From 227ec140d3c1bb9e825f32e600f0ffd04cc70873 Mon Sep 17 00:00:00 2001
From: Oneric <oneric@oneric.stub>
Date: Sat, 23 May 2020 18:01:32 +0200
Subject: Detect subs converted to ASS by ffmpeg/libav

ffmpeg/libav did rely on *VSFilter incompatible libass defaults for years.
Avoid breaking them when changing libass' default to match *VSFilter.

To this end detection of the generated by ffmpeg signature is added.
---
 libass/ass.c | 72 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 71 insertions(+), 1 deletion(-)

(limited to 'libass')

diff --git a/libass/ass.c b/libass/ass.c
index cb0e9ac..cf72ecb 100644
--- a/libass/ass.c
+++ b/libass/ass.c
@@ -55,7 +55,9 @@ typedef enum {
     SINFO_WRAPSTYLE    = 1 << 4,
     SINFO_SCALEDBORDER = 1 << 5,
     SINFO_COLOURMATRIX = 1 << 6,
-    SINFO_KERNING      = 1 << 7
+    SINFO_KERNING      = 1 << 7,
+    // for legacy detection
+    GENBY_FFMPEG       = 1 << 8
     // max 32 enumerators
 } ScriptInfo;
 
@@ -713,6 +715,9 @@ static int process_info_line(ASS_Track *track, char *str)
         while (*p && ass_isspace(*p)) p++;
         free(track->Language);
         track->Language = strndup(p, 2);
+    } else if (!strncmp(str, "; Script generated by ", 22)) {
+        if (!strncmp(str + 22,"FFmpeg/Lavc", 11))
+            track->parser_priv->header_flags |= GENBY_FFMPEG;
     }
     return 0;
 }
@@ -728,6 +733,63 @@ static void event_format_fallback(ASS_Track *track)
             "No event format found, using fallback");
 }
 
+/**
+ * \brief Return if track is post-signature and pre-SBAS ffmpeg track
+ * \param track track
+*/
+static bool detect_legacy_conv_subs(ASS_Track *track)
+{
+    /*
+     * FFmpeg and libav convert srt subtitles to ass.
+     * In legacy versions, they did not set the 'ScaledBorderAndShadow' header,
+     * but expected it to default to yes (which libass did).
+     * To avoid breaking them, we try to detect these
+     * converted subs by common properties of ffmpeg/libav's converted subs.
+     * Since files with custom format lines (-2014.10.11) default to SBAS=1
+     * regardless of being ffmpeg generated or not, we are only concerned with
+     * post-signature and pre-SBAS ffmpeg-files (2014.10.11-2020.04.17).
+     * We want to avoid matching modified ffmpeg files though.
+     *
+     * Relevant ffmpeg commits are:
+     *  2c77c90684e24ef16f7e7c4462e011434cee6a98  2010.12.29
+     *    Initial conversion format.
+     *    Style "Format:" line is mix of SSA and ASS
+     *    Event "Format:" line
+     *     "Format: Layer, Start, End, Text\r\n"
+     *    Only Header in ScriptInfo is "ScriptType: v4.00+"
+     *  0e7782c08ec77739edb0b98ba5d896b45e98235f  2012.06.15
+     *    Adds 'Style' to Event "Format:" line
+     *  5039aadf68deb9ad6dd0737ea11259fe53d3727b  2014.06.18
+     *    Adds PlayerRes(X|Y) (384x288)
+     *    (moved below ScriptType: a few minutes later)
+     *  40b9f28641b696c6bb73ce49dc97c2ce2700cbdb  2014.10.11 14:31:23 +0200
+     *    Regular full ASS Event and Style "Format:" lines
+     *  52b0a0ecaa02e17f7e01bead8c3f215f1cfd48dc  2014.10.11 18:37:43 +0200 <==
+     *    Signature comment
+     *  56bc0a6736cdc7edab837ff8f304661fd16de0e4  2015.02.08
+     *    Allow custom PlayRes(X|Y)
+     *  a8ba2a2c1294a330a0e79ae7f0d3a203a7599166  2020.04.17
+     *    Set 'ScaledBorderAndShadow: yes'
+     *
+     * libav outputs initial ffmpeg format. (no longer maintained)
+     */
+
+    // GENBY_FFMPEG and exact ffmpeg headers required
+    // Note: If there's SINFO_SCRIPTTYPE in the future this needs to be updated
+    if (track->parser_priv->header_flags
+            ^ (SINFO_PLAYRESX | SINFO_PLAYRESY | GENBY_FFMPEG))
+        return false;
+
+    // Legacy ffmpeg only ever has one style
+    // Check 2 not 1 because libass also adds a def style
+    if (track->n_styles != 2
+        || strncmp(track->styles[1].Name, "Default", 7))
+        return false;
+
+    return true;
+}
+
+
 static int process_events_line(ASS_Track *track, char *str)
 {
     if (!strncmp(str, "Format:", 7)) {
@@ -740,6 +802,14 @@ static int process_events_line(ASS_Track *track, char *str)
             custom_format_line_compatibility(track, p, ass_event_format);
         else
             custom_format_line_compatibility(track, p, ssa_event_format);
+
+        // Guess if we are dealing with legacy ffmpeg subs and change accordingly
+        // If file has no event format it was probably not created by ffmpeg/libav
+        if (detect_legacy_conv_subs(track)) {
+            track->ScaledBorderAndShadow = 1;
+            ass_msg(track->library, MSGL_INFO,
+                    "Track treated as legacy ffmpeg sub.");
+        }
     } else if (!strncmp(str, "Dialogue:", 9)) {
         // This should never be reached for embedded subtitles.
         // They have slightly different format and are parsed in ass_process_chunk,
-- 
cgit v1.2.3