[FFmpeg-devel] [PATCH 2/2] Adding closed caption decoder

Anshul anshul.ffmpeg at gmail.com
Sat Dec 6 11:52:00 CET 2014


On 12/05/2014 07:56 PM, Nicolas George wrote:
> Hi. I had time to look at the code with some more details. Comments are
> below.
>
>> >From 31f69ccfb45247a7cc203084a931b8523284aa13 Mon Sep 17 00:00:00 2001
>> From: Anshul Maheshwari <anshul.ffmpeg at gmail.com>
>> Date: Wed, 3 Dec 2014 23:37:22 +0530
>> Subject: [PATCH 2/2] Adding Closed caption Decoder
>>
>> ---
>>  libavcodec/Makefile       |   1 +
>>  libavcodec/allcodecs.c    |   1 +
>>  libavcodec/ccaption_dec.c | 318 ++++++++++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 320 insertions(+)
>>  create mode 100644 libavcodec/ccaption_dec.c
>>
>> diff --git a/libavcodec/Makefile b/libavcodec/Makefile
>> index fa0f53d..bbc516d 100644
>> --- a/libavcodec/Makefile
>> +++ b/libavcodec/Makefile
>> @@ -173,6 +173,7 @@ OBJS-$(CONFIG_BRENDER_PIX_DECODER)     += brenderpix.o
>>  OBJS-$(CONFIG_C93_DECODER)             += c93.o
>>  OBJS-$(CONFIG_CAVS_DECODER)            += cavs.o cavsdec.o cavsdsp.o \
>>                                            cavsdata.o mpeg12data.o
>> +OBJS-$(CONFIG_CCAPTION_DECODER)        += ccaption_dec.o
>>  OBJS-$(CONFIG_CDGRAPHICS_DECODER)      += cdgraphics.o
>>  OBJS-$(CONFIG_CDXL_DECODER)            += cdxl.o
>>  OBJS-$(CONFIG_CINEPAK_DECODER)         += cinepak.o
>> diff --git a/libavcodec/allcodecs.c b/libavcodec/allcodecs.c
>> index 0d39d33..8c07388 100644
>> --- a/libavcodec/allcodecs.c
>> +++ b/libavcodec/allcodecs.c
>> @@ -480,6 +480,7 @@ void avcodec_register_all(void)
>>      /* subtitles */
>>      REGISTER_ENCDEC (SSA,               ssa);
>>      REGISTER_ENCDEC (ASS,               ass);
>> +    REGISTER_DECODER(CCAPTION,          ccaption);
>>      REGISTER_ENCDEC (DVBSUB,            dvbsub);
>>      REGISTER_ENCDEC (DVDSUB,            dvdsub);
>>      REGISTER_DECODER(JACOSUB,           jacosub);
>> diff --git a/libavcodec/ccaption_dec.c b/libavcodec/ccaption_dec.c
>> new file mode 100644
>> index 0000000..0a7dfd8
>> --- /dev/null
>> +++ b/libavcodec/ccaption_dec.c
>> @@ -0,0 +1,318 @@
>> +/*
>> + * Closed Caption Decoding
>> + * Copyright (c) 2014 Anshul Maheshwari
>> + *
>> + * 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
>> + */
>> +
>> +#include "avcodec.h"
>> +#include "ass.h"
>> +
>> +#define SCREEN_ROWS 15
>> +#define SCREEN_COLUMNS 32
>> +
>> +#define SET_FLAG(var, val) ( var |= ( 1 << (val) ) )
>> +#define UNSET_FLAG(var, val) ( var &=  ~( 1 << (val)) )
>> +#define CHECK_FLAG(var, val) ( (var) & (1 << (val) ) )
>> +
>> +enum cc_mode {
>> +    CCMODE_POPON,
>> +    CCMODE_PAINTON,
>> +    CCMODE_ROLLUP_2,
>> +    CCMODE_ROLLUP_3,
>> +    CCMODE_ROLLUP_4,
>> +    CCMODE_TEXT,
>> +};
>> +
>> +struct Screen {
>> +    uint8_t characters[SCREEN_ROWS][SCREEN_COLUMNS+1];
> Maybe add a comment about the +1?
done
>> +    /*
>> +     * row used flag will be 0 when none in use other wise it will have its
>> +     * corrosponding bit high.
> Language nit. I suggest: "Bitmask of used rows; if a bit is not set, the
> corresponding row is not used."
done
>> +     * for setting row 1  use row | (1 >> 1)
>> +     * for setting row 15 use row | (1 >> 15)
> Are you sure that is ">>" and not "<<"? And is it a good idea to number
> starting from 1?
It was just commented in wrong way, corrected comment.
>> +     */
>> +    int16_t  row_used;
>> +};
>> +
>> +
>> +typedef struct CCaptionSubContext {
>> +    int parity_table[256];
>> +    int row_cnt;
>> +    struct Screen screen[2];
>> +    int active_screen;
>> +    int cursor_row;
>> +    int cursor_column;
>> +    AVBPrint buffer;
>> +    /* erase display memory */
>> +    int edm;
> It is used only a handful of times: I suggest a more meaningful name instead
> of a comment: "erase_disp_mem" for example.
done
>> +    int rollup;
>> +    enum  cc_mode mode;
>> +    int64_t start_time;
>> +    /* visible screen time */
>> +    int64_t startv_time;
> Is the v a typo?
no it was written to express v = visible, (visible screen start time)
>> +    int64_t end_time;
>> +    char prev_cmd[2];
> The code uses various types for these values: char, unsigned char, uint8_t.
> I suggest to stick with uint8_t if it works.
changed unsigned char to uint8_t
>> +}CCaptionSubContext;
>> +
>> +static void build_parity_table(int *parity_table)
>> +{
>> +        unsigned int byte;
> Inconsistent indentation.
corrected
>> +        int parity_v;
>> +        for (byte = 0; byte <= 127; byte++) {
>> +                parity_v = av_popcount(byte & 0x7f) & 1;
> The & 0x7f is redundant.
removed
>> +                parity_table[byte] = parity_v;
>> +                parity_table[byte | 0x80] = !parity_v;
>> +        }
>> +}
>> +
>> +static av_cold int init_decoder(AVCodecContext *avctx)
>> +{
>> +
>> +    CCaptionSubContext *ctx = avctx->priv_data;
>> +
>> +    build_parity_table(ctx->parity_table);
>> +    ctx->row_cnt = 0;
>> +    av_bprint_init(&ctx->buffer, 0, AV_BPRINT_SIZE_UNLIMITED);
>> +    ctx->edm = 0;
>> +    /* taking by default roll up to 2 */
>> +    ctx->rollup = 2;
>> +    /* set active screen as 0 */
>> +    ctx->active_screen = 0;
>> +    memset(ctx->prev_cmd,0,2);
>> +    ff_ass_subtitle_header_default(avctx);
> All the "= 0" and the memset are unnecessary.
removed
>> +    return 0;
>> +}
>> +
>> +static av_cold int close_decoder(AVCodecContext *avctx)
>> +{
>> +    CCaptionSubContext *ctx = avctx->priv_data;
>> +    av_bprint_finalize( &ctx->buffer, NULL);
>> +    return 0;
>> +}
>> +
>> +/**
>> + * This function after validating parity bit, also remove it from data pair.
>> + */
>> +static int validate_cc_data_pair (unsigned char *cc_data_pair, int *parity_table)
>> +{
>> +    unsigned char cc_valid = (*cc_data_pair & 4) >>2;
>> +    unsigned char cc_type = *cc_data_pair & 3;
>> +
>> +    if (!cc_valid)
>> +        return -1;
>> +
>> +    // if EIA-608 data then verify parity.
>> +    if (cc_type==0 || cc_type==1) {
>> +        if (!parity_table[cc_data_pair[2]]) {
>> +        // If the second byte doesn't pass parity, ignore pair
>> +            return -1;
> Meaningful error codes? AVERROR(EINVAL) or AVERROR_INVALID_DATA? Even when
> they are not supposed to, the -1 have a habit of reaching the user. Same
> below.
done
>> +        }
>> +        if (!parity_table[cc_data_pair[1]]) {
>> +        // The first byte doesn't pass parity, we replace it with a solid blank
>> +        // and process the pair.
>> +            cc_data_pair[1]=0x7F;
>> +        }
>> +    }
>> +
>> +    //Skip non-data
>> +    if( (cc_data_pair[0] == 0xFA || cc_data_pair[0] == 0xFC || cc_data_pair[0] == 0xFD )
>> +         && (cc_data_pair[1] & 0x7F) == 0 && (cc_data_pair[2] & 0x7F) == 0)
>> +        return -1;
>> +
>> +    //skip 708 data
>> +    if(cc_type == 3 || cc_type == 2 )
>> +        return -1;
>> +
>> +    /* remove parity bit */
>> +    cc_data_pair[1] &= 0x7F;
>> +    cc_data_pair[2] &= 0x7F;
>> +
>> +
>> +    return 0;
>> +
>> +}
>> +static void handle_pac( CCaptionSubContext *ctx, uint8_t hi, uint8_t lo )
>> +{
>> +    static const int row_map[] = {
> Could be int8_t to save memory.
done
>> +        11, -1, 1, 2, 3, 4, 12, 13, 14, 15, 5, 6, 7, 8, 9, 10
>> +    };
>> +    const int index = ( (hi<<1) & 0x0e) | ( (lo>>5) & 0x01 );
>> +
> It looks like the values come from external data with subtle arithmetic, so
> I suggest:
>
>     av_assert2((unsigned)index < sizeof(row_map));
I am using 3 bit from hi and 1 bit from lo data, so I am using total 4 bit
which mean only 0-15 possible. and for unknown value not defined in spec
I have kept the value as -1 on that index.
so no need to check. 
>> +    if( row_map[index] <= 0 )
>> +        return;
>> +
>> +    ctx->cursor_row = row_map[index] - 1;
>> +    ctx->cursor_column = 0;
>> +
>> +}
>> +
>> +/**
>> + * @param pts it is required to set end time
>> + */
>> +static void handle_edm(CCaptionSubContext *ctx,int64_t pts)
>> +{
>> +    int i;
>> +    struct Screen *screen = ctx->screen + ctx->active_screen;
>> +
>> +    ctx->start_time = ctx->startv_time;
>> +    for( i = 0; screen->row_used && i < SCREEN_ROWS; i++)
>> +    {
>> +        if(CHECK_FLAG(screen->row_used,i)) {
>> +            av_bprint_append_data(&ctx->buffer, screen->characters[i], strlen(screen->characters[i]));
>> +            av_bprint_append_data(&ctx->buffer, "\\N",2);
>> +            UNSET_FLAG(screen->row_used, i);
>> +        }
>> +    }
>> +    ctx->startv_time = pts;
>> +    ctx->edm = 1;
>> +    ctx->end_time = pts;
>> +}
>> +static void handle_eoc(CCaptionSubContext *ctx, int64_t pts)
>> +{
>> +    ctx->active_screen = !ctx->active_screen;
>> +    ctx->startv_time = pts;
>> +}
>> +static struct Screen *get_writing_screen(CCaptionSubContext *ctx)
>> +{
>> +    switch (ctx->mode) {
>> +    case CCMODE_POPON:
>> +        // use Inactive screen
>> +        return ctx->screen + !ctx->active_screen;
>> +    case CCMODE_PAINTON:
>> +    case CCMODE_ROLLUP_2:
>> +    case CCMODE_ROLLUP_3:
>> +    case CCMODE_ROLLUP_4:
>> +    case CCMODE_TEXT:
>> +        // use active screen
>> +        return ctx->screen + !ctx->active_screen;
>> +    }
>> +    /* It was never an option */
>> +    return NULL;
>> +}
>> +static void handle_char(CCaptionSubContext *ctx, char hi, char lo, int64_t pts)
>> +{
>> +    struct Screen *screen = get_writing_screen(ctx);
>> +    char *row = screen->characters[ctx->cursor_row] + ctx->cursor_column;
> Here too: av_assert0((unsigned)ctx->cursor_row < SCREEN_ROWS) and same for
> column.
We can never get row greater then SCREEN_ROWS, I have not implemented to
read column value from pac. so left it as it is.
>> +
>> +    SET_FLAG(screen->row_used,ctx->cursor_row);
>> +
>> +    *row++ = hi;
>> +    ctx->cursor_column++;
>> +    if(lo) {
>> +        *row++ = lo;
>> +        ctx->cursor_column++;
>> +    }
>> +    *row = 0;
>> +    /* reset prev command since character can repeat */
>> +    ctx->prev_cmd[0] = 0;
>> +    ctx->prev_cmd[1] = 0;
>> +}
>> +static int process_cc608(CCaptionSubContext *ctx, int64_t pts, unsigned char hi, unsigned char lo)
>> +{
>> +
>> +#define COR3(var, with1, with2, with3)  ( (var) == (with1) ||  (var) == (with2) || (var) == (with3) )
> It looks like you always use COR3(hi, 0x14, 0x15, 0x1C) exactly: is it on
> purpose, a typo, or maybe can you simplify?
These are done for purpose, I have to implement some other command which
have
different values to be compared. so not simplifying it.
>> +    if ( hi == ctx->prev_cmd[0] && lo == ctx->prev_cmd[1]) {
>> +    /* ignore redundant command */
>> +    } else if ( (hi == 0x10 && (lo >= 0x40 || lo <= 0x5f)) ||
>> +              ( (hi >= 0x11 && hi <= 0x17) && (lo >= 0x40 && lo <= 0x7f) ) ) {
>> +        handle_pac(ctx, hi, lo);
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x20 ) {
>> +    /* resume caption loading */
>> +        ctx->mode = CCMODE_POPON;
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x25 ) {
>> +        ctx->rollup = 2;
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x26 ) {
>> +        ctx->rollup = 3;
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x27 ) {
>> +        ctx->rollup = 4;
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x29 ) {
>> +    /* resume direct captioning */
>> +        ctx->mode = CCMODE_PAINTON;
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x2C ) {
>> +    /* erase display memory */
>> +        handle_edm(ctx, pts);
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x2D ) {
>> +    /* carriage return */
>> +        ctx->row_cnt++;
>> +        if(ctx->row_cnt == ctx->rollup) {
>> +            ctx->row_cnt = 0;
>> +            handle_edm(ctx, pts);
>> +        }
>> +    } else if ( COR3(hi, 0x14, 0x15, 0x1C) && lo == 0x2F ) {
>> +    /* end of caption */
>> +        handle_eoc(ctx, pts);
>> +    } else if (hi>=0x20) {
>> +    /* Standard characters (always in pairs) */
>> +        handle_char(ctx, hi, lo, pts);
>> +    } else {
>> +    /* Ignoring all other non data code */
>> +    }
>> +
>> +    /* set prev command */
>> +     ctx->prev_cmd[0] = hi;
>> +     ctx->prev_cmd[1] = lo;
>> +
>> +#undef COR3
>> +    return 0;
>> +
>> +}
>> +static int decode(AVCodecContext *avctx, void *data, int *got_sub, AVPacket *avpkt)
>> +{
>> +    CCaptionSubContext *ctx = avctx->priv_data;
>> +    AVSubtitle *sub = data;
>> +    unsigned char *bptr = avpkt->data;
>> +    int len = avpkt->size;
>> +    int ret = 0;
>> +    int i;
>> +
>> +    for (i  = 0; i < len; i += 3) {
>> +        unsigned char cc_type = *(bptr + i) & 3;
>> +        if (validate_cc_data_pair( bptr + i, ctx->parity_table ) )
>> +            continue;
>> +        /* ignoring data field 1 */
>> +        if(cc_type == 1)
>> +            continue;
>> +        else
>> +            process_cc608(ctx, avpkt->pts, *(bptr + i + 1), *(bptr + i + 2));
>> +    }
>> +    if(ctx->edm && *ctx->buffer.str)
>> +    {
>> +        int start_time = av_rescale_q(ctx->start_time, avctx->time_base, (AVRational){ 1, 100 });
>> +        int end_time = av_rescale_q(ctx->end_time, avctx->time_base, (AVRational){ 1, 100 });
>> +        ret = ff_ass_add_rect(sub, ctx->buffer.str, start_time, end_time - start_time , 0);
>> a
> You need to use av_bprint_is_complete() and return AVERROR(ENOMEM) if the
> text was truncated.
done in handle_edm
>> +        if (ret < 0)
>> +            return ret;
>> +        sub->pts = av_rescale_q(ctx->start_time, avctx->time_base, AV_TIME_BASE_Q);
>> +        ctx->edm = 0;
>> +        av_bprint_clear(&ctx->buffer);
>> +    }
>> +
>> +    *got_sub = sub->num_rects > 0;
>> +    return 0;
>> +}
>> +
>> +AVCodec ff_ccaption_decoder = {
>> +    .name           = "cc_dec",
>> +    .long_name      = NULL_IF_CONFIG_SMALL("Closed Caption Decoder"),
> I suggest to have "EIA-608" and "CEA-708" appear in the name, for people
> specifically looking for it: "Closed Caption (EIA-608 / CEA-708) Decoder"
> maybe.
done
>> +    .type           = AVMEDIA_TYPE_SUBTITLE,
>> +    .id             = AV_CODEC_ID_EIA_608,
>> +    .priv_data_size = sizeof(CCaptionSubContext),
>> +    .init           = init_decoder,
>> +    .close          = close_decoder,
>> +    .decode         = decode,
>> +};
> Regards,
>

I have also removed some error which were causing to print nothing in
subccItalic.mpg.

Though subtitle  is still not italic.

I was thinking to put extra features in next iteration of patch.

-Anshul



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