sd_lavc: match video display aspect ratio on SD video in PGS subtitles

Previous fixes in bcc3d72 make subtitles on videos with non-square pixel
aspect ratios appear incorrect. This can happen on subtitles for SD
Blu-rays where the video resolution is 720x480 but the aspect ratio is
4:3. The scaling still applies when the video frame is cropped with
filters so the previous fix will still work.

Fixes #15415.

sub/osd.c

@@ -540,7 +540,8 @@ struct mp_osd_res osd_get_vo_res(struct osd_state *osd)
 // fills the whole video area (especially if the video is magnified, e.g. on
 // fullscreen). If compensate_par is >0, adjust the way the subtitles are
 // "stretched" on the screen, and letter-box the result. If compensate_par
-// is <0, strictly letter-box the subtitles. If it is 0, stretch them.
+// is <0, strictly letter-box the subtitles according to the supplied aspect
+// ratio. If it is 0, stretch them.
 void osd_rescale_bitmaps(struct sub_bitmaps *imgs, int frame_w, int frame_h,
                          struct mp_osd_res res, double compensate_par)
 {
@@ -550,7 +551,7 @@ void osd_rescale_bitmaps(struct sub_bitmaps *imgs, int frame_w, int frame_h,
     double yscale = (double)vidh / frame_h;
     if (compensate_par < 0) {
         assert(res.display_par);
-        compensate_par = xscale / yscale / res.display_par;
+        compensate_par = xscale / yscale / res.display_par / compensate_par * -1;
     }
     if (compensate_par > 0)
         xscale /= compensate_par;

sub/sd_lavc.c

@@ -472,7 +472,24 @@ static struct sub_bitmaps *get_bitmaps(struct sd *sd, struct mp_osd_res d,
             video_par = par;
     }
     if (priv->avctx->codec_id == AV_CODEC_ID_HDMV_PGS_SUBTITLE)
-        video_par = -1;
+    {
+        // For Blu-ray subs on SD video, try to match the video PAR.
+        if (priv->video_params.w == 720 &&
+            (priv->video_params.h == 480 ||
+             priv->video_params.h == 576))
+        {
+            double par = priv->video_params.p_w / (double)priv->video_params.p_h;
+            if (isnormal(par))
+                video_par = par * -1;
+            else
+                video_par = -1;
+        }
+        else
+        {
+            // Force letter-boxing on all other Blu-ray subtitles
+            video_par = -1;
+        }
+    }
     if (opts->stretch_image_subs)
         d.ml = d.mr = d.mt = d.mb = 0;
     int w = priv->avctx->width;