Re: [FFmpeg-devel] Misc mpegvideo patches

[Ramiro Polla <ramiro.polla@gmail.com>]

On Tue, Mar 4, 2025 at 6:05 PM Andreas Rheinhardt
<andreas.rheinhardt@outlook.com> wrote:
> Ramiro Polla:
> >
> > On 3/4/25 14:42, Andreas Rheinhardt wrote:
> >> (Mostly trivial) patches attached. A branch is at
> >> https://github.com/mkver/FFmpeg/tree/mpegvideo_misc
> >
> >
> > [PATCH 10/40] avcodec/mpegvideo_enc: Move default_mv_penalty to h261enc.c
> >
> >> diff --git a/libavcodec/h261enc.c b/libavcodec/h261enc.c
> >> index dabab9d80a..e33bf35a8a 100644
> >> --- a/libavcodec/h261enc.c
> >> +++ b/libavcodec/h261enc.c
> >> @@ -46,6 +46,7 @@ static struct VLCLUT {
> >>      uint16_t code;
> >>  } vlc_lut[H261_MAX_RUN + 1][32 /* 0..2 * H261_MAX_LEN are used */];
> >>
> >> +static uint8_t mv_penalty[MAX_FCODE + 1][MAX_DMV * 2 + 1];
> >>  static uint8_t uni_h261_rl_len     [64 * 128];
> >>  static uint8_t uni_h261_rl_len_last[64 * 128];
> >>  static uint8_t h261_mv_codes[64][2];
> >> @@ -370,6 +371,8 @@ av_cold int ff_h261_encode_init(MpegEncContext *s)
> >>      s->max_qcoeff       = 127;
> >>      s->ac_esc_length    = H261_ESC_LEN;
> >>
> >> +    s->me.mv_penalty = mv_penalty;
> >> +
> >>      s->intra_ac_vlc_length      = s->inter_ac_vlc_length      =
> >> uni_h261_rl_len;
> >>      s->intra_ac_vlc_last_length = s->inter_ac_vlc_last_length =
> >> uni_h261_rl_len_last;
> >>      ff_thread_once(&init_static_once, h261_encode_init_static);
> >
> > This global mv_penalty doesn't seem to be ever initialized; it could be
> > declared const.
>
> But then it would no longer be placed in .bss, but instead in .rodata
> and increase binary size.

Wow, that's a huge array.

> > But it also makes me think that whatever code is using this mv_penalty,
> > which is always set to zero, might be calculating things wrong.
> >
>
> It is obviously done to avoid branches for the codecs that matter. H.261
> does not matter much. Apart from that, it is a very cheap workaround
> given that this table is .bss.

Could you add some comments (either next to the declaration or the
commit message) to reflect this? (save space from .rodata, and this
being a noop for h.261, which doesn't matter that much)

> > [PATCH 15/40] avcodec/ituh263enc: Make SVQ1+Snowenc stop calling
> > ff_h263_encode_init()
> >
> >> diff --git a/libavcodec/ituh263enc.c b/libavcodec/ituh263enc.c
> >> index 02da090ba4..8313b2c2c1 100644
> >> --- a/libavcodec/ituh263enc.c
> >> +++ b/libavcodec/ituh263enc.c
> >> @@ -65,6 +65,127 @@ static uint8_t  uni_h263_inter_rl_len [64*64*2*2];
> > [...]
> >> +static av_cold void h263_encode_init_static(void)
> >> +{
> >> +    static uint8_t rl_intra_table[2][2 * MAX_RUN + MAX_LEVEL + 3];
> >> +
> >> +    ff_rl_init(&ff_rl_intra_aic, rl_intra_table);
> >> +    ff_h263_init_rl_inter();
> >> +
> >> +    init_uni_h263_rl_tab(&ff_rl_intra_aic,  uni_h263_intra_aic_rl_len);
> >> +    init_uni_h263_rl_tab(&ff_h263_rl_inter, uni_h263_inter_rl_len);
> >> +
> >> +    init_mv_penalty_and_fcode();
> >> +}
> >> +
> >> +av_cold const uint8_t (*ff_h263_get_mv_penalty(void))[MAX_DMV*2+1]
> >> +{
> >> +    static AVOnce init_static_once = AV_ONCE_INIT;
> >> +
> >> +    ff_thread_once(&init_static_once, h263_encode_init_static);
> >> +
> >> +    return mv_penalty;
> >> +}
> >> +
> >
> > This approach kind of hides the rest of h263_encode_init_static() inside
> > ff_h263_get_mv_penalty(), so the name is a bit misleading. I'd expect
> > h263 to still call some init function and ff_h263_get_mv_penalty(), and
> > SVQ1 and Snow to only call ff_h263_get_mv_penalty(), which would only
> > take care of the mv_penalty table.
> >
>
> This would entail using another AVOnce etc. and this level of
> granularity is just not worth it.

Ok.

> The name is chosen for what it does for an outsider (i.e. from the
> perspective of svq1enc or snowenc, not the actual H.263 based encoders).

I'm still not quite happy with the name and how it's used, but it's
good enough so I won't insist.

> > [PATCH 20/40] avcodec/mpeg4video: Split ff_mpeg4_pred_dc()
> >
> >> diff --git a/libavcodec/mpeg4videoenc.c b/libavcodec/mpeg4videoenc.c
> >> index 64fb96a0cf..26f9b40ff7 100644
> >> --- a/libavcodec/mpeg4videoenc.c
> >> +++ b/libavcodec/mpeg4videoenc.c
> >> @@ -806,8 +806,14 @@ void ff_mpeg4_encode_mb(MpegEncContext *s,
> >> int16_t block[6][64],
> >>          const uint8_t *scan_table[6];
> >>          int i;
> >>
> >> -        for (i = 0; i < 6; i++)
> >> -            dc_diff[i] = ff_mpeg4_pred_dc(s, i, block[i][0], &dir[i],
> >> 1);
> >> +        for (int i = 0; i < 6; i++) {
> >
> > Redeclaring i inside for.
>
> There are other loops which use this i as loop variable. The shadowing
> is IMO less bad than keeping loops in their current form (with iterators
> that don't have loop-scope).

Agreed. I also prefer scoped iterators.
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