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[FFmpeg-devel,v13,10/14] avfilter/textmod: Add textmod, censor and show_speaker filters

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Series [FFmpeg-devel,v13,01/14] global: Prepare AVFrame for subtitle handling
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Soft Works Oct. 11, 2021, 4:51 a.m. UTC
- textmod {S -> S)
  Modify subtitle text in a number of ways

- censor {S -> S)
  Censor subtitles using a word list

- show_speaker {S -> S)
  Prepend speaker names from ASS subtitles to the visible text lines

Signed-off-by: softworkz <softworkz@hotmail.com>
---
 doc/filters.texi         | 211 ++++++++++++
 libavfilter/Makefile     |   5 +
 libavfilter/allfilters.c |   3 +
 libavfilter/sf_textmod.c | 697 +++++++++++++++++++++++++++++++++++++++
 4 files changed, 916 insertions(+)
 create mode 100644 libavfilter/sf_textmod.c
diff mbox series

Patch

diff --git a/doc/filters.texi b/doc/filters.texi
index 970f99366e..764c849fac 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -25230,6 +25230,150 @@  existing filters using @code{--disable-filters}.
 
 Below is a description of the currently available subtitle filters.
 
+
+@section censor
+
+Censor selected words in text subtitles.
+
+Inputs:
+@itemize
+@item 0: Subtitles[TEXT]
+@end itemize
+
+Outputs:
+@itemize
+@item 0: Subtitles[TEXT]
+@end itemize
+
+It accepts the following parameters:
+
+@table @option
+@item mode
+The censoring mode to apply.
+
+Supported censoring modes are:
+
+@table @var
+@item 0, keep_first_last
+Replace all characters with the 'censor_char' except the first and the last character of a word.
+For words with less than 4 characters, the last character will be replaced as well.
+For words with less than 3 characters, the first character will be replaced as well.
+@item 1, keep_first
+Replace all characters with the 'censor_char' except the first character of a word.
+For words with less than 3 characters, the first character will be replaced as well.
+@item 2, all
+Replace all characters with the 'censor_char'.
+@end table
+
+@item words
+A list of words to censor, separated by 'separator'.
+
+@item words_file
+Specify a file from which to load the contents for the 'words' parameter.
+
+@item censor_char
+Single character used as replacement for censoring.
+
+@item separator
+Delimiter character for words. Used with replace_words and remove_words- Must be a single character.
+The default is '.'.
+
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Change all characters to upper case while keeping all styles and animations:
+@example
+ffmpeg -i "https://streams.videolan.org/ffmpeg/mkv_subtitles.mkv" -filter_complex "[0:s]textmod=mode=to_upper" -map 0 -y out.mkv
+@end example
+@item
+Remove a set of symbol characters for am improved and smoother visual apperance:
+@example
+ffmpeg -i "https://streams.videolan.org/ffmpeg/mkv_subtitles.mkv" -filter_complex "[0:s]textmod=mode=remove_chars:find='$&#@*§'" -map 0 -y out.mkv
+@end example
+@end itemize
+
+
+@section textmod
+
+Modify subtitle text in a number of ways.
+
+Inputs:
+@itemize
+@item 0: Subtitles[TEXT]
+@end itemize
+
+Outputs:
+@itemize
+@item 0: Subtitles[TEXT]
+@end itemize
+
+It accepts the following parameters:
+
+@table @option
+@item mode
+The kind of text modification to apply
+
+Supported operation modes are:
+
+@table @var
+@item 0, leet
+Convert subtitle text to 'leet speak'. It's primarily useful for testing as the modification will be visible with almost all text lines.
+@item 1, to_upper
+Change all text to upper case. Might improve readability.
+@item 2, to_lower
+Change all text to lower case.
+@item 3, replace_chars
+Replace one or more characters. Requires the find and replace parameters to be specified. 
+Both need to be equal in length.
+The first char in find is replaced by the first char in replace, same for all subsequent chars.
+@item 4, remove_chars
+Remove certain characters. Requires the find parameter to be specified. 
+All chars in the find parameter string will be removed from all subtitle text.
+@item 5, replace_words
+Replace one or more words. Requires the find and replace parameters to be specified. Multiple words must be separated by the delimiter char specified vie the separator parameter (default: ','). 
+The number of words in the find and replace parameters needs to be equal.
+The first word in find is replaced by the first word in replace, same for all subsequent words
+@item 6, remove_words
+Remove certain words. Requires the find parameter to be specified. Multiple words must be separated by the delimiter char specified vie the separator parameter (default: ','). 
+All words in the find parameter string will be removed from all subtitle text.
+@end table
+
+@item find
+Required for replace_chars, remove_chars, replace_words and remove_words.
+
+@item find_file
+Specify a file from which to load the contents for the 'find' parameter.
+
+@item replace
+Required for replace_chars and replace_words.
+
+@item replace_file
+Specify a file from which to load the contents for the 'replace' parameter.
+
+@item separator
+Delimiter character for words. Used with replace_words and remove_words- Must be a single character.
+The default is '.'.
+
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Change all characters to upper case while keeping all styles and animations:
+@example
+ffmpeg -i "https://streams.videolan.org/ffmpeg/mkv_subtitles.mkv" -filter_complex "[0:s]textmod=mode=to_upper" -map 0 -y out.mkv
+@end example
+@item
+Remove a set of symbol characters for am improved and smoother visual apperance:
+@example
+ffmpeg -i "https://streams.videolan.org/ffmpeg/mkv_subtitles.mkv" -filter_complex "[0:s]textmod=mode=remove_chars:find='$&#@*§'" -map 0 -y out.mkv
+@end example
+@end itemize
+
 @section graphicsub2video
 
 Renders graphic subtitles as video frames. 
@@ -25397,6 +25541,73 @@  ffmpeg -i "http://streams.videolan.org/samples/sub/SSA/subtitle_testing_complex.
 @end example
 @end itemize
 
+@section showspeaker
+
+Prepend speaker names to subtitle lines (when available).
+
+Subtitles in ASS/SSA format are often including the names of the persons
+or character for each subtitle line. The showspeaker filter adds those names 
+to the actual subtitle text to make it visible on playback.
+
+Inputs:
+@itemize
+@item 0: Subtitles[TEXT]
+@end itemize
+
+Outputs:
+@itemize
+@item 0: Subtitles[TEXT]
+@end itemize
+
+It accepts the following parameters:
+
+@table @option
+@item format
+The format for prepending speaker names. Default is 'square_brackets'.
+
+Supported operation modes are:
+
+@table @var
+@item 0, square_brackets
+Enclose the speaker name in square brackets, followed by space ('[speaker] text').
+@item 1, round_brackets
+Enclose the speaker name in round brackets, followed by space ('(speaker) text').
+@item 2, colon
+Separate the speaker name with a colon and space ('speaker: text').
+@item 3, plain
+Separate the speaker name with a space only ('speaker text').
+@end table
+
+@item line_break
+Set thís parameter to insert a line break between speaker name and text instead of the space character.
+
+@item style
+Allows to set a specific style for the speaker name text. 
+
+This can be either a named style that exists in the ass subtitle header script (e.g. 'Default') or an ass style override code.
+Example:  @{\\c&HDD0000&\\be1\\i1\\bord10@}
+This sets the color to blue, enables edge blurring, italic font and a border of size 10.
+
+The behavior is as follows:
+
+- When the style parameter is not provided, the filter will find the first position in the event string that is actual text.
+  The speaker name will be inserted at this position. This allows to have the speaker name shown in the same style like the 
+  regular text, in case the string would start with a sequence of style codes.
+- When the style parameter is provided, everything will be prepended to the original text: 
+  Style Code or Style name >> Speaker Name >> Style Reset Code >> Original Text
+
+@end table
+
+@subsection Examples
+
+@itemize
+@item
+Prepend speaker names with blue text, smooth edges and blend/overlay that onto the video.
+@example
+ffmpeg -i INPUT -filter_complex "showspeaker=format=colon:style='@{\\c&HDD0000&\\be1@}',[0:v]overlay_textsubs"
+@end example
+@end itemize
+
 @section textsub2video
 
 Converts text subtitles to video frames.
diff --git a/libavfilter/Makefile b/libavfilter/Makefile
index 901095667e..1a659a8377 100644
--- a/libavfilter/Makefile
+++ b/libavfilter/Makefile
@@ -541,6 +541,11 @@  OBJS-$(CONFIG_YUVTESTSRC_FILTER)             += vsrc_testsrc.o
 
 OBJS-$(CONFIG_NULLSINK_FILTER)               += vsink_nullsink.o
 
+# subtitle filters
+OBJS-$(CONFIG_CENSOR_FILTER)                 += sf_textmod.o
+OBJS-$(CONFIG_SHOW_SPEAKER_FILTER)           += sf_textmod.o
+OBJS-$(CONFIG_TEXTMOD_FILTER)                += sf_textmod.o
+
 # multimedia filters
 OBJS-$(CONFIG_ABITSCOPE_FILTER)              += avf_abitscope.o
 OBJS-$(CONFIG_ADRAWGRAPH_FILTER)             += f_drawgraph.o
diff --git a/libavfilter/allfilters.c b/libavfilter/allfilters.c
index 78e365df3a..e9f1e88bd2 100644
--- a/libavfilter/allfilters.c
+++ b/libavfilter/allfilters.c
@@ -531,6 +531,9 @@  extern const AVFilter ff_avf_showvolume;
 extern const AVFilter ff_avf_showwaves;
 extern const AVFilter ff_avf_showwavespic;
 extern const AVFilter ff_vaf_spectrumsynth;
+extern const AVFilter ff_sf_censor;
+extern const AVFilter ff_sf_showspeaker;
+extern const AVFilter ff_sf_textmod;
 extern const AVFilter ff_svf_graphicsub2video;
 extern const AVFilter ff_svf_textsub2video;
 
diff --git a/libavfilter/sf_textmod.c b/libavfilter/sf_textmod.c
new file mode 100644
index 0000000000..434e98e4a4
--- /dev/null
+++ b/libavfilter/sf_textmod.c
@@ -0,0 +1,697 @@ 
+/*
+ * Copyright (c) 2021 softworkz
+ *
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * text subtitle filter which allows to modify subtitle text in several ways
+ */
+
+#include <libavutil/ass_internal.h>
+
+#include "libavutil/opt.h"
+#include "internal.h"
+#include "libavutil/ass_split_internal.h"
+#include "libavutil/bprint.h"
+#include "libavutil/file.h"
+
+static const char* leet_src = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
+static const char* leet_dst = "abcd3f6#1jklmn0pq257uvwxyzAB(D3F6#1JKLMN0PQ257UVWXYZ";
+
+enum TextModFilterType {
+    TM_TEXTMOD,
+    TM_CENSOR,
+    TM_SHOW_SPEAKER,
+};
+
+enum TextModOperation {
+    OP_LEET,
+    OP_TO_UPPER,
+    OP_TO_LOWER,
+    OP_REPLACE_CHARS,
+    OP_REMOVE_CHARS,
+    OP_REPLACE_WORDS,
+    OP_REMOVE_WORDS,
+    NB_OPS,
+};
+
+enum CensorMode {
+    CM_KEEP_FIRST_LAST,
+    CM_KEEP_FIRST,
+    CM_ALL,
+};
+
+enum ShowSpeakerMode {
+    SM_SQUARE_BRACKETS,
+    SM_ROUND_BRACKETS,
+    SM_COLON,
+    SM_PLAIN,
+};
+
+typedef struct TextModContext {
+    const AVClass *class;
+    enum AVSubtitleType format;
+    enum TextModFilterType filter_type;
+    enum TextModOperation operation;
+    enum CensorMode censor_mode;
+    enum ShowSpeakerMode speaker_mode;
+    char *find;
+    char *find_file;
+    char *style;
+    char *replace;
+    char *replace_file;
+    char *separator;
+    char *censor_char;
+    char **find_list;
+    int  line_break;
+    int  nb_find_list;
+    char **replace_list;
+    int  nb_replace_list;
+} TextModContext;
+
+static char **split_string(char *source, int *nb_elems, const char *delim)
+{
+    char **list = NULL;
+    char *temp = NULL;
+    char *ptr = av_strtok(source, delim, &temp);
+
+    while (ptr) {
+        if (strlen(ptr)) {
+            av_dynarray_add(&list, nb_elems, ptr);
+            if (!list)
+                return NULL;
+        }
+
+        ptr = av_strtok(NULL, delim, &temp);
+    }
+
+    if (!list)
+        return NULL;
+
+    for (int i = 0; i < *nb_elems; i++) {
+        list[i] = av_strdup(list[i]);
+        if (!list[i]) {
+            for (int n = 0; n < i; n++)
+                av_free(list[n]);
+            av_free(list);
+            return NULL;
+        }
+    }
+
+    return list;
+}
+
+static int load_text_from_file(AVFilterContext *ctx, const char *file_name, char **text, char separator)
+{
+    int err;
+    uint8_t *textbuf;
+    char *tmp;
+    size_t textbuf_size;
+    int offset = 0;
+
+    if ((err = av_file_map(file_name, &textbuf, &textbuf_size, 0, ctx)) < 0) {
+        av_log(ctx, AV_LOG_ERROR, "The text file '%s' could not be read or is empty\n", file_name);
+        return err;
+    }
+
+    if (textbuf_size > 1 && 
+        (textbuf[0] == 0xFF && textbuf[1] == 0xFE
+        || textbuf[0] == 0xFE && textbuf[1] == 0xFF)) {
+        av_log(ctx, AV_LOG_ERROR, "UTF text files are not supported. File: %s\n", file_name);
+        return AVERROR(EINVAL);
+    }
+
+    if (textbuf_size > 2 && textbuf[0] == 0xEF && textbuf[1] == 0xBB && textbuf[2] == 0xBF)
+        offset = 3; // UTF-8
+
+    if (textbuf_size > SIZE_MAX - 1 || !((tmp = av_strndup((char *)textbuf + offset, textbuf_size - offset)))) {
+        av_file_unmap(textbuf, textbuf_size);
+        return AVERROR(ENOMEM);
+    }
+
+    av_file_unmap(textbuf, textbuf_size);
+
+    for (size_t i = 0; i < strlen(tmp); i++) {
+        switch (tmp[i]) {
+        case '\n':
+        case '\r':
+        case '\f':
+        case '\v':
+            tmp[i] = separator;
+        }
+    }
+
+    *text = tmp;
+
+    return 0;
+}
+
+static int load_files(AVFilterContext *ctx)
+{
+    TextModContext *s = ctx->priv;
+    int ret;
+
+    if (!s->separator || strlen(s->separator) != 1) {
+        av_log(ctx, AV_LOG_ERROR, "A single character needs to be specified for the separator parameter.\n");
+        return AVERROR(EINVAL);
+    }
+
+    if (s->find_file && strlen(s->find_file)) {
+        ret = load_text_from_file(ctx, s->find_file, &s->find, s->separator[0]);
+        if (ret < 0 )
+            return ret;
+    }
+
+    if (s->replace_file && strlen(s->replace_file)) {
+        ret = load_text_from_file(ctx, s->replace_file, &s->replace, s->separator[0]);
+        if (ret < 0 )
+            return ret;
+    }
+
+    return 0;
+}
+
+static int init_censor(AVFilterContext *ctx)
+{
+    TextModContext *s = ctx->priv;
+    int ret;
+
+    s->filter_type = TM_CENSOR;
+    s->operation = OP_REPLACE_WORDS;
+
+    ret = load_files(ctx);
+    if (ret < 0 )
+        return ret;
+
+    if (!s->find || !strlen(s->find)) {
+        av_log(ctx, AV_LOG_ERROR, "Either the 'words' or the 'words_file' parameter needs to be specified\n");
+        return AVERROR(EINVAL);
+    }
+
+    if (!s->censor_char || strlen(s->censor_char) != 1) {
+        av_log(ctx, AV_LOG_ERROR, "A single character needs to be specified for the censor_char parameter\n");
+        return AVERROR(EINVAL);
+    }
+
+    s->find_list = split_string(s->find, &s->nb_find_list, s->separator);
+    if (!s->find_list)
+        return AVERROR(ENOMEM);
+
+    s->replace_list = av_calloc(s->nb_find_list, sizeof(char *));
+    if (!s->replace_list)
+        return AVERROR(ENOMEM);
+
+    for (int i = 0; i < s->nb_find_list; i++) {
+        size_t len, start = 0, end;
+        char *item = av_strdup(s->find_list[i]);
+        if (!item)
+            return AVERROR(ENOMEM);
+
+        len = end = strlen(item);
+
+        switch (s->censor_mode) {
+        case CM_KEEP_FIRST_LAST:
+
+            if (len > 2)
+                start = 1;
+            if (len > 3)
+                end--;
+
+            break;
+        case CM_KEEP_FIRST:
+
+            if (len > 2)
+                start = 1;
+
+            break;
+        }
+
+        for (size_t n = start; n < end; n++)
+            item[n] = s->censor_char[0];
+
+        s->replace_list[i] = item;
+    }
+
+    return 0;
+}
+
+static int init_showspeaker(AVFilterContext *ctx)
+{
+    TextModContext *s = ctx->priv;
+    s->filter_type = TM_SHOW_SPEAKER;
+
+    return 0;
+}
+
+static int init(AVFilterContext *ctx)
+{
+    TextModContext *s = ctx->priv;
+    int ret;
+
+    ret = load_files(ctx);
+    if (ret < 0 )
+        return ret;
+
+    switch (s->operation) {
+    case OP_REPLACE_CHARS:
+    case OP_REMOVE_CHARS:
+    case OP_REPLACE_WORDS:
+    case OP_REMOVE_WORDS:
+        if (!s->find || !strlen(s->find)) {
+            av_log(ctx, AV_LOG_ERROR, "Selected mode requires the 'find' parameter to be specified\n");
+            return AVERROR(EINVAL);
+        }
+        break;
+    }
+
+    switch (s->operation) {
+    case OP_REPLACE_CHARS:
+    case OP_REPLACE_WORDS:
+        if (!s->replace || !strlen(s->replace)) {
+            av_log(ctx, AV_LOG_ERROR, "Selected mode requires the 'replace' parameter to be specified\n");
+            return AVERROR(EINVAL);
+        }
+        break;
+    }
+
+    if (s->operation == OP_REPLACE_CHARS && strlen(s->find) != strlen(s->replace)) {
+        av_log(ctx, AV_LOG_ERROR, "Selected mode requires the 'find' and 'replace' parameters to have the same length\n");
+        return AVERROR(EINVAL);
+    }
+
+    if (s->operation == OP_REPLACE_WORDS || s->operation == OP_REMOVE_WORDS) {
+        if (!s->separator || strlen(s->separator) != 1) {
+            av_log(ctx, AV_LOG_ERROR, "Selected mode requires a single separator char to be specified\n");
+            return AVERROR(EINVAL);
+        }
+
+        s->find_list = split_string(s->find, &s->nb_find_list, s->separator);
+        if (!s->find_list)
+            return AVERROR(ENOMEM);
+
+        if (s->operation == OP_REPLACE_WORDS) {
+
+            s->replace_list = split_string(s->replace, &s->nb_replace_list, s->separator);
+            if (!s->replace_list)
+                return AVERROR(ENOMEM);
+
+            if (s->nb_find_list != s->nb_replace_list) {
+                av_log(ctx, AV_LOG_ERROR, "The number of words in 'find' and 'replace' needs to be equal\n");
+                return AVERROR(EINVAL);
+            }
+        }
+    }
+
+    return 0;
+}
+
+static void uninit(AVFilterContext *ctx)
+{
+    TextModContext *s = ctx->priv;
+
+    for (int i = 0; i < s->nb_find_list; i++)
+        av_freep(&s->find_list[i]);
+
+    s->nb_find_list = 0;
+    av_freep(&s->find_list);
+
+    for (int i = 0; i < s->nb_replace_list; i++)
+        av_freep(&s->replace_list[i]);
+
+    s->nb_replace_list = 0;
+    av_freep(&s->replace_list);
+}
+
+static char *process_text(TextModContext *s, char *text)
+{
+    const char *char_src = s->find;
+    const char *char_dst = s->replace;
+    char *result         = NULL;
+    int escape_level     = 0, k = 0;
+
+    switch (s->operation) {
+    case OP_LEET:
+    case OP_REPLACE_CHARS:
+
+        if (s->operation == OP_LEET) {
+            char_src = leet_src;
+            char_dst = leet_dst;
+        }
+
+        result = av_strdup(text);
+        if (!result)
+            return NULL;
+
+        for (size_t n = 0; n < strlen(result); n++) {
+            if (result[n] == '{')
+                escape_level++;
+
+            if (!escape_level) {
+                size_t len = strlen(char_src);
+                for (size_t t = 0; t < len; t++) {
+                    if (result[n] == char_src[t]) {
+                        result[n] = char_dst[t];
+                        break;
+                    }
+                }
+            }
+
+            if (result[n] == '}')
+                escape_level--;
+        }
+
+        break;
+    case OP_TO_UPPER:
+    case OP_TO_LOWER:
+
+        result = av_strdup(text);
+        if (!result)
+            return NULL;
+
+        for (size_t n = 0; n < strlen(result); n++) {
+            if (result[n] == '{')
+                escape_level++;
+            if (!escape_level)
+                result[n] = s->operation == OP_TO_LOWER ? av_tolower(result[n]) : av_toupper(result[n]);
+            if (result[n] == '}')
+                escape_level--;
+        }
+
+        break;
+    case OP_REMOVE_CHARS:
+
+        result = av_strdup(text);
+        if (!result)
+            return NULL;
+
+        for (size_t n = 0; n < strlen(result); n++) {
+            int skip_char = 0;
+
+            if (result[n] == '{')
+                escape_level++;
+
+            if (!escape_level) {
+                size_t len = strlen(char_src);
+                for (size_t t = 0; t < len; t++) {
+                    if (result[n] == char_src[t]) {
+                        skip_char = 1;
+                        break;
+                    }
+                }
+            }
+
+            if (!skip_char)
+                result[k++] = result[n];
+
+            if (result[n] == '}')
+                escape_level--;
+        }
+
+        result[k] = 0;
+
+        break;
+    case OP_REPLACE_WORDS:
+    case OP_REMOVE_WORDS:
+
+        result = av_strdup(text);
+        if (!result)
+            return NULL;
+
+        for (int n = 0; n < s->nb_find_list; n++) {
+            char *tmp           = result;
+            const char *replace = (s->operation == OP_REPLACE_WORDS) ? s->replace_list[n] : "";
+
+            result = av_strireplace(result, s->find_list[n], replace);
+            if (!result)
+                return NULL;
+
+            av_free(tmp);
+        }
+
+        break;
+    }
+
+    return result;
+}
+
+static char *process_dialog_show_speaker(TextModContext *s, char *ass_line)
+{
+    ASSDialog *dialog = avpriv_ass_split_dialog(NULL, ass_line);
+    int escape_level = 0;
+    unsigned pos = 0, len;
+    char *result, *text;
+    AVBPrint pbuf;
+
+    if (!dialog)
+        return NULL;
+
+    text = process_text(s, dialog->text);
+    if (!text)
+        return NULL;
+
+    if (!dialog->name || !strlen(dialog->name) || !dialog->text || !strlen(dialog->text))
+        return av_strdup(ass_line);
+
+    // Find insertion point in case the line starts with style codes
+    len = (unsigned)strlen(dialog->text);
+    for (unsigned i = 0; i < len; i++) {
+
+        if (dialog->text[i] == '{')
+            escape_level++;
+
+        if (dialog->text[i] == '}')
+            escape_level--;
+
+        if (escape_level == 0) {
+            pos = i;
+            break;
+        }
+    }
+
+    if (s->style && strlen(s->style))
+        // When a style is specified reset the insertion point 
+        // (always add speaker plus style at the start in that case)
+        pos = 0;
+
+    if (pos >= len - 1)
+        return av_strdup(ass_line);
+
+    av_bprint_init(&pbuf, 1, AV_BPRINT_SIZE_UNLIMITED);
+
+    if (pos > 0) {
+        av_bprint_append_data(&pbuf, dialog->text, pos);
+    }
+
+    if (s->style && strlen(s->style)) {
+        if (s->style[0] == '{')
+            // Assume complete and valid style code, e.g. {\c&HFF0000&}
+            av_bprintf(&pbuf, "%s", s->style);
+        else
+            // Otherwise it must be a style name
+            av_bprintf(&pbuf, "{\\r%s}", s->style);
+    }
+
+    switch (s->speaker_mode) {
+    case SM_SQUARE_BRACKETS:
+        av_bprintf(&pbuf, "[%s]", dialog->name);
+        break;
+    case SM_ROUND_BRACKETS:
+        av_bprintf(&pbuf, "(%s)", dialog->name);
+        break;
+    case SM_COLON:
+        av_bprintf(&pbuf, "%s:", dialog->name);
+        break;
+    case SM_PLAIN:
+        av_bprintf(&pbuf, "%s", dialog->name);
+        break;
+    }
+
+    if (s->style && strlen(s->style)) {
+        // Reset line style
+        if (dialog->style && strlen(dialog->style) && !av_strcasecmp(dialog->style, "default"))
+            av_bprintf(&pbuf, "{\\r%s}", dialog->style);
+        else
+            av_bprintf(&pbuf, "{\\r}");
+    }
+
+    if (s->line_break)
+        av_bprintf(&pbuf, "\\N");
+    else
+        av_bprintf(&pbuf, " ");
+
+    av_bprint_append_data(&pbuf, dialog->text + pos, len - pos);
+
+    av_bprint_finalize(&pbuf, &text);
+
+    result = avpriv_ass_get_dialog(dialog->readorder, dialog->layer, dialog->style, dialog->name, text);
+
+    av_free(text);
+    avpriv_ass_free_dialog(&dialog);
+    return result;
+}
+
+static char *process_dialog(TextModContext *s, char *ass_line)
+{
+    ASSDialog *dialog;
+    char *result, *text;
+
+    if (s->filter_type == TM_SHOW_SPEAKER)
+        return process_dialog_show_speaker(s, ass_line);
+
+    dialog = avpriv_ass_split_dialog(NULL, ass_line);
+    if (!dialog)
+        return NULL;
+
+    text = process_text(s, dialog->text);
+    if (!text)
+        return NULL;
+
+    result = avpriv_ass_get_dialog(dialog->readorder, dialog->layer, dialog->style, dialog->name, text);
+
+    av_free(text);
+    avpriv_ass_free_dialog(&dialog);
+    return result;
+}
+
+static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
+{
+    TextModContext *s = inlink->dst->priv;
+    AVFilterLink *outlink = inlink->dst->outputs[0];
+    int ret;
+
+    outlink->format = inlink->format;
+
+    ret = av_frame_make_writable(frame);
+    if (ret < 0)
+        return ret;
+
+    for (unsigned i = 0; i < frame->num_subtitle_areas; i++) {
+
+        AVSubtitleArea *area = frame->subtitle_areas[i];
+
+        if (area->ass) {
+            char *tmp = area->ass;
+            area->ass = process_dialog(s, area->ass);
+            av_free(tmp);
+            if (!area->ass)
+                return AVERROR(ENOMEM);
+        }
+    }
+
+    return ff_filter_frame(outlink, frame);
+}
+
+#define OFFSET(x) offsetof(TextModContext, x)
+#define FLAGS (AV_OPT_FLAG_SUBTITLE_PARAM | AV_OPT_FLAG_FILTERING_PARAM)
+
+static const AVOption textmod_options[] = {
+    { "mode",             "set operation mode",              OFFSET(operation),    AV_OPT_TYPE_INT,    {.i64=OP_LEET},          OP_LEET, NB_OPS-1, FLAGS, "mode" },
+    {   "leet",           "convert text to 'leet speak'",    0,                    AV_OPT_TYPE_CONST,  {.i64=OP_LEET},          0,       0,        FLAGS, "mode" },
+    {   "to_upper",       "change to upper case",            0,                    AV_OPT_TYPE_CONST,  {.i64=OP_TO_UPPER},      0,       0,        FLAGS, "mode" },
+    {   "to_lower",       "change to lower case",            0,                    AV_OPT_TYPE_CONST,  {.i64=OP_TO_LOWER},      0,       0,        FLAGS, "mode" },
+    {   "replace_chars",  "replace characters",              0,                    AV_OPT_TYPE_CONST,  {.i64=OP_REPLACE_CHARS}, 0,       0,        FLAGS, "mode" },
+    {   "remove_chars",   "remove characters",               0,                    AV_OPT_TYPE_CONST,  {.i64=OP_REMOVE_CHARS},  0,       0,        FLAGS, "mode" },
+    {   "replace_words",  "replace words",                   0,                    AV_OPT_TYPE_CONST,  {.i64=OP_REPLACE_WORDS}, 0,       0,        FLAGS, "mode" },
+    {   "remove_words",   "remove words",                    0,                    AV_OPT_TYPE_CONST,  {.i64=OP_REMOVE_WORDS},  0,       0,        FLAGS, "mode" },
+    { "find",             "chars/words to find or remove",   OFFSET(find),         AV_OPT_TYPE_STRING, {.str = NULL},           0,       0,        FLAGS, NULL   },
+    { "find_file",        "load find param from file",       OFFSET(find_file),    AV_OPT_TYPE_STRING, {.str = NULL},           0,       0,        FLAGS, NULL   },
+    { "replace",          "chars/words to replace",          OFFSET(replace),      AV_OPT_TYPE_STRING, {.str = NULL},           0,       0,        FLAGS, NULL   },
+    { "replace_file",     "load replace param from file",    OFFSET(replace_file), AV_OPT_TYPE_STRING, {.str = NULL},           0,       0,        FLAGS, NULL   },
+    { "separator",        "word separator",                  OFFSET(separator),    AV_OPT_TYPE_STRING, {.str = ","},            0,       0,        FLAGS, NULL   },
+    { NULL },
+};
+
+
+static const AVOption censor_options[] = {
+    { "mode",               "set censoring mode",        OFFSET(censor_mode), AV_OPT_TYPE_INT,    {.i64=CM_KEEP_FIRST_LAST}, 0, 2, FLAGS, "mode" },
+    {   "keep_first_last",  "censor inner chars",        0,                   AV_OPT_TYPE_CONST,  {.i64=CM_KEEP_FIRST_LAST}, 0, 0, FLAGS, "mode" },
+    {   "keep_first",       "censor all but first char", 0,                   AV_OPT_TYPE_CONST,  {.i64=CM_KEEP_FIRST},      0, 0, FLAGS, "mode" },
+    {   "all",              "censor all chars",          0,                   AV_OPT_TYPE_CONST,  {.i64=CM_ALL},             0, 0, FLAGS, "mode" },
+    { "words",              "list of words to censor",   OFFSET(find),        AV_OPT_TYPE_STRING, {.str = NULL},             0, 0, FLAGS, NULL   },
+    { "words_file",         "path to word list file",    OFFSET(find_file),   AV_OPT_TYPE_STRING, {.str = NULL},             0, 0, FLAGS, NULL   },
+    { "separator",          "word separator",            OFFSET(separator),   AV_OPT_TYPE_STRING, {.str = ","},              0, 0, FLAGS, NULL   },
+    { "censor_char",        "replacement character",     OFFSET(censor_char), AV_OPT_TYPE_STRING, {.str = "*"},              0, 0, FLAGS, NULL   },
+    { NULL },
+};
+
+static const AVOption showspeaker_options[] = {
+    { "format",             "speaker name formatting",        OFFSET(speaker_mode), AV_OPT_TYPE_INT,    {.i64=SM_SQUARE_BRACKETS}, 0, 2, FLAGS, "format" },
+    {   "square_brackets",  "[speaker] text",                 0,                    AV_OPT_TYPE_CONST,  {.i64=SM_SQUARE_BRACKETS}, 0, 0, FLAGS, "format" },
+    {   "round_brackets",   "(speaker) text",                 0,                    AV_OPT_TYPE_CONST,  {.i64=SM_ROUND_BRACKETS},  0, 0, FLAGS, "format" },
+    {   "colon",            "speaker: text",                  0,                    AV_OPT_TYPE_CONST,  {.i64=SM_COLON},           0, 0, FLAGS, "format" },
+    {   "plain",            "speaker text",                   0,                    AV_OPT_TYPE_CONST,  {.i64=SM_PLAIN},           0, 0, FLAGS, "format" },
+    { "line_break",         "insert line break",              OFFSET(line_break),   AV_OPT_TYPE_BOOL,   {.i64=0},                  0, 1, FLAGS, NULL     },
+    { "style",              "ass type name or style code",    OFFSET(style),        AV_OPT_TYPE_STRING, {.str = NULL},             0, 0, FLAGS, NULL     },
+    { NULL },
+};
+
+AVFILTER_DEFINE_CLASS(textmod);
+AVFILTER_DEFINE_CLASS(censor);
+AVFILTER_DEFINE_CLASS(showspeaker);
+
+static const AVFilterPad inputs[] = {
+    {
+        .name         = "default",
+        .type         = AVMEDIA_TYPE_SUBTITLE,
+        .filter_frame = filter_frame,
+    },
+};
+
+static const AVFilterPad outputs[] = {
+    {
+        .name          = "default",
+        .type          = AVMEDIA_TYPE_SUBTITLE,
+    },
+};
+
+const AVFilter ff_sf_textmod = {
+    .name          = "textmod",
+    .description   = NULL_IF_CONFIG_SMALL("Modify subtitle text in several ways"),
+    .init          = init,
+    .uninit        = uninit,
+    .priv_size     = sizeof(TextModContext),
+    .priv_class    = &textmod_class,
+    FILTER_INPUTS(inputs),
+    FILTER_OUTPUTS(outputs),
+    FILTER_SINGLE_SUBFMT(AV_SUBTITLE_FMT_ASS),
+};
+
+const AVFilter ff_sf_censor = {
+    .name          = "censor",
+    .description   = NULL_IF_CONFIG_SMALL("Censor words in subtitle text"),
+    .init          = init_censor,
+    .uninit        = uninit,
+    .priv_size     = sizeof(TextModContext),
+    .priv_class    = &censor_class,
+    FILTER_INPUTS(inputs),
+    FILTER_OUTPUTS(outputs),
+    FILTER_SINGLE_SUBFMT(AV_SUBTITLE_FMT_ASS),
+};
+
+const AVFilter ff_sf_showspeaker = {
+    .name          = "showspeaker",
+    .description   = NULL_IF_CONFIG_SMALL("Prepend speaker names to text subtitles (when available)"),
+    .init          = init_showspeaker,
+    .uninit        = uninit,
+    .priv_size     = sizeof(TextModContext),
+    .priv_class    = &showspeaker_class,
+    FILTER_INPUTS(inputs),
+    FILTER_OUTPUTS(outputs),
+    FILTER_SINGLE_SUBFMT(AV_SUBTITLE_FMT_ASS),
+};