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Smoother intensity stereo decision thresholds

We now have rates for each possible threshold. This also adds some
histeresis but it's not that useful for now because the effective rate
doesn't changes from frame to frame (unless one changes the target rate).
This change also has the side effect of lowering the intensity stereo
point for 64 kb/s to band 15 (instead of 16).

--- a/celt/celt.c

+++ b/celt/celt.c

@@ -190,6 +190,7 @@

    opus_int32 vbr_count;

    opus_val16 overlap_max;

    opus_val16 stereo_saving;

+   int intensity;

 #ifdef RESYNTH

    celt_sig syn_mem[2][2*MAX_PERIOD];

@@ -986,7 +987,6 @@

    int alloc_trim;

    int pitch_index=COMBFILTER_MINPERIOD;

    opus_val16 gain1 = 0;

-   int intensity=0;

    int dual_stereo=0;

    int effectiveBytes;

    opus_val16 pf_threshold;

@@ -1569,26 +1569,20 @@

       if (LM!=0)

          dual_stereo = stereo_analysis(st->mode, X, LM, N);

+      static const opus_val16 intensity_thresholds[21]=

+      /* 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19  20  off*/

+        { 16,21,23,25,27,29,31,33,35,38,42,46,50,54,58,63,68,75,84,102,130};

+      static const opus_val16 intensity_histeresis[21]=

+        {  2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 4, 5, 6,  8, 12};

+

       /* Account for coarse energy */

       effectiveRate = (8*effectiveBytes - 80)>>LM;

       /* effectiveRate in kb/s */

       effectiveRate = 2*effectiveRate/5;

-      if (effectiveRate<35)

-         intensity = 8;

-      else if (effectiveRate<50)

-         intensity = 12;

-      else if (effectiveRate<68)

-         intensity = 16;

-      else if (effectiveRate<84)

-         intensity = 18;

-      else if (effectiveRate<102)

-         intensity = 19;

-      else if (effectiveRate<130)

-         intensity = 20;

-      else

-         intensity = 100;

-      intensity = IMIN(st->end,IMAX(st->start, intensity));

+

+      st->intensity = hysteresis_decision(effectiveRate, intensity_thresholds, intensity_histeresis, 21, st->intensity);

+      st->intensity = IMIN(st->end,IMAX(st->start, st->intensity));

    }

    alloc_trim = 5;

@@ -1595,7 +1589,7 @@

    if (tell+(6<<BITRES) <= total_bits - total_boost)

    {

       alloc_trim = alloc_trim_analysis(st->mode, X, bandLogE,

-            st->end, LM, C, N, &st->analysis, &st->stereo_saving, tf_estimate, intensity);

+            st->end, LM, C, N, &st->analysis, &st->stereo_saving, tf_estimate, st->intensity);

       ec_enc_icdf(enc, alloc_trim, trim_icdf, 7);

       tell = ec_tell_frac(enc);

    }

@@ -1614,7 +1608,7 @@

      coded_bands = st->lastCodedBands ? st->lastCodedBands : st->mode->nbEBands;

      coded_bins = st->mode->eBands[coded_bands]<<LM;

      if (C==2)

-        coded_bins += st->mode->eBands[IMIN(intensity, coded_bands)]<<LM;

+        coded_bins += st->mode->eBands[IMIN(st->intensity, coded_bands)]<<LM;

      /* Don't attempt to use more than 510 kb/s, even for frames smaller than 20 ms.

         The CELT allocator will just not be able to use more than that anyway. */

@@ -1632,7 +1626,7 @@

      {

         int coded_stereo_bands;

         int coded_stereo_dof;

-        coded_stereo_bands = IMIN(intensity, coded_bands);

+        coded_stereo_bands = IMIN(st->intensity, coded_bands);

         coded_stereo_dof = (st->mode->eBands[coded_stereo_bands]<<LM)-coded_stereo_bands;

         /*printf("%d %d %d ", coded_stereo_dof, coded_bins, tot_boost);*/

         target -= MIN32(target/3, SHR16(MULT16_16(st->stereo_saving,(coded_stereo_dof<<BITRES)),8));

@@ -1744,7 +1738,7 @@

    anti_collapse_rsv = isTransient&&LM>=2&&bits>=((LM+2)<<BITRES) ? (1<<BITRES) : 0;

    bits -= anti_collapse_rsv;

    codedBands = compute_allocation(st->mode, st->start, st->end, offsets, cap,

-         alloc_trim, &intensity, &dual_stereo, bits, &balance, pulses,

+         alloc_trim, &st->intensity, &dual_stereo, bits, &balance, pulses,

          fine_quant, fine_priority, C, LM, enc, 1, st->lastCodedBands);

    st->lastCodedBands = codedBands;

@@ -1764,7 +1758,7 @@

    /* Residual quantisation */

    ALLOC(collapse_masks, C*st->mode->nbEBands, unsigned char);

    quant_all_bands(1, st->mode, st->start, st->end, X, C==2 ? X+N : NULL, collapse_masks,

-         bandE, pulses, shortBlocks, st->spread_decision, dual_stereo, intensity, tf_res,

+         bandE, pulses, shortBlocks, st->spread_decision, dual_stereo, st->intensity, tf_res,

          nbCompressedBytes*(8<<BITRES)-anti_collapse_rsv, balance, enc, LM, codedBands, &st->rng);

    if (anti_collapse_rsv > 0)