third_party/libwebp/enc/quant.c

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This source file includes following definitions.
PrintBlockInfo
clip
ExpandMatrix
SetupMatrices
SetupFilterStrength
QualityToCompression
QualityToJPEGCompression
SegmentsAreEquivalent
SimplifySegments
VP8SetSegmentParams
VP8MakeLuma16Preds
VP8MakeChroma8Preds
VP8MakeIntra4Preds
InitScore
CopyScore
AddScore
SetRDScore
RDScoreTrellis
TrellisQuantizeBlock
ReconstructIntra16
ReconstructIntra4
ReconstructUV
StoreMaxDelta
SwapPtr
SwapOut
IsFlat
PickBestIntra16
GetCostModeI4
PickBestIntra4
PickBestUV
SimpleQuantize
DistoRefine
VP8Decimate
// Copyright 2011 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
//   Quantization
//
// Author: Skal (pascal.massimino@gmail.com)

#include <assert.h>
#include <math.h>
#include <stdlib.h>  // for abs()

#include "./vp8enci.h"
#include "./cost.h"

#define DO_TRELLIS_I4  1
#define DO_TRELLIS_I16 1   // not a huge gain, but ok at low bitrate.
#define DO_TRELLIS_UV  0   // disable trellis for UV. Risky. Not worth.
#define USE_TDISTO 1

#define MID_ALPHA 64      // neutral value for susceptibility
#define MIN_ALPHA 30      // lowest usable value for susceptibility
#define MAX_ALPHA 100     // higher meaningful value for susceptibility

#define SNS_TO_DQ 0.9     // Scaling constant between the sns value and the QP
                          // power-law modulation. Must be strictly less than 1.

#define I4_PENALTY 4000   // Rate-penalty for quick i4/i16 decision

// number of non-zero coeffs below which we consider the block very flat
// (and apply a penalty to complex predictions)
#define FLATNESS_LIMIT_I16 10      // I16 mode
#define FLATNESS_LIMIT_I4  3       // I4 mode
#define FLATNESS_LIMIT_UV  2       // UV mode
#define FLATNESS_PENALTY   140     // roughly ~1bit per block

#define MULT_8B(a, b) (((a) * (b) + 128) >> 8)

// #define DEBUG_BLOCK

//------------------------------------------------------------------------------

#if defined(DEBUG_BLOCK)

#include <stdio.h>
#include <stdlib.h>

static void PrintBlockInfo(const VP8EncIterator* const it,
                           const VP8ModeScore* const rd) {
  int i, j;
  const int is_i16 = (it->mb_->type_ == 1);
  printf("SOURCE / OUTPUT / ABS DELTA\n");
  for (j = 0; j < 24; ++j) {
    if (j == 16) printf("\n");   // newline before the U/V block
    for (i = 0; i < 16; ++i) printf("%3d ", it->yuv_in_[i + j * BPS]);
    printf("     ");
    for (i = 0; i < 16; ++i) printf("%3d ", it->yuv_out_[i + j * BPS]);
    printf("     ");
    for (i = 0; i < 16; ++i) {
      printf("%1d ", abs(it->yuv_out_[i + j * BPS] - it->yuv_in_[i + j * BPS]));
    }
    printf("\n");
  }
  printf("\nD:%d SD:%d R:%d H:%d nz:0x%x score:%d\n",
    (int)rd->D, (int)rd->SD, (int)rd->R, (int)rd->H, (int)rd->nz,
    (int)rd->score);
  if (is_i16) {
    printf("Mode: %d\n", rd->mode_i16);
    printf("y_dc_levels:");
    for (i = 0; i < 16; ++i) printf("%3d ", rd->y_dc_levels[i]);
    printf("\n");
  } else {
    printf("Modes[16]: ");
    for (i = 0; i < 16; ++i) printf("%d ", rd->modes_i4[i]);
    printf("\n");
  }
  printf("y_ac_levels:\n");
  for (j = 0; j < 16; ++j) {
    for (i = is_i16 ? 1 : 0; i < 16; ++i) {
      printf("%4d ", rd->y_ac_levels[j][i]);
    }
    printf("\n");
  }
  printf("\n");
  printf("uv_levels (mode=%d):\n", rd->mode_uv);
  for (j = 0; j < 8; ++j) {
    for (i = 0; i < 16; ++i) {
      printf("%4d ", rd->uv_levels[j][i]);
    }
    printf("\n");
  }
}

#endif   // DEBUG_BLOCK

//------------------------------------------------------------------------------

static WEBP_INLINE int clip(int v, int m, int M) {
  return v < m ? m : v > M ? M : v;
}

static const uint8_t kZigzag[16] = {
  0, 1, 4, 8, 5, 2, 3, 6, 9, 12, 13, 10, 7, 11, 14, 15
};

static const uint8_t kDcTable[128] = {
  4,     5,   6,   7,   8,   9,  10,  10,
  11,   12,  13,  14,  15,  16,  17,  17,
  18,   19,  20,  20,  21,  21,  22,  22,
  23,   23,  24,  25,  25,  26,  27,  28,
  29,   30,  31,  32,  33,  34,  35,  36,
  37,   37,  38,  39,  40,  41,  42,  43,
  44,   45,  46,  46,  47,  48,  49,  50,
  51,   52,  53,  54,  55,  56,  57,  58,
  59,   60,  61,  62,  63,  64,  65,  66,
  67,   68,  69,  70,  71,  72,  73,  74,
  75,   76,  76,  77,  78,  79,  80,  81,
  82,   83,  84,  85,  86,  87,  88,  89,
  91,   93,  95,  96,  98, 100, 101, 102,
  104, 106, 108, 110, 112, 114, 116, 118,
  122, 124, 126, 128, 130, 132, 134, 136,
  138, 140, 143, 145, 148, 151, 154, 157
};

static const uint16_t kAcTable[128] = {
  4,     5,   6,   7,   8,   9,  10,  11,
  12,   13,  14,  15,  16,  17,  18,  19,
  20,   21,  22,  23,  24,  25,  26,  27,
  28,   29,  30,  31,  32,  33,  34,  35,
  36,   37,  38,  39,  40,  41,  42,  43,
  44,   45,  46,  47,  48,  49,  50,  51,
  52,   53,  54,  55,  56,  57,  58,  60,
  62,   64,  66,  68,  70,  72,  74,  76,
  78,   80,  82,  84,  86,  88,  90,  92,
  94,   96,  98, 100, 102, 104, 106, 108,
  110, 112, 114, 116, 119, 122, 125, 128,
  131, 134, 137, 140, 143, 146, 149, 152,
  155, 158, 161, 164, 167, 170, 173, 177,
  181, 185, 189, 193, 197, 201, 205, 209,
  213, 217, 221, 225, 229, 234, 239, 245,
  249, 254, 259, 264, 269, 274, 279, 284
};

static const uint16_t kAcTable2[128] = {
  8,     8,   9,  10,  12,  13,  15,  17,
  18,   20,  21,  23,  24,  26,  27,  29,
  31,   32,  34,  35,  37,  38,  40,  41,
  43,   44,  46,  48,  49,  51,  52,  54,
  55,   57,  58,  60,  62,  63,  65,  66,
  68,   69,  71,  72,  74,  75,  77,  79,
  80,   82,  83,  85,  86,  88,  89,  93,
  96,   99, 102, 105, 108, 111, 114, 117,
  120, 124, 127, 130, 133, 136, 139, 142,
  145, 148, 151, 155, 158, 161, 164, 167,
  170, 173, 176, 179, 184, 189, 193, 198,
  203, 207, 212, 217, 221, 226, 230, 235,
  240, 244, 249, 254, 258, 263, 268, 274,
  280, 286, 292, 299, 305, 311, 317, 323,
  330, 336, 342, 348, 354, 362, 370, 379,
  385, 393, 401, 409, 416, 424, 432, 440
};

static const uint8_t kBiasMatrices[3][2] = {  // [luma-ac,luma-dc,chroma][dc,ac]
  { 96, 110 }, { 96, 108 }, { 110, 115 }
};

// Sharpening by (slightly) raising the hi-frequency coeffs.
// Hack-ish but helpful for mid-bitrate range. Use with care.
#define SHARPEN_BITS 11  // number of descaling bits for sharpening bias
static const uint8_t kFreqSharpening[16] = {
  0,  30, 60, 90,
  30, 60, 90, 90,
  60, 90, 90, 90,
  90, 90, 90, 90
};

//------------------------------------------------------------------------------
// Initialize quantization parameters in VP8Matrix

// Returns the average quantizer
static int ExpandMatrix(VP8Matrix* const m, int type) {
  int i, sum;
  for (i = 0; i < 2; ++i) {
    const int is_ac_coeff = (i > 0);
    const int bias = kBiasMatrices[type][is_ac_coeff];
    m->iq_[i] = (1 << QFIX) / m->q_[i];
    m->bias_[i] = BIAS(bias);
    // zthresh_ is the exact value such that QUANTDIV(coeff, iQ, B) is:
    //   * zero if coeff <= zthresh
    //   * non-zero if coeff > zthresh
    m->zthresh_[i] = ((1 << QFIX) - 1 - m->bias_[i]) / m->iq_[i];
  }
  for (i = 2; i < 16; ++i) {
    m->q_[i] = m->q_[1];
    m->iq_[i] = m->iq_[1];
    m->bias_[i] = m->bias_[1];
    m->zthresh_[i] = m->zthresh_[1];
  }
  for (sum = 0, i = 0; i < 16; ++i) {
    if (type == 0) {  // we only use sharpening for AC luma coeffs
      m->sharpen_[i] = (kFreqSharpening[i] * m->q_[i]) >> SHARPEN_BITS;
    } else {
      m->sharpen_[i] = 0;
    }
    sum += m->q_[i];
  }
  return (sum + 8) >> 4;
}

static void SetupMatrices(VP8Encoder* enc) {
  int i;
  const int tlambda_scale =
    (enc->method_ >= 4) ? enc->config_->sns_strength
                        : 0;
  const int num_segments = enc->segment_hdr_.num_segments_;
  for (i = 0; i < num_segments; ++i) {
    VP8SegmentInfo* const m = &enc->dqm_[i];
    const int q = m->quant_;
    int q4, q16, quv;
    m->y1_.q_[0] = kDcTable[clip(q + enc->dq_y1_dc_, 0, 127)];
    m->y1_.q_[1] = kAcTable[clip(q,                  0, 127)];

    m->y2_.q_[0] = kDcTable[ clip(q + enc->dq_y2_dc_, 0, 127)] * 2;
    m->y2_.q_[1] = kAcTable2[clip(q + enc->dq_y2_ac_, 0, 127)];

    m->uv_.q_[0] = kDcTable[clip(q + enc->dq_uv_dc_, 0, 117)];
    m->uv_.q_[1] = kAcTable[clip(q + enc->dq_uv_ac_, 0, 127)];

    q4  = ExpandMatrix(&m->y1_, 0);
    q16 = ExpandMatrix(&m->y2_, 1);
    quv = ExpandMatrix(&m->uv_, 2);

    m->lambda_i4_          = (3 * q4 * q4) >> 7;
    m->lambda_i16_         = (3 * q16 * q16);
    m->lambda_uv_          = (3 * quv * quv) >> 6;
    m->lambda_mode_        = (1 * q4 * q4) >> 7;
    m->lambda_trellis_i4_  = (7 * q4 * q4) >> 3;
    m->lambda_trellis_i16_ = (q16 * q16) >> 2;
    m->lambda_trellis_uv_  = (quv *quv) << 1;
    m->tlambda_            = (tlambda_scale * q4) >> 5;

    m->min_disto_ = 10 * m->y1_.q_[0];   // quantization-aware min disto
    m->max_edge_  = 0;
  }
}

//------------------------------------------------------------------------------
// Initialize filtering parameters

// Very small filter-strength values have close to no visual effect. So we can
// save a little decoding-CPU by turning filtering off for these.
#define FSTRENGTH_CUTOFF 2

static void SetupFilterStrength(VP8Encoder* const enc) {
  int i;
  // level0 is in [0..500]. Using '-f 50' as filter_strength is mid-filtering.
  const int level0 = 5 * enc->config_->filter_strength;
  for (i = 0; i < NUM_MB_SEGMENTS; ++i) {
    VP8SegmentInfo* const m = &enc->dqm_[i];
    // We focus on the quantization of AC coeffs.
    const int qstep = kAcTable[clip(m->quant_, 0, 127)] >> 2;
    const int base_strength =
        VP8FilterStrengthFromDelta(enc->filter_hdr_.sharpness_, qstep);
    // Segments with lower complexity ('beta') will be less filtered.
    const int f = base_strength * level0 / (256 + m->beta_);
    m->fstrength_ = (f < FSTRENGTH_CUTOFF) ? 0 : (f > 63) ? 63 : f;
  }
  // We record the initial strength (mainly for the case of 1-segment only).
  enc->filter_hdr_.level_ = enc->dqm_[0].fstrength_;
  enc->filter_hdr_.simple_ = (enc->config_->filter_type == 0);
  enc->filter_hdr_.sharpness_ = enc->config_->filter_sharpness;
}

//------------------------------------------------------------------------------

// Note: if you change the values below, remember that the max range
// allowed by the syntax for DQ_UV is [-16,16].
#define MAX_DQ_UV (6)
#define MIN_DQ_UV (-4)

// We want to emulate jpeg-like behaviour where the expected "good" quality
// is around q=75. Internally, our "good" middle is around c=50. So we
// map accordingly using linear piece-wise function
static double QualityToCompression(double c) {
  const double linear_c = (c < 0.75) ? c * (2. / 3.) : 2. * c - 1.;
  // The file size roughly scales as pow(quantizer, 3.). Actually, the
  // exponent is somewhere between 2.8 and 3.2, but we're mostly interested
  // in the mid-quant range. So we scale the compressibility inversely to
  // this power-law: quant ~= compression ^ 1/3. This law holds well for
  // low quant. Finer modeling for high-quant would make use of kAcTable[]
  // more explicitly.
  const double v = pow(linear_c, 1 / 3.);
  return v;
}

static double QualityToJPEGCompression(double c, double alpha) {
  // We map the complexity 'alpha' and quality setting 'c' to a compression
  // exponent empirically matched to the compression curve of libjpeg6b.
  // On average, the WebP output size will be roughly similar to that of a
  // JPEG file compressed with same quality factor.
  const double amin = 0.30;
  const double amax = 0.85;
  const double exp_min = 0.4;
  const double exp_max = 0.9;
  const double slope = (exp_min - exp_max) / (amax - amin);
  // Linearly interpolate 'expn' from exp_min to exp_max
  // in the [amin, amax] range.
  const double expn = (alpha > amax) ? exp_min
                    : (alpha < amin) ? exp_max
                    : exp_max + slope * (alpha - amin);
  const double v = pow(c, expn);
  return v;
}

static int SegmentsAreEquivalent(const VP8SegmentInfo* const S1,
                                 const VP8SegmentInfo* const S2) {
  return (S1->quant_ == S2->quant_) && (S1->fstrength_ == S2->fstrength_);
}

static void SimplifySegments(VP8Encoder* const enc) {
  int map[NUM_MB_SEGMENTS] = { 0, 1, 2, 3 };
  const int num_segments = enc->segment_hdr_.num_segments_;
  int num_final_segments = 1;
  int s1, s2;
  for (s1 = 1; s1 < num_segments; ++s1) {    // find similar segments
    const VP8SegmentInfo* const S1 = &enc->dqm_[s1];
    int found = 0;
    // check if we already have similar segment
    for (s2 = 0; s2 < num_final_segments; ++s2) {
      const VP8SegmentInfo* const S2 = &enc->dqm_[s2];
      if (SegmentsAreEquivalent(S1, S2)) {
        found = 1;
        break;
      }
    }
    map[s1] = s2;
    if (!found) {
      if (num_final_segments != s1) {
        enc->dqm_[num_final_segments] = enc->dqm_[s1];
      }
      ++num_final_segments;
    }
  }
  if (num_final_segments < num_segments) {  // Remap
    int i = enc->mb_w_ * enc->mb_h_;
    while (i-- > 0) enc->mb_info_[i].segment_ = map[enc->mb_info_[i].segment_];
    enc->segment_hdr_.num_segments_ = num_final_segments;
    // Replicate the trailing segment infos (it's mostly cosmetics)
    for (i = num_final_segments; i < num_segments; ++i) {
      enc->dqm_[i] = enc->dqm_[num_final_segments - 1];
    }
  }
}

void VP8SetSegmentParams(VP8Encoder* const enc, float quality) {
  int i;
  int dq_uv_ac, dq_uv_dc;
  const int num_segments = enc->segment_hdr_.num_segments_;
  const double amp = SNS_TO_DQ * enc->config_->sns_strength / 100. / 128.;
  const double Q = quality / 100.;
  const double c_base = enc->config_->emulate_jpeg_size ?
      QualityToJPEGCompression(Q, enc->alpha_ / 255.) :
      QualityToCompression(Q);
  for (i = 0; i < num_segments; ++i) {
    // We modulate the base coefficient to accommodate for the quantization
    // susceptibility and allow denser segments to be quantized more.
    const double expn = 1. - amp * enc->dqm_[i].alpha_;
    const double c = pow(c_base, expn);
    const int q = (int)(127. * (1. - c));
    assert(expn > 0.);
    enc->dqm_[i].quant_ = clip(q, 0, 127);
  }

  // purely indicative in the bitstream (except for the 1-segment case)
  enc->base_quant_ = enc->dqm_[0].quant_;

  // fill-in values for the unused segments (required by the syntax)
  for (i = num_segments; i < NUM_MB_SEGMENTS; ++i) {
    enc->dqm_[i].quant_ = enc->base_quant_;
  }

  // uv_alpha_ is normally spread around ~60. The useful range is
  // typically ~30 (quite bad) to ~100 (ok to decimate UV more).
  // We map it to the safe maximal range of MAX/MIN_DQ_UV for dq_uv.
  dq_uv_ac = (enc->uv_alpha_ - MID_ALPHA) * (MAX_DQ_UV - MIN_DQ_UV)
                                          / (MAX_ALPHA - MIN_ALPHA);
  // we rescale by the user-defined strength of adaptation
  dq_uv_ac = dq_uv_ac * enc->config_->sns_strength / 100;
  // and make it safe.
  dq_uv_ac = clip(dq_uv_ac, MIN_DQ_UV, MAX_DQ_UV);
  // We also boost the dc-uv-quant a little, based on sns-strength, since
  // U/V channels are quite more reactive to high quants (flat DC-blocks
  // tend to appear, and are displeasant).
  dq_uv_dc = -4 * enc->config_->sns_strength / 100;
  dq_uv_dc = clip(dq_uv_dc, -15, 15);   // 4bit-signed max allowed

  enc->dq_y1_dc_ = 0;       // TODO(skal): dq-lum
  enc->dq_y2_dc_ = 0;
  enc->dq_y2_ac_ = 0;
  enc->dq_uv_dc_ = dq_uv_dc;
  enc->dq_uv_ac_ = dq_uv_ac;

  SetupFilterStrength(enc);   // initialize segments' filtering, eventually

  if (num_segments > 1) SimplifySegments(enc);

  SetupMatrices(enc);         // finalize quantization matrices
}

//------------------------------------------------------------------------------
// Form the predictions in cache

// Must be ordered using {DC_PRED, TM_PRED, V_PRED, H_PRED} as index
const int VP8I16ModeOffsets[4] = { I16DC16, I16TM16, I16VE16, I16HE16 };
const int VP8UVModeOffsets[4] = { C8DC8, C8TM8, C8VE8, C8HE8 };

// Must be indexed using {B_DC_PRED -> B_HU_PRED} as index
const int VP8I4ModeOffsets[NUM_BMODES] = {
  I4DC4, I4TM4, I4VE4, I4HE4, I4RD4, I4VR4, I4LD4, I4VL4, I4HD4, I4HU4
};

void VP8MakeLuma16Preds(const VP8EncIterator* const it) {
  const uint8_t* const left = it->x_ ? it->y_left_ : NULL;
  const uint8_t* const top = it->y_ ? it->y_top_ : NULL;
  VP8EncPredLuma16(it->yuv_p_, left, top);
}

void VP8MakeChroma8Preds(const VP8EncIterator* const it) {
  const uint8_t* const left = it->x_ ? it->u_left_ : NULL;
  const uint8_t* const top = it->y_ ? it->uv_top_ : NULL;
  VP8EncPredChroma8(it->yuv_p_, left, top);
}

void VP8MakeIntra4Preds(const VP8EncIterator* const it) {
  VP8EncPredLuma4(it->yuv_p_, it->i4_top_);
}

//------------------------------------------------------------------------------
// Quantize

// Layout:
// +----+
// |YYYY| 0
// |YYYY| 4
// |YYYY| 8
// |YYYY| 12
// +----+
// |UUVV| 16
// |UUVV| 20
// +----+

const int VP8Scan[16 + 4 + 4] = {
  // Luma
  0 +  0 * BPS,  4 +  0 * BPS, 8 +  0 * BPS, 12 +  0 * BPS,
  0 +  4 * BPS,  4 +  4 * BPS, 8 +  4 * BPS, 12 +  4 * BPS,
  0 +  8 * BPS,  4 +  8 * BPS, 8 +  8 * BPS, 12 +  8 * BPS,
  0 + 12 * BPS,  4 + 12 * BPS, 8 + 12 * BPS, 12 + 12 * BPS,

  0 + 0 * BPS,   4 + 0 * BPS, 0 + 4 * BPS,  4 + 4 * BPS,    // U
  8 + 0 * BPS,  12 + 0 * BPS, 8 + 4 * BPS, 12 + 4 * BPS     // V
};

//------------------------------------------------------------------------------
// Distortion measurement

static const uint16_t kWeightY[16] = {
  38, 32, 20, 9, 32, 28, 17, 7, 20, 17, 10, 4, 9, 7, 4, 2
};

static const uint16_t kWeightTrellis[16] = {
#if USE_TDISTO == 0
  16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16, 16
#else
  30, 27, 19, 11,
  27, 24, 17, 10,
  19, 17, 12,  8,
  11, 10,  8,  6
#endif
};

// Init/Copy the common fields in score.
static void InitScore(VP8ModeScore* const rd) {
  rd->D  = 0;
  rd->SD = 0;
  rd->R  = 0;
  rd->H  = 0;
  rd->nz = 0;
  rd->score = MAX_COST;
}

static void CopyScore(VP8ModeScore* const dst, const VP8ModeScore* const src) {
  dst->D  = src->D;
  dst->SD = src->SD;
  dst->R  = src->R;
  dst->H  = src->H;
  dst->nz = src->nz;      // note that nz is not accumulated, but just copied.
  dst->score = src->score;
}

static void AddScore(VP8ModeScore* const dst, const VP8ModeScore* const src) {
  dst->D  += src->D;
  dst->SD += src->SD;
  dst->R  += src->R;
  dst->H  += src->H;
  dst->nz |= src->nz;     // here, new nz bits are accumulated.
  dst->score += src->score;
}

//------------------------------------------------------------------------------
// Performs trellis-optimized quantization.

// Trellis

typedef struct {
  int prev;        // best previous
  int level;       // level
  int sign;        // sign of coeff_i
  score_t cost;    // bit cost
  score_t error;   // distortion = sum of (|coeff_i| - level_i * Q_i)^2
  int ctx;         // context (only depends on 'level'. Could be spared.)
} Node;

// If a coefficient was quantized to a value Q (using a neutral bias),
// we test all alternate possibilities between [Q-MIN_DELTA, Q+MAX_DELTA]
// We don't test negative values though.
#define MIN_DELTA 0   // how much lower level to try
#define MAX_DELTA 1   // how much higher
#define NUM_NODES (MIN_DELTA + 1 + MAX_DELTA)
#define NODE(n, l) (nodes[(n) + 1][(l) + MIN_DELTA])

static WEBP_INLINE void SetRDScore(int lambda, VP8ModeScore* const rd) {
  // TODO: incorporate the "* 256" in the tables?
  rd->score = (rd->R + rd->H) * lambda + 256 * (rd->D + rd->SD);
}

static WEBP_INLINE score_t RDScoreTrellis(int lambda, score_t rate,
                                          score_t distortion) {
  return rate * lambda + 256 * distortion;
}

static int TrellisQuantizeBlock(const VP8EncIterator* const it,
                                int16_t in[16], int16_t out[16],
                                int ctx0, int coeff_type,
                                const VP8Matrix* const mtx,
                                int lambda) {
  ProbaArray* const last_costs = it->enc_->proba_.coeffs_[coeff_type];
  CostArray* const costs = it->enc_->proba_.level_cost_[coeff_type];
  const int first = (coeff_type == 0) ? 1 : 0;
  Node nodes[17][NUM_NODES];
  int best_path[3] = {-1, -1, -1};   // store best-last/best-level/best-previous
  score_t best_score;
  int best_node;
  int last = first - 1;
  int n, m, p, nz;

  {
    score_t cost;
    score_t max_error;
    const int thresh = mtx->q_[1] * mtx->q_[1] / 4;
    const int last_proba = last_costs[VP8EncBands[first]][ctx0][0];

    // compute maximal distortion.
    max_error = 0;
    for (n = first; n < 16; ++n) {
      const int j  = kZigzag[n];
      const int err = in[j] * in[j];
      max_error += kWeightTrellis[j] * err;
      if (err > thresh) last = n;
    }
    // we don't need to go inspect up to n = 16 coeffs. We can just go up
    // to last + 1 (inclusive) without losing much.
    if (last < 15) ++last;

    // compute 'skip' score. This is the max score one can do.
    cost = VP8BitCost(0, last_proba);
    best_score = RDScoreTrellis(lambda, cost, max_error);

    // initialize source node.
    n = first - 1;
    for (m = -MIN_DELTA; m <= MAX_DELTA; ++m) {
      NODE(n, m).cost = 0;
      NODE(n, m).error = max_error;
      NODE(n, m).ctx = ctx0;
    }
  }

  // traverse trellis.
  for (n = first; n <= last; ++n) {
    const int j  = kZigzag[n];
    const int Q  = mtx->q_[j];
    const int iQ = mtx->iq_[j];
    const int B = BIAS(0x00);     // neutral bias
    // note: it's important to take sign of the _original_ coeff,
    // so we don't have to consider level < 0 afterward.
    const int sign = (in[j] < 0);
    const int coeff0 = (sign ? -in[j] : in[j]) + mtx->sharpen_[j];
    int level0 = QUANTDIV(coeff0, iQ, B);
    if (level0 > MAX_LEVEL) level0 = MAX_LEVEL;

    // test all alternate level values around level0.
    for (m = -MIN_DELTA; m <= MAX_DELTA; ++m) {
      Node* const cur = &NODE(n, m);
      int delta_error, new_error;
      score_t cur_score = MAX_COST;
      int level = level0 + m;
      int last_proba;

      cur->sign = sign;
      cur->level = level;
      cur->ctx = (level == 0) ? 0 : (level == 1) ? 1 : 2;
      if (level > MAX_LEVEL || level < 0) {   // node is dead?
        cur->cost = MAX_COST;
        continue;
      }
      last_proba = last_costs[VP8EncBands[n + 1]][cur->ctx][0];

      // Compute delta_error = how much coding this level will
      // subtract as distortion to max_error
      new_error = coeff0 - level * Q;
      delta_error =
        kWeightTrellis[j] * (coeff0 * coeff0 - new_error * new_error);

      // Inspect all possible non-dead predecessors. Retain only the best one.
      for (p = -MIN_DELTA; p <= MAX_DELTA; ++p) {
        const Node* const prev = &NODE(n - 1, p);
        const int prev_ctx = prev->ctx;
        const uint16_t* const tcost = costs[VP8EncBands[n]][prev_ctx];
        const score_t total_error = prev->error - delta_error;
        score_t cost, base_cost, score;

        if (prev->cost >= MAX_COST) {   // dead node?
          continue;
        }

        // Base cost of both terminal/non-terminal
        base_cost = prev->cost + VP8LevelCost(tcost, level);

        // Examine node assuming it's a non-terminal one.
        cost = base_cost;
        if (level && n < 15) {
          cost += VP8BitCost(1, last_proba);
        }
        score = RDScoreTrellis(lambda, cost, total_error);
        if (score < cur_score) {
          cur_score = score;
          cur->cost  = cost;
          cur->error = total_error;
          cur->prev  = p;
        }

        // Now, record best terminal node (and thus best entry in the graph).
        if (level) {
          cost = base_cost;
          if (n < 15) cost += VP8BitCost(0, last_proba);
          score = RDScoreTrellis(lambda, cost, total_error);
          if (score < best_score) {
            best_score = score;
            best_path[0] = n;   // best eob position
            best_path[1] = m;   // best level
            best_path[2] = p;   // best predecessor
          }
        }
      }
    }
  }

  // Fresh start
  memset(in + first, 0, (16 - first) * sizeof(*in));
  memset(out + first, 0, (16 - first) * sizeof(*out));
  if (best_path[0] == -1) {
    return 0;   // skip!
  }

  // Unwind the best path.
  // Note: best-prev on terminal node is not necessarily equal to the
  // best_prev for non-terminal. So we patch best_path[2] in.
  n = best_path[0];
  best_node = best_path[1];
  NODE(n, best_node).prev = best_path[2];   // force best-prev for terminal
  nz = 0;

  for (; n >= first; --n) {
    const Node* const node = &NODE(n, best_node);
    const int j = kZigzag[n];
    out[n] = node->sign ? -node->level : node->level;
    nz |= (node->level != 0);
    in[j] = out[n] * mtx->q_[j];
    best_node = node->prev;
  }
  return nz;
}

#undef NODE

//------------------------------------------------------------------------------
// Performs: difference, transform, quantize, back-transform, add
// all at once. Output is the reconstructed block in *yuv_out, and the
// quantized levels in *levels.

static int ReconstructIntra16(VP8EncIterator* const it,
                              VP8ModeScore* const rd,
                              uint8_t* const yuv_out,
                              int mode) {
  VP8Encoder* const enc = it->enc_;
  const uint8_t* const ref = it->yuv_p_ + VP8I16ModeOffsets[mode];
  const uint8_t* const src = it->yuv_in_ + Y_OFF;
  VP8SegmentInfo* const dqm = &enc->dqm_[it->mb_->segment_];
  int nz = 0;
  int n;
  int16_t tmp[16][16], dc_tmp[16];

  for (n = 0; n < 16; ++n) {
    VP8FTransform(src + VP8Scan[n], ref + VP8Scan[n], tmp[n]);
  }
  VP8FTransformWHT(tmp[0], dc_tmp);
  nz |= VP8EncQuantizeBlockWHT(dc_tmp, rd->y_dc_levels, &dqm->y2_) << 24;

  if (DO_TRELLIS_I16 && it->do_trellis_) {
    int x, y;
    VP8IteratorNzToBytes(it);
    for (y = 0, n = 0; y < 4; ++y) {
      for (x = 0; x < 4; ++x, ++n) {
        const int ctx = it->top_nz_[x] + it->left_nz_[y];
        const int non_zero =
           TrellisQuantizeBlock(it, tmp[n], rd->y_ac_levels[n], ctx, 0,
                                &dqm->y1_, dqm->lambda_trellis_i16_);
        it->top_nz_[x] = it->left_nz_[y] = non_zero;
        nz |= non_zero << n;
      }
    }
  } else {
    for (n = 0; n < 16; ++n) {
      nz |= VP8EncQuantizeBlock(tmp[n], rd->y_ac_levels[n], 1, &dqm->y1_) << n;
    }
  }

  // Transform back
  VP8ITransformWHT(dc_tmp, tmp[0]);
  for (n = 0; n < 16; n += 2) {
    VP8ITransform(ref + VP8Scan[n], tmp[n], yuv_out + VP8Scan[n], 1);
  }

  return nz;
}

static int ReconstructIntra4(VP8EncIterator* const it,
                             int16_t levels[16],
                             const uint8_t* const src,
                             uint8_t* const yuv_out,
                             int mode) {
  const VP8Encoder* const enc = it->enc_;
  const uint8_t* const ref = it->yuv_p_ + VP8I4ModeOffsets[mode];
  const VP8SegmentInfo* const dqm = &enc->dqm_[it->mb_->segment_];
  int nz = 0;
  int16_t tmp[16];

  VP8FTransform(src, ref, tmp);
  if (DO_TRELLIS_I4 && it->do_trellis_) {
    const int x = it->i4_ & 3, y = it->i4_ >> 2;
    const int ctx = it->top_nz_[x] + it->left_nz_[y];
    nz = TrellisQuantizeBlock(it, tmp, levels, ctx, 3, &dqm->y1_,
                              dqm->lambda_trellis_i4_);
  } else {
    nz = VP8EncQuantizeBlock(tmp, levels, 0, &dqm->y1_);
  }
  VP8ITransform(ref, tmp, yuv_out, 0);
  return nz;
}

static int ReconstructUV(VP8EncIterator* const it, VP8ModeScore* const rd,
                         uint8_t* const yuv_out, int mode) {
  const VP8Encoder* const enc = it->enc_;
  const uint8_t* const ref = it->yuv_p_ + VP8UVModeOffsets[mode];
  const uint8_t* const src = it->yuv_in_ + U_OFF;
  const VP8SegmentInfo* const dqm = &enc->dqm_[it->mb_->segment_];
  int nz = 0;
  int n;
  int16_t tmp[8][16];

  for (n = 0; n < 8; ++n) {
    VP8FTransform(src + VP8Scan[16 + n], ref + VP8Scan[16 + n], tmp[n]);
  }
  if (DO_TRELLIS_UV && it->do_trellis_) {
    int ch, x, y;
    for (ch = 0, n = 0; ch <= 2; ch += 2) {
      for (y = 0; y < 2; ++y) {
        for (x = 0; x < 2; ++x, ++n) {
          const int ctx = it->top_nz_[4 + ch + x] + it->left_nz_[4 + ch + y];
          const int non_zero =
            TrellisQuantizeBlock(it, tmp[n], rd->uv_levels[n], ctx, 2,
                                 &dqm->uv_, dqm->lambda_trellis_uv_);
          it->top_nz_[4 + ch + x] = it->left_nz_[4 + ch + y] = non_zero;
          nz |= non_zero << n;
        }
      }
    }
  } else {
    for (n = 0; n < 8; ++n) {
      nz |= VP8EncQuantizeBlock(tmp[n], rd->uv_levels[n], 0, &dqm->uv_) << n;
    }
  }

  for (n = 0; n < 8; n += 2) {
    VP8ITransform(ref + VP8Scan[16 + n], tmp[n], yuv_out + VP8Scan[16 + n], 1);
  }
  return (nz << 16);
}

//------------------------------------------------------------------------------
// RD-opt decision. Reconstruct each modes, evalue distortion and bit-cost.
// Pick the mode is lower RD-cost = Rate + lambda * Distortion.

static void StoreMaxDelta(VP8SegmentInfo* const dqm, const int16_t DCs[16]) {
  // We look at the first three AC coefficients to determine what is the average
  // delta between each sub-4x4 block.
  const int v0 = abs(DCs[1]);
  const int v1 = abs(DCs[4]);
  const int v2 = abs(DCs[5]);
  int max_v = (v0 > v1) ? v1 : v0;
  max_v = (v2 > max_v) ? v2 : max_v;
  if (max_v > dqm->max_edge_) dqm->max_edge_ = max_v;
}

static void SwapPtr(uint8_t** a, uint8_t** b) {
  uint8_t* const tmp = *a;
  *a = *b;
  *b = tmp;
}

static void SwapOut(VP8EncIterator* const it) {
  SwapPtr(&it->yuv_out_, &it->yuv_out2_);
}

static score_t IsFlat(const int16_t* levels, int num_blocks, score_t thresh) {
  score_t score = 0;
  while (num_blocks-- > 0) {      // TODO(skal): refine positional scoring?
    int i;
    for (i = 1; i < 16; ++i) {    // omit DC, we're only interested in AC
      score += (levels[i] != 0);
      if (score > thresh) return 0;
    }
    levels += 16;
  }
  return 1;
}

static void PickBestIntra16(VP8EncIterator* const it, VP8ModeScore* const rd) {
  const int kNumBlocks = 16;
  VP8Encoder* const enc = it->enc_;
  VP8SegmentInfo* const dqm = &enc->dqm_[it->mb_->segment_];
  const int lambda = dqm->lambda_i16_;
  const int tlambda = dqm->tlambda_;
  const uint8_t* const src = it->yuv_in_ + Y_OFF;
  VP8ModeScore rd16;
  int mode;

  rd->mode_i16 = -1;
  for (mode = 0; mode < NUM_PRED_MODES; ++mode) {
    uint8_t* const tmp_dst = it->yuv_out2_ + Y_OFF;  // scratch buffer
    int nz;

    // Reconstruct
    nz = ReconstructIntra16(it, &rd16, tmp_dst, mode);

    // Measure RD-score
    rd16.D = VP8SSE16x16(src, tmp_dst);
    rd16.SD = tlambda ? MULT_8B(tlambda, VP8TDisto16x16(src, tmp_dst, kWeightY))
            : 0;
    rd16.H = VP8FixedCostsI16[mode];
    rd16.R = VP8GetCostLuma16(it, &rd16);
    if (mode > 0 &&
        IsFlat(rd16.y_ac_levels[0], kNumBlocks, FLATNESS_LIMIT_I16)) {
      // penalty to avoid flat area to be mispredicted by complex mode
      rd16.R += FLATNESS_PENALTY * kNumBlocks;
    }

    // Since we always examine Intra16 first, we can overwrite *rd directly.
    SetRDScore(lambda, &rd16);
    if (mode == 0 || rd16.score < rd->score) {
      CopyScore(rd, &rd16);
      rd->mode_i16 = mode;
      rd->nz = nz;
      memcpy(rd->y_ac_levels, rd16.y_ac_levels, sizeof(rd16.y_ac_levels));
      memcpy(rd->y_dc_levels, rd16.y_dc_levels, sizeof(rd16.y_dc_levels));
      SwapOut(it);
    }
  }
  SetRDScore(dqm->lambda_mode_, rd);   // finalize score for mode decision.
  VP8SetIntra16Mode(it, rd->mode_i16);

  // we have a blocky macroblock (only DCs are non-zero) with fairly high
  // distortion, record max delta so we can later adjust the minimal filtering
  // strength needed to smooth these blocks out.
  if ((rd->nz & 0xffff) == 0 && rd->D > dqm->min_disto_) {
    StoreMaxDelta(dqm, rd->y_dc_levels);
  }
}

//------------------------------------------------------------------------------

// return the cost array corresponding to the surrounding prediction modes.
static const uint16_t* GetCostModeI4(VP8EncIterator* const it,
                                     const uint8_t modes[16]) {
  const int preds_w = it->enc_->preds_w_;
  const int x = (it->i4_ & 3), y = it->i4_ >> 2;
  const int left = (x == 0) ? it->preds_[y * preds_w - 1] : modes[it->i4_ - 1];
  const int top = (y == 0) ? it->preds_[-preds_w + x] : modes[it->i4_ - 4];
  return VP8FixedCostsI4[top][left];
}

static int PickBestIntra4(VP8EncIterator* const it, VP8ModeScore* const rd) {
  const VP8Encoder* const enc = it->enc_;
  const VP8SegmentInfo* const dqm = &enc->dqm_[it->mb_->segment_];
  const int lambda = dqm->lambda_i4_;
  const int tlambda = dqm->tlambda_;
  const uint8_t* const src0 = it->yuv_in_ + Y_OFF;
  uint8_t* const best_blocks = it->yuv_out2_ + Y_OFF;
  int total_header_bits = 0;
  VP8ModeScore rd_best;

  if (enc->max_i4_header_bits_ == 0) {
    return 0;
  }

  InitScore(&rd_best);
  rd_best.H = 211;  // '211' is the value of VP8BitCost(0, 145)
  SetRDScore(dqm->lambda_mode_, &rd_best);
  VP8IteratorStartI4(it);
  do {
    const int kNumBlocks = 1;
    VP8ModeScore rd_i4;
    int mode;
    int best_mode = -1;
    const uint8_t* const src = src0 + VP8Scan[it->i4_];
    const uint16_t* const mode_costs = GetCostModeI4(it, rd->modes_i4);
    uint8_t* best_block = best_blocks + VP8Scan[it->i4_];
    uint8_t* tmp_dst = it->yuv_p_ + I4TMP;    // scratch buffer.

    InitScore(&rd_i4);
    VP8MakeIntra4Preds(it);
    for (mode = 0; mode < NUM_BMODES; ++mode) {
      VP8ModeScore rd_tmp;
      int16_t tmp_levels[16];

      // Reconstruct
      rd_tmp.nz =
          ReconstructIntra4(it, tmp_levels, src, tmp_dst, mode) << it->i4_;

      // Compute RD-score
      rd_tmp.D = VP8SSE4x4(src, tmp_dst);
      rd_tmp.SD =
          tlambda ? MULT_8B(tlambda, VP8TDisto4x4(src, tmp_dst, kWeightY))
                  : 0;
      rd_tmp.H = mode_costs[mode];
      rd_tmp.R = VP8GetCostLuma4(it, tmp_levels);
      if (mode > 0 && IsFlat(tmp_levels, kNumBlocks, FLATNESS_LIMIT_I4)) {
        rd_tmp.R += FLATNESS_PENALTY * kNumBlocks;
      }

      SetRDScore(lambda, &rd_tmp);
      if (best_mode < 0 || rd_tmp.score < rd_i4.score) {
        CopyScore(&rd_i4, &rd_tmp);
        best_mode = mode;
        SwapPtr(&tmp_dst, &best_block);
        memcpy(rd_best.y_ac_levels[it->i4_], tmp_levels, sizeof(tmp_levels));
      }
    }
    SetRDScore(dqm->lambda_mode_, &rd_i4);
    AddScore(&rd_best, &rd_i4);
    if (rd_best.score >= rd->score) {
      return 0;
    }
    total_header_bits += (int)rd_i4.H;   // <- equal to mode_costs[best_mode];
    if (total_header_bits > enc->max_i4_header_bits_) {
      return 0;
    }
    // Copy selected samples if not in the right place already.
    if (best_block != best_blocks + VP8Scan[it->i4_]) {
      VP8Copy4x4(best_block, best_blocks + VP8Scan[it->i4_]);
    }
    rd->modes_i4[it->i4_] = best_mode;
    it->top_nz_[it->i4_ & 3] = it->left_nz_[it->i4_ >> 2] = (rd_i4.nz ? 1 : 0);
  } while (VP8IteratorRotateI4(it, best_blocks));

  // finalize state
  CopyScore(rd, &rd_best);
  VP8SetIntra4Mode(it, rd->modes_i4);
  SwapOut(it);
  memcpy(rd->y_ac_levels, rd_best.y_ac_levels, sizeof(rd->y_ac_levels));
  return 1;   // select intra4x4 over intra16x16
}

//------------------------------------------------------------------------------

static void PickBestUV(VP8EncIterator* const it, VP8ModeScore* const rd) {
  const int kNumBlocks = 8;
  const VP8Encoder* const enc = it->enc_;
  const VP8SegmentInfo* const dqm = &enc->dqm_[it->mb_->segment_];
  const int lambda = dqm->lambda_uv_;
  const uint8_t* const src = it->yuv_in_ + U_OFF;
  uint8_t* const tmp_dst = it->yuv_out2_ + U_OFF;  // scratch buffer
  uint8_t* const dst0 = it->yuv_out_ + U_OFF;
  VP8ModeScore rd_best;
  int mode;

  rd->mode_uv = -1;
  InitScore(&rd_best);
  for (mode = 0; mode < NUM_PRED_MODES; ++mode) {
    VP8ModeScore rd_uv;

    // Reconstruct
    rd_uv.nz = ReconstructUV(it, &rd_uv, tmp_dst, mode);

    // Compute RD-score
    rd_uv.D  = VP8SSE16x8(src, tmp_dst);
    rd_uv.SD = 0;    // TODO: should we call TDisto? it tends to flatten areas.
    rd_uv.H  = VP8FixedCostsUV[mode];
    rd_uv.R  = VP8GetCostUV(it, &rd_uv);
    if (mode > 0 && IsFlat(rd_uv.uv_levels[0], kNumBlocks, FLATNESS_LIMIT_UV)) {
      rd_uv.R += FLATNESS_PENALTY * kNumBlocks;
    }

    SetRDScore(lambda, &rd_uv);
    if (mode == 0 || rd_uv.score < rd_best.score) {
      CopyScore(&rd_best, &rd_uv);
      rd->mode_uv = mode;
      memcpy(rd->uv_levels, rd_uv.uv_levels, sizeof(rd->uv_levels));
      memcpy(dst0, tmp_dst, UV_SIZE);   //  TODO: SwapUVOut() ?
    }
  }
  VP8SetIntraUVMode(it, rd->mode_uv);
  AddScore(rd, &rd_best);
}

//------------------------------------------------------------------------------
// Final reconstruction and quantization.

static void SimpleQuantize(VP8EncIterator* const it, VP8ModeScore* const rd) {
  const VP8Encoder* const enc = it->enc_;
  const int is_i16 = (it->mb_->type_ == 1);
  int nz = 0;

  if (is_i16) {
    nz = ReconstructIntra16(it, rd, it->yuv_out_ + Y_OFF, it->preds_[0]);
  } else {
    VP8IteratorStartI4(it);
    do {
      const int mode =
          it->preds_[(it->i4_ & 3) + (it->i4_ >> 2) * enc->preds_w_];
      const uint8_t* const src = it->yuv_in_ + Y_OFF + VP8Scan[it->i4_];
      uint8_t* const dst = it->yuv_out_ + Y_OFF + VP8Scan[it->i4_];
      VP8MakeIntra4Preds(it);
      nz |= ReconstructIntra4(it, rd->y_ac_levels[it->i4_],
                              src, dst, mode) << it->i4_;
    } while (VP8IteratorRotateI4(it, it->yuv_out_ + Y_OFF));
  }

  nz |= ReconstructUV(it, rd, it->yuv_out_ + U_OFF, it->mb_->uv_mode_);
  rd->nz = nz;
}

// Refine intra16/intra4 sub-modes based on distortion only (not rate).
static void DistoRefine(VP8EncIterator* const it, int try_both_i4_i16) {
  const int is_i16 = (it->mb_->type_ == 1);
  score_t best_score = MAX_COST;

  if (try_both_i4_i16 || is_i16) {
    int mode;
    int best_mode = -1;
    for (mode = 0; mode < NUM_PRED_MODES; ++mode) {
      const uint8_t* const ref = it->yuv_p_ + VP8I16ModeOffsets[mode];
      const uint8_t* const src = it->yuv_in_ + Y_OFF;
      const score_t score = VP8SSE16x16(src, ref);
      if (score < best_score) {
        best_mode = mode;
        best_score = score;
      }
    }
    VP8SetIntra16Mode(it, best_mode);
  }
  if (try_both_i4_i16 || !is_i16) {
    uint8_t modes_i4[16];
    // We don't evaluate the rate here, but just account for it through a
    // constant penalty (i4 mode usually needs more bits compared to i16).
    score_t score_i4 = (score_t)I4_PENALTY;

    VP8IteratorStartI4(it);
    do {
      int mode;
      int best_sub_mode = -1;
      score_t best_sub_score = MAX_COST;
      const uint8_t* const src = it->yuv_in_ + Y_OFF + VP8Scan[it->i4_];

      // TODO(skal): we don't really need the prediction pixels here,
      // but just the distortion against 'src'.
      VP8MakeIntra4Preds(it);
      for (mode = 0; mode < NUM_BMODES; ++mode) {
        const uint8_t* const ref = it->yuv_p_ + VP8I4ModeOffsets[mode];
        const score_t score = VP8SSE4x4(src, ref);
        if (score < best_sub_score) {
          best_sub_mode = mode;
          best_sub_score = score;
        }
      }
      modes_i4[it->i4_] = best_sub_mode;
      score_i4 += best_sub_score;
      if (score_i4 >= best_score) break;
    } while (VP8IteratorRotateI4(it, it->yuv_in_ + Y_OFF));
    if (score_i4 < best_score) {
      VP8SetIntra4Mode(it, modes_i4);
    }
  }
}

//------------------------------------------------------------------------------
// Entry point

int VP8Decimate(VP8EncIterator* const it, VP8ModeScore* const rd,
                VP8RDLevel rd_opt) {
  int is_skipped;
  const int method = it->enc_->method_;

  InitScore(rd);

  // We can perform predictions for Luma16x16 and Chroma8x8 already.
  // Luma4x4 predictions needs to be done as-we-go.
  VP8MakeLuma16Preds(it);
  VP8MakeChroma8Preds(it);

  if (rd_opt > RD_OPT_NONE) {
    it->do_trellis_ = (rd_opt >= RD_OPT_TRELLIS_ALL);
    PickBestIntra16(it, rd);
    if (method >= 2) {
      PickBestIntra4(it, rd);
    }
    PickBestUV(it, rd);
    if (rd_opt == RD_OPT_TRELLIS) {   // finish off with trellis-optim now
      it->do_trellis_ = 1;
      SimpleQuantize(it, rd);
    }
  } else {
    // For method == 2, pick the best intra4/intra16 based on SSE (~tad slower).
    // For method <= 1, we refine intra4 or intra16 (but don't re-examine mode).
    DistoRefine(it, (method >= 2));
    SimpleQuantize(it, rd);
  }
  is_skipped = (rd->nz == 0);
  VP8SetSkip(it, is_skipped);
  return is_skipped;
}
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root/third_party/libwebp/enc/quant.c

DEFINITIONS
