diff --git a/libavcodec/ass.c b/libavcodec/ass.c
index 725e4d42ba..a1e560d7ff 100644
--- a/libavcodec/ass.c
+++ b/libavcodec/ass.c
@@ -114,17 +114,34 @@ char *ff_ass_get_dialog(int readorder, int layer, const char *style,
                        speaker ? speaker : "", text);
 }
 
-int ff_ass_add_rect(AVSubtitle *sub, const char *dialog,
+int ff_ass_add_rect2(AVSubtitle *sub, const char *dialog,
                     int readorder, int layer, const char *style,
-                    const char *speaker)
+                    const char *speaker, unsigned *nb_rect_allocated)
 {
-    AVSubtitleRect **rects, *rect;
+    AVSubtitleRect **rects = sub->rects, *rect;
     char *ass_str;
+    uint64_t new_nb = 0;
 
-    rects = av_realloc_array(sub->rects, sub->num_rects+1, sizeof(*sub->rects));
-    if (!rects)
+    if (sub->num_rects >= UINT_MAX)
         return AVERROR(ENOMEM);
-    sub->rects = rects;
+
+    if (nb_rect_allocated && *nb_rect_allocated <= sub->num_rects) {
+        if (sub->num_rects < UINT_MAX / 17 * 16) {
+            new_nb = sub->num_rects + sub->num_rects/16 + 1;
+        } else
+            new_nb = UINT_MAX;
+    } else if (!nb_rect_allocated)
+        new_nb = sub->num_rects + 1;
+
+    if (new_nb) {
+        rects = av_realloc_array(rects, new_nb, sizeof(*sub->rects));
+        if (!rects)
+            return AVERROR(ENOMEM);
+        if (nb_rect_allocated)
+            *nb_rect_allocated = new_nb;
+        sub->rects = rects;
+    }
+
     rect       = av_mallocz(sizeof(*rect));
     if (!rect)
         return AVERROR(ENOMEM);
@@ -137,6 +154,13 @@ int ff_ass_add_rect(AVSubtitle *sub, const char *dialog,
     return 0;
 }
 
+int ff_ass_add_rect(AVSubtitle *sub, const char *dialog,
+                    int readorder, int layer, const char *style,
+                    const char *speaker)
+{
+    return ff_ass_add_rect2(sub, dialog, readorder, layer, style, speaker, NULL);
+}
+
 void ff_ass_decoder_flush(AVCodecContext *avctx)
 {
     FFASSDecoderContext *s = avctx->priv_data;
diff --git a/libavcodec/ass.h b/libavcodec/ass.h
index 2c260e4e78..4dffe923d9 100644
--- a/libavcodec/ass.h
+++ b/libavcodec/ass.h
@@ -118,6 +118,13 @@ int ff_ass_add_rect(AVSubtitle *sub, const char *dialog,
                     int readorder, int layer, const char *style,
                     const char *speaker);
 
+/**
+ * Add an ASS dialog to a subtitle.
+ */
+int ff_ass_add_rect2(AVSubtitle *sub, const char *dialog,
+                     int readorder, int layer, const char *style,
+                     const char *speaker, unsigned *nb_rect_allocated);
+
 /**
  * Helper to flush a text subtitles decoder making use of the
  * FFASSDecoderContext.