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FFmpeg/libavcodec/h261dec.c
Andreas Rheinhardt 9f38fac053 avcodec/h261dec: Make VLC smaller 
The VLC for the macroblock address increment uses nine bits;
yet there is no code with this length: All codes are either shorter or
longer. So one can make the table smaller without changing the amount of
codes that need more than one round of parsing.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
2021-01-27 11:53:22 +01:00

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/*
* H.261 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 "libavutil/thread.h"
#include "avcodec.h"
#include "mpeg_er.h"
#include "mpegutils.h"
#include "mpegvideo.h"
#include "h263.h"
#include "h261.h"
#include "internal.h"
#define H261_MBA_VLC_BITS 8
#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_static(void)
{
INIT_VLC_STATIC(&h261_mba_vlc, H261_MBA_VLC_BITS, 35,
ff_h261_mba_bits, 1, 1,
ff_h261_mba_code, 1, 1, 540);
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_FIRST_VLC_RL(ff_h261_rl_tcoeff, 552);
}
static av_cold int h261_decode_init(AVCodecContext *avctx)
{
static AVOnce init_static_once = AV_ONCE_INIT;
H261Context *h = avctx->priv_data;
MpegEncContext *const s = &h->s;
// set defaults
ff_mpv_decode_init(s, avctx);
s->out_format = FMT_H261;
s->low_delay = 1;
avctx->pix_fmt = AV_PIX_FMT_YUV420P;
h->gob_start_code_skipped = 0;
ff_thread_once(&init_static_once, h261_decode_init_static);
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 */
if (skip_1stop_8data_bits(&s->gb) < 0)
return AVERROR_INVALIDDATA;
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 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;
if (s->current_picture.motion_val[0]) {
int b_stride = 2*s->mb_width + 1;
int b_xy = 2 * s->mb_x + (2 * s->mb_y) * b_stride;
s->current_picture.motion_val[0][b_xy][0] = s->mv[0][0][0];
s->current_picture.motion_val[0][b_xy][1] = s->mv[0][0][1];
}
ff_mpv_reconstruct_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 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;
}
{
OPEN_READER(re, &s->gb);
i--; // offset by -1 to allow direct indexing of scan_table
for (;;) {
UPDATE_CACHE(re, &s->gb);
GET_RL_VLC(level, run, re, &s->gb, rl->rl_vlc[0], TCOEFF_VLC_BITS, 2, 0);
if (run == 66) {
if (level) {
CLOSE_READER(re, &s->gb);
av_log(s->avctx, AV_LOG_ERROR, "illegal ac vlc code at %dx%d\n",
s->mb_x, s->mb_y);
return -1;
}
/* 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 = SHOW_UBITS(re, &s->gb, 6) + 1;
SKIP_CACHE(re, &s->gb, 6);
level = SHOW_SBITS(re, &s->gb, 8);
SKIP_COUNTER(re, &s->gb, 6 + 8);
} else if (level == 0) {
break;
} else {
if (SHOW_UBITS(re, &s->gb, 1))
level = -level;
SKIP_COUNTER(re, &s->gb, 1);
}
i += run;
if (i >= 64) {
CLOSE_READER(re, &s->gb);
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;
}
CLOSE_READER(re, &s->gb);
}
s->block_last_index[n] = i;
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, "Invalid mtype index %d\n",
h->mtype);
return SLICE_ERROR;
}
av_assert0(h->mtype < FF_ARRAY_ELEMS(ff_h261_mtype_map));
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, 1) + 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;
if (s->current_picture.motion_val[0]) {
int b_stride = 2*s->mb_width + 1;
int b_xy = 2 * s->mb_x + (2 * s->mb_y) * b_stride;
s->current_picture.motion_val[0][b_xy][0] = s->mv[0][0][0];
s->current_picture.motion_val[0][b_xy][1] = s->mv[0][0][1];
}
intra:
/* decode each block */
if (s->mb_intra || HAS_CBP(h->mtype)) {
s->bdsp.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_reconstruct_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->framerate = (AVRational) { 30000, 1001 };
/* 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 */
if (skip_1stop_8data_bits(&s->gb) < 0)
return AVERROR_INVALIDDATA;
/* 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;
ff_dlog(avctx, "*****frame %d size=%d\n", avctx->frame_number, buf_size);
ff_dlog(avctx, "bytes=%x %x %x %x\n", buf[0], buf[1], buf[2], buf[3]);
h->gob_start_code_skipped = 0;
retry:
init_get_bits(&s->gb, buf, buf_size * 8);
if (!s->context_initialized)
// we need the IDCT permutation for reading a custom matrix
ff_mpv_idct_init(s);
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) {
if ((ret = ff_mpv_common_init(s)) < 0)
return ret;
ret = ff_set_dimensions(avctx, s->width, s->height);
if (ret < 0)
return ret;
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 (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",
.long_name = NULL_IF_CONFIG_SMALL("H.261"),
.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 = AV_CODEC_CAP_DR1,
.caps_internal = FF_CODEC_CAP_INIT_THREADSAFE | FF_CODEC_CAP_INIT_CLEANUP,
.max_lowres = 3,
};
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