From 42e9f302df65ee02598f6efe92bde1541654a5b4 Mon Sep 17 00:00:00 2001
From: nanahi <130121847+na-na-hi@users.noreply.github.com>
Date: Mon, 10 Jun 2024 01:03:02 -0400
Subject: [PATCH] sd_ass: fix sub scale with window when use_margins is enabled

By default, libass scales subtitle with video size.
When --sub-scale-with-window is enabled, mpv attempts to undo this scale
and use window size instead. However, the current bahavior is incorrect
when use_margins is enabled, because in this case libass uses the size
of video as if it's "fit" to the window, instead of the displayed size.
As a result, subtitle scale is broken when video-zoom is used in this case:
when zooming out video, the subtitle is scaled up.

Fix this by using the correct factor when use_margins is enabled.
---
 sub/sd_ass.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/sub/sd_ass.c b/sub/sd_ass.c
index d04a40a1de..0333216379 100644
--- a/sub/sd_ass.c
+++ b/sub/sd_ass.c
@@ -470,6 +470,21 @@ static void decode(struct sd *sd, struct demux_packet *packet)
     }
 }
 
+// Calculate the height used for scaling subtitle text size so --sub-scale-with-window
+// can undo this scale and use frame size instead. The algorithm used is the following:
+// - If use_margins is disabled, the text is scaled with the visual size of the video.
+// - If use_margins is enabled, the text is scaled with the size of the video
+//   as if the video is resized to "fit" the size of the frame.
+static float get_libass_scale_height(struct mp_osd_res *dim, bool use_margins)
+{
+    float vidw = dim->w - (dim->ml + dim->mr);
+    float vidh = dim->h - (dim->mt + dim->mb);
+    if (!use_margins || vidw < 1.0)
+        return vidh;
+    else
+        return MPMIN(dim->h, dim->w / vidw * vidh);
+}
+
 static void configure_ass(struct sd *sd, struct mp_osd_res *dim,
                           bool converted, ASS_Track *track)
 {
@@ -508,8 +523,7 @@ static void configure_ass(struct sd *sd, struct mp_osd_res *dim,
         set_font_scale = opts->sub_scale;
     }
     if (set_scale_with_window) {
-        int vidh = dim->h - (dim->mt + dim->mb);
-        set_font_scale *= dim->h / (float)MPMAX(vidh, 1);
+        set_font_scale *= dim->h / MPMAX(get_libass_scale_height(dim, set_use_margins), 1);
     }
     if (!set_scale_by_window) {
         double factor = dim->h / 720.0;