Message ID | 20190619234521.15619-15-andreas.rheinhardt@gmail.com |
---|---|

State | New |

Headers | show |

On 20/06/2019 00:45, Andreas Rheinhardt wrote: > Remove superfluous trailing zeros from slices. Because MPEG-2 slices > can end with zero bits a safe number of trailing zero bits is always > kept. > > More explicitly, 6 + max{f_code[i][1] - 1, i = 0,1, f_code[i][1] != 0xf} > is an upper bound for the number of possible trailing zeros that are > part of the slice. Here f_code[i][1] is the relevant value of the > picture coding extension the slice belongs to and the maximum of the > empty set is zero. > It is this number of trailing zero bits that is actually kept. > > That this is really an upper bound can be seen as follows: > > a) Every slice actually ends with a macroblock. > > b) If the last macroblock of a slice ends with a block(i) structure > with pattern_code[i] != 0, then the slice ends with an "End of block" > VLC code (namely the "End of block" code of the last block with > pattern_code[i] != 0). > These codes are 10 and 0110, so that in this case there is exactly one > trailing zero bit. > > c) Otherwise, all pattern_code[i] are zero. In this case, > if macroblock_pattern is set for the last macroblock of the slice, then > by the definition of pattern_code[i] in 6.3.17.4 cbp (derived > according to table B.9) must be zero and also the > coded_block_pattern_1/2 (if existing) must consist of zeros alone. The > value zero for cbp is coded by 0000 0000 1 so that the maximum number of > trailing zeros in this case is the length of coded_block_pattern_1/2 which > have a length of two resp. six bits. So six trailing zero bits at most. > > d) Otherwise, if the slice actually ends with the marker bit of the > last macroblock, then there are certainly no trailing zero bits at > all. > > e) Otherwise, if the slice ends with a motion_vectors(s) structure > (with s = 0 or 1 -- it doesn't matter which one), then it ends with a > motion_vector(r,s) (r, s = 0, 1 -- it doesn't matter) structure. This > structure ends with motion_code[r][s][1] (always existing) potentially > followed by motion_residual[r][s][1] and dmvector[1]. If dmvector[1] > exists, and contains a bit different from 0, there is at most one > trailing zero bit; if dmvector[1] consists of zeros alone, its length > is one according to B.11. motion_residual[r][s][1] (if it exists) has > a length of f_code[s][1] - 1 bits and can consist of zero bits alone. > Given that the value 0xf for f_code indicates that there is no motion > vector of the mentioned type, the length of motion_residual[r][s][1] is > bounded by max{f_code[i][1] - 1, i=1,2, f_code[i][1] != 0xf}. The > motion_code[r][s][1] can end with at most five zero bits (see B.10) > and always contains a bit set to one, so that in this case there are > at most 5 + max{f_code[i][1] - 1, i=1,2, f_code[i][1] != 0xf} + 1 > zero trailing bits. > > f) Otherwise, if the last macroblock of the slice ends with a > quantiser_scale_code, then there are at most four trailing zero bits, > because quantiser_scale_code has a length of five bits and must not > attain the value zero. > > g) Otherwise, the last macroblock ends with the macroblock_modes > syntax structure. The potentially existing dct_type at the end might > be a zero bit; the frame/field_motion_type isn't present here, because > otherwise we would have a motion_vectors(i) (i = 0 or 1 or both) syntax > structure, so that e) (or b)-d)) would have applied. > spatial_temporal_weight_code might entirely consist of two zero bits. > The macroblock_type VLC code always contains a 1 bit and ends with two > zero bits at most (see B.2-B.8 for this), so we have maximally 2+2+1 > trailing zero bits. > > The fate test cbs-mpeg2-sony-ct3 had to be adapted because the input > file contains trailing zeros that were stripped away; the filesize is > reduced from 135 KB to 117 KB. Of course, decoding the smaller output > still produces the same frames. > Most of these savings happen in between slices rather than after the > last slice: The chomp bitstream filter can only reduce the filesize > by 50 bytes. > > Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com> > --- > libavcodec/cbs_mpeg2.c | 26 ++++++++++++++++++++++++-- > tests/ref/fate/cbs-mpeg2-sony-ct3 | 2 +- > 2 files changed, 25 insertions(+), 3 deletions(-) > > diff --git a/libavcodec/cbs_mpeg2.c b/libavcodec/cbs_mpeg2.c > index 516b728a64..0630fe17d8 100644 > --- a/libavcodec/cbs_mpeg2.c > +++ b/libavcodec/cbs_mpeg2.c > @@ -170,7 +170,7 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx, > uint8_t *unit_data; > uint32_t start_code = -1, next_start_code = -1; > size_t unit_size; > - int err, i, unit_type; > + int err, i, unit_type, max_trailing_bits = 14; > > start = avpriv_find_start_code(frag->data, frag->data + frag->data_size, > &start_code); > @@ -187,10 +187,32 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx, > unit_size = end - (start - 1); > } else { > // Unit runs from start to the beginning of the start code > - // pointed to by end (including any padding zeroes). > + // pointed to by end (preliminarily including any padding zeroes). > unit_size = (end - 4) - (start - 1); > } The start pointer being offset by one from the start of the unit makes the following code more confusing. Perhaps you could adjust start here before using it? > > + if (unit_type == MPEG2_START_EXTENSION && unit_size >= 4 && > + *start >> 4 == MPEG2_EXTENSION_PICTURE_CODING) { > + // The values f_code[0][1], f_code[1][1] are used to improve > + // the upper bound for the number of trailing zero bits. > + // 6 + max{f_code[i][1] - 1, i = 0,1, f_code[i][1] != 0xf} is > + // an upper bound. An f_code value of 0xf means that there is > + // no motion vector of the respective type. > + max_trailing_bits = start[1] >> 4 == 0xf ? 0 : (start[1] >> 4) - 1; > + max_trailing_bits = FFMAX(start[2] >> 4 == 0xf ? > + 0 : (start[2] >> 4) - 1, > + max_trailing_bits) + 6; The f_code values aren't yet verified, so could be invalid. Does that matter? The symmetry between the two start[] expressions is a bit hidden - maybe make sure they have matching code alignment. > + } > + > + if (MPEG2_START_IS_SLICE(unit_type)) { > + const uint8_t *tmp = start + unit_size - 2; > + > + while (tmp > start && *tmp == 0) > + tmp--; > + unit_size = FFMIN(unit_size, tmp - start + max_trailing_bits / 8 + > + !!(*tmp & 0xff >> 8 - max_trailing_bits % 8) + 2); I think a calculation based on max_trailing_bits - ctz(*tmp) might be clearer? > + } > + > unit_data = (uint8_t *)start - 1; > > err = ff_cbs_insert_unit_data(ctx, frag, i, unit_type, Tbh the tricksiness here feels excessive to me, though I have no objection to it. Thanks, - Mark

diff --git a/libavcodec/cbs_mpeg2.c b/libavcodec/cbs_mpeg2.c index 516b728a64..0630fe17d8 100644 --- a/libavcodec/cbs_mpeg2.c +++ b/libavcodec/cbs_mpeg2.c @@ -170,7 +170,7 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx, uint8_t *unit_data; uint32_t start_code = -1, next_start_code = -1; size_t unit_size; - int err, i, unit_type; + int err, i, unit_type, max_trailing_bits = 14; start = avpriv_find_start_code(frag->data, frag->data + frag->data_size, &start_code); @@ -187,10 +187,32 @@ static int cbs_mpeg2_split_fragment(CodedBitstreamContext *ctx, unit_size = end - (start - 1); } else { // Unit runs from start to the beginning of the start code - // pointed to by end (including any padding zeroes). + // pointed to by end (preliminarily including any padding zeroes). unit_size = (end - 4) - (start - 1); } + if (unit_type == MPEG2_START_EXTENSION && unit_size >= 4 && + *start >> 4 == MPEG2_EXTENSION_PICTURE_CODING) { + // The values f_code[0][1], f_code[1][1] are used to improve + // the upper bound for the number of trailing zero bits. + // 6 + max{f_code[i][1] - 1, i = 0,1, f_code[i][1] != 0xf} is + // an upper bound. An f_code value of 0xf means that there is + // no motion vector of the respective type. + max_trailing_bits = start[1] >> 4 == 0xf ? 0 : (start[1] >> 4) - 1; + max_trailing_bits = FFMAX(start[2] >> 4 == 0xf ? + 0 : (start[2] >> 4) - 1, + max_trailing_bits) + 6; + } + + if (MPEG2_START_IS_SLICE(unit_type)) { + const uint8_t *tmp = start + unit_size - 2; + + while (tmp > start && *tmp == 0) + tmp--; + unit_size = FFMIN(unit_size, tmp - start + max_trailing_bits / 8 + + !!(*tmp & 0xff >> 8 - max_trailing_bits % 8) + 2); + } + unit_data = (uint8_t *)start - 1; err = ff_cbs_insert_unit_data(ctx, frag, i, unit_type, diff --git a/tests/ref/fate/cbs-mpeg2-sony-ct3 b/tests/ref/fate/cbs-mpeg2-sony-ct3 index b5b4b12f07..ab226794d5 100644 --- a/tests/ref/fate/cbs-mpeg2-sony-ct3 +++ b/tests/ref/fate/cbs-mpeg2-sony-ct3 @@ -1 +1 @@ -b1e15a09cfffbad801810af0928736ab +033db56a4632c7bbcf729eddd7d5300b

`Remove superfluous trailing zeros from slices. Because MPEG-2 slices can end with zero bits a safe number of trailing zero bits is always kept. More explicitly, 6 + max{f_code[i][1] - 1, i = 0,1, f_code[i][1] != 0xf} is an upper bound for the number of possible trailing zeros that are part of the slice. Here f_code[i][1] is the relevant value of the picture coding extension the slice belongs to and the maximum of the empty set is zero. It is this number of trailing zero bits that is actually kept. That this is really an upper bound can be seen as follows: a) Every slice actually ends with a macroblock. b) If the last macroblock of a slice ends with a block(i) structure with pattern_code[i] != 0, then the slice ends with an "End of block" VLC code (namely the "End of block" code of the last block with pattern_code[i] != 0). These codes are 10 and 0110, so that in this case there is exactly one trailing zero bit. c) Otherwise, all pattern_code[i] are zero. In this case, if macroblock_pattern is set for the last macroblock of the slice, then by the definition of pattern_code[i] in 6.3.17.4 cbp (derived according to table B.9) must be zero and also the coded_block_pattern_1/2 (if existing) must consist of zeros alone. The value zero for cbp is coded by 0000 0000 1 so that the maximum number of trailing zeros in this case is the length of coded_block_pattern_1/2 which have a length of two resp. six bits. So six trailing zero bits at most. d) Otherwise, if the slice actually ends with the marker bit of the last macroblock, then there are certainly no trailing zero bits at all. e) Otherwise, if the slice ends with a motion_vectors(s) structure (with s = 0 or 1 -- it doesn't matter which one), then it ends with a motion_vector(r,s) (r, s = 0, 1 -- it doesn't matter) structure. This structure ends with motion_code[r][s][1] (always existing) potentially followed by motion_residual[r][s][1] and dmvector[1]. If dmvector[1] exists, and contains a bit different from 0, there is at most one trailing zero bit; if dmvector[1] consists of zeros alone, its length is one according to B.11. motion_residual[r][s][1] (if it exists) has a length of f_code[s][1] - 1 bits and can consist of zero bits alone. Given that the value 0xf for f_code indicates that there is no motion vector of the mentioned type, the length of motion_residual[r][s][1] is bounded by max{f_code[i][1] - 1, i=1,2, f_code[i][1] != 0xf}. The motion_code[r][s][1] can end with at most five zero bits (see B.10) and always contains a bit set to one, so that in this case there are at most 5 + max{f_code[i][1] - 1, i=1,2, f_code[i][1] != 0xf} + 1 zero trailing bits. f) Otherwise, if the last macroblock of the slice ends with a quantiser_scale_code, then there are at most four trailing zero bits, because quantiser_scale_code has a length of five bits and must not attain the value zero. g) Otherwise, the last macroblock ends with the macroblock_modes syntax structure. The potentially existing dct_type at the end might be a zero bit; the frame/field_motion_type isn't present here, because otherwise we would have a motion_vectors(i) (i = 0 or 1 or both) syntax structure, so that e) (or b)-d)) would have applied. spatial_temporal_weight_code might entirely consist of two zero bits. The macroblock_type VLC code always contains a 1 bit and ends with two zero bits at most (see B.2-B.8 for this), so we have maximally 2+2+1 trailing zero bits. The fate test cbs-mpeg2-sony-ct3 had to be adapted because the input file contains trailing zeros that were stripped away; the filesize is reduced from 135 KB to 117 KB. Of course, decoding the smaller output still produces the same frames. Most of these savings happen in between slices rather than after the last slice: The chomp bitstream filter can only reduce the filesize by 50 bytes. Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com> --- libavcodec/cbs_mpeg2.c | 26 ++++++++++++++++++++++++-- tests/ref/fate/cbs-mpeg2-sony-ct3 | 2 +- 2 files changed, 25 insertions(+), 3 deletions(-)`