[FIX] Use packet timestamps for tx3g subtitle times on the FFmpeg MP4 path

src/lib_ccx/mp4_rust_bridge.c

@@ -222,14 +222,14 @@ int ccx_mp4_process_cc_packet(struct lib_ccx_ctx *ctx, int sub_type_c608,
 
 int ccx_mp4_process_tx3g_packet(struct lib_ccx_ctx *ctx,
 				unsigned char *data, unsigned int data_length,
-				long long dts, int cts_offset,
-				unsigned int timescale,
+				long long start_ms,
 				struct cc_subtitle *sub)
 {
 	struct lib_cc_decode *dec_ctx = update_decoder_list(ctx);
 	struct encoder_ctx *enc_ctx = update_encoder_list(ctx);
 
-	set_current_pts(dec_ctx->timing, (dts + cts_offset) * MPEG_CLOCK_FREQ / timescale);
+	/* Keep the global timing pointer roughly in sync with this track. */
+	set_current_pts(dec_ctx->timing, start_ms * MPEG_CLOCK_FREQ / 1000);
 	set_fts(dec_ctx->timing);
 
 	/* tx3g format: 16-bit text length + UTF-8 text */
@@ -242,7 +242,7 @@ int ccx_mp4_process_tx3g_packet(struct lib_ccx_ctx *ctx,
 
 	/* Set up subtitle for deferred encoding */
 	sub->type = CC_TEXT;
-	sub->start_time = dec_ctx->timing->fts_now;
+	sub->start_time = start_ms;
 	if (sub->data != NULL)
 		free(sub->data);
 	sub->data = malloc(text_length + 1);
@@ -255,14 +255,13 @@ int ccx_mp4_process_tx3g_packet(struct lib_ccx_ctx *ctx,
 	return 0;
 }
 
-void ccx_mp4_flush_tx3g(struct lib_ccx_ctx *ctx, struct cc_subtitle *sub)
+void ccx_mp4_flush_tx3g(struct lib_ccx_ctx *ctx, struct cc_subtitle *sub, long long end_ms)
 {
-	struct lib_cc_decode *dec_ctx = update_decoder_list(ctx);
 	struct encoder_ctx *enc_ctx = update_encoder_list(ctx);
 
 	if (sub->data != NULL)
 	{
-		sub->end_time = dec_ctx->timing->fts_now;
+		sub->end_time = end_ms;
 		encode_sub(enc_ctx, sub);
 	}
 }

src/lib_ccx/mp4_rust_bridge.h

@@ -59,28 +59,30 @@ extern "C"
 	/*
 	 * Process a tx3g (3GPP timed text) subtitle sample.
 	 *
+	 * Subtitle times are taken explicitly from the packet timestamps instead of
+	 * the global timing pointer, which can be far ahead of this track because
+	 * packets from every track are interleaved.
+	 *
 	 * @param ctx           CCExtractor library context
 	 * @param data          tx3g sample data
 	 * @param data_length   Length of the data
-	 * @param dts           Decoding timestamp
-	 * @param cts_offset    Composition time offset
-	 * @param timescale     Media timescale
+	 * @param start_ms      Subtitle start time in milliseconds
 	 * @param sub           Output subtitle structure
 	 * @return 0 on success, non-zero on error
 	 */
 	int ccx_mp4_process_tx3g_packet(struct lib_ccx_ctx *ctx,
 					unsigned char *data, unsigned int data_length,
-					long long dts, int cts_offset,
-					unsigned int timescale,
+					long long start_ms,
 					struct cc_subtitle *sub);
 
 	/*
 	 * Flush pending tx3g subtitle (encode the last one).
 	 *
-	 * @param ctx  CCExtractor library context
-	 * @param sub  Output subtitle structure
+	 * @param ctx     CCExtractor library context
+	 * @param sub     Output subtitle structure
+	 * @param end_ms  Subtitle end time in milliseconds
 	 */
-	void ccx_mp4_flush_tx3g(struct lib_ccx_ctx *ctx, struct cc_subtitle *sub);
+	void ccx_mp4_flush_tx3g(struct lib_ccx_ctx *ctx, struct cc_subtitle *sub, long long end_ms);
 
 	/*
 	 * Report progress for the MP4 demuxer.

src/rust/src/demuxer/mp4.rs

@@ -54,13 +54,11 @@ extern "C" {
         ctx: *mut lib_ccx_ctx,
         data: *mut u8,
         data_length: c_uint,
-        dts: i64,
-        cts_offset: c_int,
-        timescale: c_uint,
+        start_ms: i64,
         sub: *mut cc_subtitle,
     ) -> c_int;
 
-    fn ccx_mp4_flush_tx3g(ctx: *mut lib_ccx_ctx, sub: *mut cc_subtitle);
+    fn ccx_mp4_flush_tx3g(ctx: *mut lib_ccx_ctx, sub: *mut cc_subtitle, end_ms: i64);
 
     fn ccx_mp4_report_progress(ctx: *mut lib_ccx_ctx, cur: c_uint, total: c_uint);
 
@@ -313,6 +311,8 @@ pub unsafe fn processmp4_rust(ctx: *mut lib_ccx_ctx, path: &CStr, sub: *mut cc_s
 
     let mut packet_count: u32 = 0;
     let mut has_tx3g = false;
+    let mut prev_tx3g_start_ms: i64 = -1;
+    let mut prev_tx3g_end_ms: i64 = -1;
     let mut prev_dvdsub_pts: i64 = -1;
     let mut prev_dvdsub_data: Vec<u8> = Vec::new();
 
@@ -397,20 +397,36 @@ pub unsafe fn processmp4_rust(ctx: *mut lib_ccx_ctx, path: &CStr, sub: *mut cc_s
                 }
                 TrackType::Tx3g => {
                     if pkt.size > 0 && !pkt.data.is_null() {
+                        // Subtitle times are taken from the packet timestamps directly:
+                        // the global timing pointer may already be past this subtitle
+                        // because packets from every track are interleaved.
+                        let start_ms = pts * 1000 / timescale as i64;
+                        let end_ms = if pkt.duration > 0 {
+                            start_ms + pkt.duration * 1000 / timescale as i64
+                        } else {
+                            -1
+                        };
                         if has_tx3g {
-                            ccx_mp4_flush_tx3g(ctx, sub);
+                            // Fall back to the new subtitle's start when the previous
+                            // sample had no duration information.
+                            let flush_end_ms = if prev_tx3g_end_ms >= 0 {
+                                prev_tx3g_end_ms
+                            } else {
+                                start_ms
+                            };
+                            ccx_mp4_flush_tx3g(ctx, sub, flush_end_ms);
                         }
                         let r = ccx_mp4_process_tx3g_packet(
                             ctx,
                             pkt.data,
                             pkt.size as c_uint,
-                            dts,
-                            cts_offset,
-                            timescale,
+                            start_ms,
                             sub,
                         );
                         if r == 0 {
                             has_tx3g = true;
+                            prev_tx3g_start_ms = start_ms;
+                            prev_tx3g_end_ms = end_ms;
                             mp4_ret = 1;
                         }
                     }
@@ -459,7 +475,13 @@ pub unsafe fn processmp4_rust(ctx: *mut lib_ccx_ctx, path: &CStr, sub: *mut cc_s
 
     // Flush last tx3g subtitle
     if has_tx3g {
-        ccx_mp4_flush_tx3g(ctx, sub);
+        let end_ms = if prev_tx3g_end_ms >= 0 {
+            prev_tx3g_end_ms
+        } else {
+            // Match the 5 second default used for the other subtitle paths.
+            prev_tx3g_start_ms + 5000
+        };
+        ccx_mp4_flush_tx3g(ctx, sub, end_ms);
     }
 
     // Flush last dvdsub packet (use 5s default duration)