player/loadfile: possibly do a reset of sd_ass when turning on video

It's a pretty niche case, but if you start a file with no video with
animated ass subtitles and then turned on the video later, the subtitles
would not actually be rendered animated. This because the subtitles were
already decoded, and we do not do a check for animations expect in the
specific case of an image being played back on the video since the check
can be quite expensive. To fix this edge case, we just need to reset the
ass before doing the track switch in the case where we go from no video
to a track that's an image. SD_CTRL_RESET_SOFT is added as a a sub_ctrl
to accomplish this. Fixes #17268.

Author: Dudemanguy

player/loadfile.c

@@ -745,6 +745,20 @@ void mp_switch_track_n(struct MPContext *mpctx, int order, enum stream_type type
         reselect_demux_stream(mpctx, current, false);
     }
 
+    // For the edge case of switching from no video to a still image, we need to
+    // flush the ass events of any sub track for potential animations.
+    if (type == STREAM_VIDEO && order == 0 && !current && track->image) {
+        for (int n = 0; n < num_ptracks[STREAM_SUB]; n++) {
+            struct track *sub_track = mpctx->current_track[n][STREAM_SUB];
+            if (sub_track && sub_track->d_sub) {
+                sub_control(sub_track->d_sub, SD_CTRL_RESET_SOFT, NULL);
+                sub_reset(sub_track->d_sub);
+                if (sub_track->selected)
+                    reselect_demux_stream(mpctx, sub_track, true);
+            }
+        }
+    }
+
     mpctx->current_track[order][type] = track;
 
     if (track) {

sub/dec_sub.h

@@ -19,6 +19,7 @@ enum sd_ctrl {
     SD_CTRL_SET_ANIMATED_CHECK,
     SD_CTRL_SET_VIDEO_PARAMS,
     SD_CTRL_SET_VIDEO_DEF_FPS,
+    SD_CTRL_RESET_SOFT,
     SD_CTRL_UPDATE_OPTS,
 };
 

sub/sd_ass.c

@@ -1034,6 +1034,10 @@ static int control(struct sd *sd, enum sd_ctrl cmd, void *arg)
     case SD_CTRL_SET_ANIMATED_CHECK:
         ctx->check_animated = *(bool *)arg;
         return CONTROL_OK;
+    case SD_CTRL_RESET_SOFT:
+        ctx->clear_once = true;
+        reset(sd);
+        return CONTROL_OK;
     case SD_CTRL_SET_VIDEO_PARAMS:
         ctx->video_params = *(struct mp_image_params *)arg;
         return CONTROL_OK;