libavcodec/h261dec.c

Go to the documentation of this file.

/*
 * H261 decoder
 * Copyright (c) 2002-2004 Michael Niedermayer <michaelni@gmx.at>
 * Copyright (c) 2004 Maarten Daniels
 *
 * 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
 * H.261 decoder.
 */

#include "libavutil/avassert.h"
#include "avcodec.h"
#include "mpegvideo.h"
#include "h263.h"
#include "h261.h"

#define H261_MBA_VLC_BITS 9
#define H261_MTYPE_VLC_BITS 6
#define H261_MV_VLC_BITS 7
#define H261_CBP_VLC_BITS 9
#define TCOEFF_VLC_BITS 9
#define MBA_STUFFING 33
#define MBA_STARTCODE 34

static VLC h261_mba_vlc;
static VLC h261_mtype_vlc;
static VLC h261_mv_vlc;
static VLC h261_cbp_vlc;

static av_cold void h261_decode_init_vlc(H261Context *h)
{
    static int done = 0;

    if (!done) {
        done = 1;
        INIT_VLC_STATIC(&h261_mba_vlc, H261_MBA_VLC_BITS, 35,
                        ff_h261_mba_bits, 1, 1,
                        ff_h261_mba_code, 1, 1, 662);
        INIT_VLC_STATIC(&h261_mtype_vlc, H261_MTYPE_VLC_BITS, 10,
                        ff_h261_mtype_bits, 1, 1,
                        ff_h261_mtype_code, 1, 1, 80);
        INIT_VLC_STATIC(&h261_mv_vlc, H261_MV_VLC_BITS, 17,
                        &ff_h261_mv_tab[0][1], 2, 1,
                        &ff_h261_mv_tab[0][0], 2, 1, 144);
        INIT_VLC_STATIC(&h261_cbp_vlc, H261_CBP_VLC_BITS, 63,
                        &ff_h261_cbp_tab[0][1], 2, 1,
                        &ff_h261_cbp_tab[0][0], 2, 1, 512);
        INIT_VLC_RL(ff_h261_rl_tcoeff, 552);
    }
}

static av_cold int h261_decode_init(AVCodecContext *avctx)
{
    H261Context *h          = avctx->priv_data;
    MpegEncContext *const s = &h->s;

    // set defaults
    ff_MPV_decode_defaults(s);
    s->avctx       = avctx;
    s->width       = s->avctx->coded_width;
    s->height      = s->avctx->coded_height;
    s->codec_id    = s->avctx->codec->id;
    s->out_format  = FMT_H261;
    s->low_delay   = 1;
    avctx->pix_fmt = AV_PIX_FMT_YUV420P;
    s->codec_id    = avctx->codec->id;

    ff_h261_common_init();
    h261_decode_init_vlc(h);

    h->gob_start_code_skipped = 0;

    return 0;
}

/**
 * Decode the group of blocks header or slice header.
 * @return <0 if an error occurred
 */
static int h261_decode_gob_header(H261Context *h)
{
    unsigned int val;
    MpegEncContext *const s = &h->s;

    if (!h->gob_start_code_skipped) {
        /* Check for GOB Start Code */
        val = show_bits(&s->gb, 15);
        if (val)
            return -1;

        /* We have a GBSC */
        skip_bits(&s->gb, 16);
    }

    h->gob_start_code_skipped = 0;

    h->gob_number = get_bits(&s->gb, 4); /* GN */
    s->qscale     = get_bits(&s->gb, 5); /* GQUANT */

    /* Check if gob_number is valid */
    if (s->mb_height == 18) { // CIF
        if ((h->gob_number <= 0) || (h->gob_number > 12))
            return -1;
    } else { // QCIF
        if ((h->gob_number != 1) && (h->gob_number != 3) &&
            (h->gob_number != 5))
            return -1;
    }

    /* GEI */
    while (get_bits1(&s->gb) != 0)
        skip_bits(&s->gb, 8);

    if (s->qscale == 0) {
        av_log(s->avctx, AV_LOG_ERROR, "qscale has forbidden 0 value\n");
        if (s->avctx->err_recognition & (AV_EF_BITSTREAM | AV_EF_COMPLIANT))
            return -1;
    }

    /* For the first transmitted macroblock in a GOB, MBA is the absolute
     * address. For subsequent macroblocks, MBA is the difference between
     * the absolute addresses of the macroblock and the last transmitted
     * macroblock. */
    h->current_mba = 0;
    h->mba_diff    = 0;

    return 0;
}

/**
 * Decode the group of blocks / video packet header.
 * @return <0 if no resync found
 */
static int ff_h261_resync(H261Context *h)
{
    MpegEncContext *const s = &h->s;
    int left, ret;

    if (h->gob_start_code_skipped) {
        ret = h261_decode_gob_header(h);
        if (ret >= 0)
            return 0;
    } else {
        if (show_bits(&s->gb, 15) == 0) {
            ret = h261_decode_gob_header(h);
            if (ret >= 0)
                return 0;
        }
        // OK, it is not where it is supposed to be ...
        s->gb = s->last_resync_gb;
        align_get_bits(&s->gb);
        left = get_bits_left(&s->gb);

        for (; left > 15 + 1 + 4 + 5; left -= 8) {
            if (show_bits(&s->gb, 15) == 0) {
                GetBitContext bak = s->gb;

                ret = h261_decode_gob_header(h);
                if (ret >= 0)
                    return 0;

                s->gb = bak;
            }
            skip_bits(&s->gb, 8);
        }
    }

    return -1;
}

/**
 * Decode skipped macroblocks.
 * @return 0
 */
static int h261_decode_mb_skipped(H261Context *h, int mba1, int mba2)
{
    MpegEncContext *const s = &h->s;
    int i;

    s->mb_intra = 0;

    for (i = mba1; i < mba2; i++) {
        int j, xy;

        s->mb_x = ((h->gob_number - 1) % 2) * 11 + i % 11;
        s->mb_y = ((h->gob_number - 1) / 2) * 3 + i / 11;
        xy      = s->mb_x + s->mb_y * s->mb_stride;
        ff_init_block_index(s);
        ff_update_block_index(s);

        for (j = 0; j < 6; j++)
            s->block_last_index[j] = -1;

        s->mv_dir                      = MV_DIR_FORWARD;
        s->mv_type                     = MV_TYPE_16X16;
        s->current_picture.mb_type[xy] = MB_TYPE_SKIP | MB_TYPE_16x16 | MB_TYPE_L0;
        s->mv[0][0][0]                 = 0;
        s->mv[0][0][1]                 = 0;
        s->mb_skipped                  = 1;
        h->mtype                      &= ~MB_TYPE_H261_FIL;

        ff_MPV_decode_mb(s, s->block);
    }

    return 0;
}

static const int mvmap[17] = {
    0, -1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -15, -16
};

static int decode_mv_component(GetBitContext *gb, int v)
{
    int mv_diff = get_vlc2(gb, h261_mv_vlc.table, H261_MV_VLC_BITS, 2);

    /* check if mv_diff is valid */
    if (mv_diff < 0)
        return v;

    mv_diff = mvmap[mv_diff];

    if (mv_diff && !get_bits1(gb))
        mv_diff = -mv_diff;

    v += mv_diff;
    if (v <= -16)
        v += 32;
    else if (v >= 16)
        v -= 32;

    return v;
}

/**
 * Decode a macroblock.
 * @return <0 if an error occurred
 */
static int h261_decode_block(H261Context *h, int16_t *block, int n, int coded)
{
    MpegEncContext *const s = &h->s;
    int code, level, i, j, run;
    RLTable *rl = &ff_h261_rl_tcoeff;
    const uint8_t *scan_table;

    /* For the variable length encoding there are two code tables, one being
     * used for the first transmitted LEVEL in INTER, INTER + MC and
     * INTER + MC + FIL blocks, the second for all other LEVELs except the
     * first one in INTRA blocks which is fixed length coded with 8 bits.
     * NOTE: The two code tables only differ in one VLC so we handle that
     * manually. */
    scan_table = s->intra_scantable.permutated;
    if (s->mb_intra) {
        /* DC coef */
        level = get_bits(&s->gb, 8);
        // 0 (00000000b) and -128 (10000000b) are FORBIDDEN
        if ((level & 0x7F) == 0) {
            av_log(s->avctx, AV_LOG_ERROR, "illegal dc %d at %d %d\n",
                   level, s->mb_x, s->mb_y);
            return -1;
        }
        /* The code 1000 0000 is not used, the reconstruction level of 1024
         * being coded as 1111 1111. */
        if (level == 255)
            level = 128;
        block[0] = level;
        i        = 1;
    } else if (coded) {
        // Run  Level   Code
        // EOB          Not possible for first level when cbp is available (that's why the table is different)
        // 0    1       1s
        // *    *       0*
        int check = show_bits(&s->gb, 2);
        i = 0;
        if (check & 0x2) {
            skip_bits(&s->gb, 2);
            block[0] = (check & 0x1) ? -1 : 1;
            i        = 1;
        }
    } else {
        i = 0;
    }
    if (!coded) {
        s->block_last_index[n] = i - 1;
        return 0;
    }
    for (;;) {
        code = get_vlc2(&s->gb, rl->vlc.table, TCOEFF_VLC_BITS, 2);
        if (code < 0) {
            av_log(s->avctx, AV_LOG_ERROR, "illegal ac vlc code at %dx%d\n",
                   s->mb_x, s->mb_y);
            return -1;
        }
        if (code == rl->n) {
            /* escape */
            /* The remaining combinations of (run, level) are encoded with a
             * 20-bit word consisting of 6 bits escape, 6 bits run and 8 bits
             * level. */
            run   = get_bits(&s->gb, 6);
            level = get_sbits(&s->gb, 8);
        } else if (code == 0) {
            break;
        } else {
            run   = rl->table_run[code];
            level = rl->table_level[code];
            if (get_bits1(&s->gb))
                level = -level;
        }
        i += run;
        if (i >= 64) {
            av_log(s->avctx, AV_LOG_ERROR, "run overflow at %dx%d\n",
                   s->mb_x, s->mb_y);
            return -1;
        }
        j        = scan_table[i];
        block[j] = level;
        i++;
    }
    s->block_last_index[n] = i - 1;
    return 0;
}

static int h261_decode_mb(H261Context *h)
{
    MpegEncContext *const s = &h->s;
    int i, cbp, xy;

    cbp = 63;
    // Read mba
    do {
        h->mba_diff = get_vlc2(&s->gb, h261_mba_vlc.table,
                               H261_MBA_VLC_BITS, 2);

        /* Check for slice end */
        /* NOTE: GOB can be empty (no MB data) or exist only of MBA_stuffing */
        if (h->mba_diff == MBA_STARTCODE) { // start code
            h->gob_start_code_skipped = 1;
            return SLICE_END;
        }
    } while (h->mba_diff == MBA_STUFFING); // stuffing

    if (h->mba_diff < 0) {
        if (get_bits_left(&s->gb) <= 7)
            return SLICE_END;

        av_log(s->avctx, AV_LOG_ERROR, "illegal mba at %d %d\n", s->mb_x, s->mb_y);
        return SLICE_ERROR;
    }

    h->mba_diff    += 1;
    h->current_mba += h->mba_diff;

    if (h->current_mba > MBA_STUFFING)
        return SLICE_ERROR;

    s->mb_x = ((h->gob_number - 1) % 2) * 11 + ((h->current_mba - 1) % 11);
    s->mb_y = ((h->gob_number - 1) / 2) * 3 + ((h->current_mba - 1) / 11);
    xy      = s->mb_x + s->mb_y * s->mb_stride;
    ff_init_block_index(s);
    ff_update_block_index(s);

    // Read mtype
    h->mtype = get_vlc2(&s->gb, h261_mtype_vlc.table, H261_MTYPE_VLC_BITS, 2);
    if (h->mtype < 0) {
        av_log(s->avctx, AV_LOG_ERROR, "illegal mtype %d\n", h->mtype);
        return SLICE_ERROR;
    }
    h->mtype = ff_h261_mtype_map[h->mtype];

    // Read mquant
    if (IS_QUANT(h->mtype))
        ff_set_qscale(s, get_bits(&s->gb, 5));

    s->mb_intra = IS_INTRA4x4(h->mtype);

    // Read mv
    if (IS_16X16(h->mtype)) {
        /* Motion vector data is included for all MC macroblocks. MVD is
         * obtained from the macroblock vector by subtracting the vector
         * of the preceding macroblock. For this calculation the vector
         * of the preceding macroblock is regarded as zero in the
         * following three situations:
         * 1) evaluating MVD for macroblocks 1, 12 and 23;
         * 2) evaluating MVD for macroblocks in which MBA does not represent a difference of 1;
         * 3) MTYPE of the previous macroblock was not MC. */
        if ((h->current_mba ==  1) || (h->current_mba == 12) ||
            (h->current_mba == 23) || (h->mba_diff != 1)) {
            h->current_mv_x = 0;
            h->current_mv_y = 0;
        }

        h->current_mv_x = decode_mv_component(&s->gb, h->current_mv_x);
        h->current_mv_y = decode_mv_component(&s->gb, h->current_mv_y);
    } else {
        h->current_mv_x = 0;
        h->current_mv_y = 0;
    }

    // Read cbp
    if (HAS_CBP(h->mtype))
        cbp = get_vlc2(&s->gb, h261_cbp_vlc.table, H261_CBP_VLC_BITS, 2) + 1;

    if (s->mb_intra) {
        s->current_picture.mb_type[xy] = MB_TYPE_INTRA;
        goto intra;
    }

    //set motion vectors
    s->mv_dir                      = MV_DIR_FORWARD;
    s->mv_type                     = MV_TYPE_16X16;
    s->current_picture.mb_type[xy] = MB_TYPE_16x16 | MB_TYPE_L0;
    s->mv[0][0][0]                 = h->current_mv_x * 2; // gets divided by 2 in motion compensation
    s->mv[0][0][1]                 = h->current_mv_y * 2;

intra:
    /* decode each block */
    if (s->mb_intra || HAS_CBP(h->mtype)) {
        s->dsp.clear_blocks(s->block[0]);
        for (i = 0; i < 6; i++) {
            if (h261_decode_block(h, s->block[i], i, cbp & 32) < 0)
                return SLICE_ERROR;
            cbp += cbp;
        }
    } else {
        for (i = 0; i < 6; i++)
            s->block_last_index[i] = -1;
    }

    ff_MPV_decode_mb(s, s->block);

    return SLICE_OK;
}

/**
 * Decode the H.261 picture header.
 * @return <0 if no startcode found
 */
static int h261_decode_picture_header(H261Context *h)
{
    MpegEncContext *const s = &h->s;
    int format, i;
    uint32_t startcode = 0;

    for (i = get_bits_left(&s->gb); i > 24; i -= 1) {
        startcode = ((startcode << 1) | get_bits(&s->gb, 1)) & 0x000FFFFF;

        if (startcode == 0x10)
            break;
    }

    if (startcode != 0x10) {
        av_log(s->avctx, AV_LOG_ERROR, "Bad picture start code\n");
        return -1;
    }

    /* temporal reference */
    i = get_bits(&s->gb, 5); /* picture timestamp */
    if (i < (s->picture_number & 31))
        i += 32;
    s->picture_number = (s->picture_number & ~31) + i;

    s->avctx->time_base      = (AVRational) { 1001, 30000 };
    s->current_picture.f.pts = s->picture_number;

    /* PTYPE starts here */
    skip_bits1(&s->gb); /* split screen off */
    skip_bits1(&s->gb); /* camera  off */
    skip_bits1(&s->gb); /* freeze picture release off */

    format = get_bits1(&s->gb);

    // only 2 formats possible
    if (format == 0) { // QCIF
        s->width     = 176;
        s->height    = 144;
        s->mb_width  = 11;
        s->mb_height = 9;
    } else { // CIF
        s->width     = 352;
        s->height    = 288;
        s->mb_width  = 22;
        s->mb_height = 18;
    }

    s->mb_num = s->mb_width * s->mb_height;

    skip_bits1(&s->gb); /* still image mode off */
    skip_bits1(&s->gb); /* Reserved */

    /* PEI */
    while (get_bits1(&s->gb) != 0)
        skip_bits(&s->gb, 8);

    /* H.261 has no I-frames, but if we pass AV_PICTURE_TYPE_I for the first
     * frame, the codec crashes if it does not contain all I-blocks
     * (e.g. when a packet is lost). */
    s->pict_type = AV_PICTURE_TYPE_P;

    h->gob_number = 0;
    return 0;
}

static int h261_decode_gob(H261Context *h)
{
    MpegEncContext *const s = &h->s;

    ff_set_qscale(s, s->qscale);

    /* decode mb's */
    while (h->current_mba <= MBA_STUFFING) {
        int ret;
        /* DCT & quantize */
        ret = h261_decode_mb(h);
        if (ret < 0) {
            if (ret == SLICE_END) {
                h261_decode_mb_skipped(h, h->current_mba, 33);
                return 0;
            }
            av_log(s->avctx, AV_LOG_ERROR, "Error at MB: %d\n",
                   s->mb_x + s->mb_y * s->mb_stride);
            return -1;
        }

        h261_decode_mb_skipped(h,
                               h->current_mba - h->mba_diff,
                               h->current_mba - 1);
    }

    return -1;
}

/**
 * returns the number of bytes consumed for building the current frame
 */
static int get_consumed_bytes(MpegEncContext *s, int buf_size)
{
    int pos = get_bits_count(&s->gb) >> 3;
    if (pos == 0)
        pos = 1;      // avoid infinite loops (i doubt that is needed but ...)
    if (pos + 10 > buf_size)
        pos = buf_size;               // oops ;)

    return pos;
}

static int h261_decode_frame(AVCodecContext *avctx, void *data,
                             int *got_frame, AVPacket *avpkt)
{
    const uint8_t *buf = avpkt->data;
    int buf_size       = avpkt->size;
    H261Context *h     = avctx->priv_data;
    MpegEncContext *s  = &h->s;
    int ret;
    AVFrame *pict = data;

    av_dlog(avctx, "*****frame %d size=%d\n", avctx->frame_number, buf_size);
    av_dlog(avctx, "bytes=%x %x %x %x\n", buf[0], buf[1], buf[2], buf[3]);
    s->flags  = avctx->flags;
    s->flags2 = avctx->flags2;

    h->gob_start_code_skipped = 0;

retry:
    init_get_bits(&s->gb, buf, buf_size * 8);

    if (!s->context_initialized)
        // we need the IDCT permutaton for reading a custom matrix
        if (ff_MPV_common_init(s) < 0)
            return -1;

    /* We need to set current_picture_ptr before reading the header,
     * otherwise we cannot store anything in there. */
    if (s->current_picture_ptr == NULL || s->current_picture_ptr->f.data[0]) {
        int i = ff_find_unused_picture(s, 0);
        if (i < 0)
            return i;
        s->current_picture_ptr = &s->picture[i];
    }

    ret = h261_decode_picture_header(h);

    /* skip if the header was thrashed */
    if (ret < 0) {
        av_log(s->avctx, AV_LOG_ERROR, "header damaged\n");
        return -1;
    }

    if (s->width != avctx->coded_width || s->height != avctx->coded_height) {
        ParseContext pc = s->parse_context; // FIXME move this demuxing hack to libavformat
        s->parse_context.buffer = 0;
        ff_MPV_common_end(s);
        s->parse_context = pc;
    }
    if (!s->context_initialized) {
        avcodec_set_dimensions(avctx, s->width, s->height);

        goto retry;
    }

    // for skipping the frame
    s->current_picture.f.pict_type = s->pict_type;
    s->current_picture.f.key_frame = s->pict_type == AV_PICTURE_TYPE_I;

    if ((avctx->skip_frame >= AVDISCARD_NONREF && s->pict_type == AV_PICTURE_TYPE_B) ||
        (avctx->skip_frame >= AVDISCARD_NONKEY && s->pict_type != AV_PICTURE_TYPE_I) ||
         avctx->skip_frame >= AVDISCARD_ALL)
        return get_consumed_bytes(s, buf_size);

    if (ff_MPV_frame_start(s, avctx) < 0)
        return -1;

    ff_mpeg_er_frame_start(s);

    /* decode each macroblock */
    s->mb_x = 0;
    s->mb_y = 0;

    while (h->gob_number < (s->mb_height == 18 ? 12 : 5)) {
        if (ff_h261_resync(h) < 0)
            break;
        h261_decode_gob(h);
    }
    ff_MPV_frame_end(s);

    av_assert0(s->current_picture.f.pict_type == s->current_picture_ptr->f.pict_type);
    av_assert0(s->current_picture.f.pict_type == s->pict_type);

    if ((ret = av_frame_ref(pict, &s->current_picture_ptr->f)) < 0)
        return ret;
    ff_print_debug_info(s, s->current_picture_ptr, pict);

    *got_frame = 1;

    return get_consumed_bytes(s, buf_size);
}

static av_cold int h261_decode_end(AVCodecContext *avctx)
{
    H261Context *h    = avctx->priv_data;
    MpegEncContext *s = &h->s;

    ff_MPV_common_end(s);
    return 0;
}

AVCodec ff_h261_decoder = {
    .name           = "h261",
    .type           = AVMEDIA_TYPE_VIDEO,
    .id             = AV_CODEC_ID_H261,
    .priv_data_size = sizeof(H261Context),
    .init           = h261_decode_init,
    .close          = h261_decode_end,
    .decode         = h261_decode_frame,
    .capabilities   = CODEC_CAP_DR1,
    .max_lowres     = 3,
    .long_name      = NULL_IF_CONFIG_SMALL("H.261"),
};