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avcodec/h261enc: Avoid RLTable when writing macroblock

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The RLTable API in rl.c is not well designed for codecs with
an explicit end-of-block code. ff_h261_rl_tcoeff's vlc has
the EOB code as first element (presumably so that the decoder
can check for it via "if (level == 0)") and this implies
that the indices returned by get_rl_index() are off by one
for run == 0 which is therefore explicitly checked.

This commit changes this by adding a simple LUT for the
values not requiring escaping. It is easy to directly
include the sign bit into this, so this has also been done.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
This commit is contained in:
Andreas Rheinhardt
2024-06-15 17:26:14 +02:00
parent 3874442db2
commit f3829cc72d
1 changed files with 31 additions and 20 deletions
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51
libavcodec/h261enc.c
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@@ -36,6 +36,14 @@
#include "h261enc.h"
#include "mpegvideoenc.h"
#define H261_MAX_RUN 26
#define H261_MAX_LEVEL 15
static struct VLCLUT {
uint8_t len;
uint16_t code;
} vlc_lut[H261_MAX_RUN + 1][32 /* 0..2 * H261_MAX_LEN are used */];
static uint8_t uni_h261_rl_len [64*64*2*2];
#define UNI_ENC_INDEX(last,run,level) ((last)*128*64 + (run)*128 + (level))
@@ -165,10 +173,8 @@ static inline int get_cbp(MpegEncContext *s, int16_t block[6][64])
static void h261_encode_block(H261EncContext *h, int16_t *block, int n)
{
MpegEncContext *const s = &h->s;
int level, run, i, j, last_index, last_non_zero, sign, slevel, code;
const RLTable *rl;
int level, run, i, j, last_index, last_non_zero;
rl = &ff_h261_rl_tcoeff;
if (s->mb_intra) {
/* DC coef */
level = block[0];
@@ -204,24 +210,18 @@ static void h261_encode_block(H261EncContext *h, int16_t *block, int n)
level = block[j];
if (level) {
run = i - last_non_zero - 1;
sign = 0;
slevel = level;
if (level < 0) {
sign = 1;
level = -level;
}
code = get_rl_index(rl, 0 /*no last in H.261, EOB is used*/,
run, level);
if (run == 0 && level < 16)
code += 1;
put_bits(&s->pb, rl->table_vlc[code][1], rl->table_vlc[code][0]);
if (code == rl->n) {
put_bits(&s->pb, 6, run);
av_assert1(slevel != 0);
av_assert1(level <= 127);
put_sbits(&s->pb, 8, slevel);
if (run <= H261_MAX_RUN &&
(unsigned)(level + H261_MAX_LEVEL) <= 2 * H261_MAX_LEVEL &&
vlc_lut[run][level + H261_MAX_LEVEL].len) {
put_bits(&s->pb, vlc_lut[run][level + H261_MAX_LEVEL].len,
vlc_lut[run][level + H261_MAX_LEVEL].code);
} else {
put_bits(&s->pb, 1, sign);
/* Escape */
put_bits(&s->pb, 6 + 6, (1 << 6) | run);
av_assert1(level != 0);
av_assert1(FFABS(level) <= 127);
put_sbits(&s->pb, 8, level);
}
last_non_zero = i;
}
@@ -365,6 +365,17 @@ static av_cold void h261_encode_init_static(void)
ff_rl_init(&ff_h261_rl_tcoeff, h261_rl_table_store);
init_uni_h261_rl_tab(&ff_h261_rl_tcoeff, uni_h261_rl_len);
// The following loop is over the ordinary elements, not EOB or escape.
for (size_t i = 1; i < FF_ARRAY_ELEMS(ff_h261_tcoeff_vlc) - 1; i++) {
unsigned run = ff_h261_tcoeff_run[i];
unsigned level = ff_h261_tcoeff_level[i];
unsigned len = ff_h261_tcoeff_vlc[i][1] + 1 /* sign */;
unsigned code = ff_h261_tcoeff_vlc[i][0];
vlc_lut[run][H261_MAX_LEVEL + level] = (struct VLCLUT){ len, code << 1 };
vlc_lut[run][H261_MAX_LEVEL - level] = (struct VLCLUT){ len, (code << 1) | 1 };
}
}
av_cold int ff_h261_encode_init(MpegEncContext *s)