avcodec/libaribcaption: only clear on CS in text and ass mode

The text and ass paths treated every caption without regions as a clear
and emitted an empty ("") subtitle. A statement containing only TIME
also decodes to zero regions, without the clear-screen flag, so it
erased the caption that was being displayed.

Test ARIBCC_CAPTIONFLAGS_CLEARSCREEN as the bitmap path already does,
and emit nothing for other regionless captions.

Add text and ass mode tests on the existing aribcaption_clear.ts sample
(visible caption, TIME-only statement, CS-only statement). Each
reference has two events; before this change there was a third, empty
one for the TIME-only statement.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DdDiDRusmi6eD7qSrKBjHb

libavcodec/libaribcaption.c

@@ -618,13 +618,19 @@ static int aribcaption_trans_ass_subtitle(ARIBCaptionContext *ctx)
     if (ctx->avctx->profile == AV_PROFILE_ARIB_PROFILE_C)
         single_rect = true;
 
-    sub->format = 1; /* text */
     if (ctx->caption.region_count == 0) {
+        /* Only an explicit clear screen (CS) erases the previous caption;
+           a statement containing only TIME also has no region. */
+        if (!(ctx->caption.flags & ARIBCC_CAPTIONFLAGS_CLEARSCREEN))
+            return 0;
         /* clear previous caption for indefinite duration  */
+        sub->format = 1; /* text */
         ff_ass_add_rect(sub, "", ctx->readorder++, 0, NULL, NULL);
         return 1;
     }
 
+    sub->format = 1; /* text */
+
     av_bprint_init(&buf, ARIBC_BPRINT_SIZE_INIT, ARIBC_BPRINT_SIZE_MAX);
 
     if (single_rect && ctx->avctx->profile != AV_PROFILE_ARIB_PROFILE_C) {
@@ -814,6 +820,12 @@ static int aribcaption_trans_text_subtitle(ARIBCaptionContext *ctx)
     int ret = 0;
     const char *text;
 
+    /* Only an explicit clear screen (CS) erases the previous caption;
+       a statement containing only TIME also has no region. */
+    if (ctx->caption.region_count == 0 &&
+        !(ctx->caption.flags & ARIBCC_CAPTIONFLAGS_CLEARSCREEN))
+        return 0;
+
     sub->rects = av_calloc(1, sizeof(*sub->rects));
     if (!sub->rects) {
         ret = AVERROR(ENOMEM);

tests/fate/subtitles.mak

@@ -172,6 +172,14 @@ FATE_SUBTITLES_PROBE-$(call DEMDEC, MPEGTS, LIBARIBCAPTION) += fate-sub-aribcapt
 fate-sub-aribcaption-clear: CMD = probeframes -sub_type bitmap $(TARGET_SAMPLES)/sub/aribcaption_clear.ts
 fate-sub-aribcaption-clear: CMP = diff
 
+# same sample in text and ass mode: only CS must emit a clear event, which
+# the ass dialogue shows as an empty text
+FATE_SUBTITLES_PROBE-$(call DEMDEC, MPEGTS, LIBARIBCAPTION) += fate-sub-aribcaption-clear-text
+fate-sub-aribcaption-clear-text: CMD = probeframes -sub_type text $(TARGET_SAMPLES)/sub/aribcaption_clear.ts
+FATE_SUBTITLES_ASS-$(call DEMDEC, MPEGTS, LIBARIBCAPTION) += fate-sub-aribcaption-clear-ass
+fate-sub-aribcaption-clear-ass: CMD = fmtstdout ass -sub_type ass -font sans-serif -i $(TARGET_SAMPLES)/sub/aribcaption_clear.ts
+fate-sub-aribcaption-clear-text fate-sub-aribcaption-clear-ass: CMP = diff
+
 FATE_SUBTITLES-$(call ENCMUX, ASS, ASS) += $(FATE_SUBTITLES_ASS-yes)
 FATE_SUBTITLES += $(FATE_SUBTITLES-yes)
 FATE_SUBTITLES := $(if $(CONFIG_PIPE_PROTOCOL), $(FATE_SUBTITLES))

tests/ref/fate/sub-aribcaption-clear-ass

@@ -0,0 +1,14 @@
+[Script Info]
+ScriptType: v4.00+
+PlayResX: 960
+PlayResY: 540
+WrapStyle: 2
+
+[V4+ Styles]
+Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
+Style: Default,sans-serif,36,&Hffffff,&Hffffff,&H0,&H0,0,0,0,0,100,100,0,0,4,0,4,2,10,10,10,0
+
+[Events]
+Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
+Dialogue: 0,0:00:01.00,9:59:59.99,Default,,0,0,0,,{\an7}{\pos(200,300)}{\fsp4}{\3a&Hff&}{\4a&Hff&}〓
+Dialogue: 0,0:00:03.00,9:59:59.99,Default,,0,0,0,,

tests/ref/fate/sub-aribcaption-clear-text

@@ -0,0 +1,18 @@
+[SUBTITLE]
+media_type=subtitle
+pts=2000000
+pts_time=2.000000
+format=1
+start_display_time=0
+end_display_time=4294967295
+num_rects=1
+[/SUBTITLE]
+[SUBTITLE]
+media_type=subtitle
+pts=4000000
+pts_time=4.000000
+format=1
+start_display_time=0
+end_display_time=4294967295
+num_rects=1
+[/SUBTITLE]