver->control(vo, VOCTRL_VO_OPTS_CHANGED, NULL);
             // "Legacy" update of video position related options.
@@ -599,6 +621,8 @@ static void run_reconfig(void *p)
     vo->params = talloc_dup(vo, params);
     mp_mutex_unlock(&vo->params_mutex);
 
+    recenter(vo);
+
     if (vo->driver->reconfig2) {
         *ret = vo->driver->reconfig2(vo, img);
     } else {