libwebp: update snapshot to v0.2.0-rc1

third_party/libwebp/enc/vp8l.c

+// Copyright 2012 Google Inc. All Rights Reserved.
+//
+// This code is licensed under the same terms as WebM:
+//  Software License Agreement:  http://www.webmproject.org/license/software/
+//  Additional IP Rights Grant:  http://www.webmproject.org/license/additional/
+// -----------------------------------------------------------------------------
+//
+// main entry for the lossless encoder.
+//
+// Author: Vikas Arora (vikaas.arora@gmail.com)
+//
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+#include "./backward_references.h"
+#include "./vp8enci.h"
+#include "./vp8li.h"
+#include "../dsp/lossless.h"
+#include "../utils/bit_writer.h"
+#include "../utils/huffman_encode.h"
+#include "../utils/utils.h"
+#include "../webp/format_constants.h"
+
+#if defined(__cplusplus) || defined(c_plusplus)
+extern "C" {
+#endif
+
+#define PALETTE_KEY_RIGHT_SHIFT   22  // Key for 1K buffer.
+#define MAX_HUFF_IMAGE_SIZE       (16 * 1024 * 1024)
+
+// -----------------------------------------------------------------------------
+// Palette
+
+static int CompareColors(const void* p1, const void* p2) {
+  const uint32_t a = *(const uint32_t*)p1;
+  const uint32_t b = *(const uint32_t*)p2;
+  return (a < b) ? -1 : (a > b) ? 1 : 0;
+}
+
+// If number of colors in the image is less than or equal to MAX_PALETTE_SIZE,
+// creates a palette and returns true, else returns false.
+static int AnalyzeAndCreatePalette(const WebPPicture* const pic,
+                                   uint32_t palette[MAX_PALETTE_SIZE],
+                                   int* const palette_size) {
+  int i, x, y, key;
+  int num_colors = 0;
+  uint8_t in_use[MAX_PALETTE_SIZE * 4] = { 0 };
+  uint32_t colors[MAX_PALETTE_SIZE * 4];
+  static const uint32_t kHashMul = 0x1e35a7bd;
+  const uint32_t* argb = pic->argb;
+  const int width = pic->width;
+  const int height = pic->height;
+  uint32_t last_pix = ~argb[0];   // so we're sure that last_pix != argb[0]
+
+  for (y = 0; y < height; ++y) {
+    for (x = 0; x < width; ++x) {
+      if (argb[x] == last_pix) {
+        continue;
+      }
+      last_pix = argb[x];
+      key = (kHashMul * last_pix) >> PALETTE_KEY_RIGHT_SHIFT;
+      while (1) {
+        if (!in_use[key]) {
+          colors[key] = last_pix;
+          in_use[key] = 1;
+          ++num_colors;
+          if (num_colors > MAX_PALETTE_SIZE) {
+            return 0;
+          }
+          break;
+        } else if (colors[key] == last_pix) {
+          // The color is already there.
+          break;
+        } else {
+          // Some other color sits there.
+          // Do linear conflict resolution.
+          ++key;
+          key &= (MAX_PALETTE_SIZE * 4 - 1);  // key mask for 1K buffer.
+        }
+      }
+    }
+    argb += pic->argb_stride;
+  }
+
+  // TODO(skal): could we reuse in_use[] to speed up ApplyPalette()?
+  num_colors = 0;
+  for (i = 0; i < (int)(sizeof(in_use) / sizeof(in_use[0])); ++i) {
+    if (in_use[i]) {
+      palette[num_colors] = colors[i];
+      ++num_colors;
+    }
+  }
+
+  qsort(palette, num_colors, sizeof(*palette), CompareColors);
+  *palette_size = num_colors;
+  return 1;
+}
+
+static int AnalyzeEntropy(const WebPPicture* const pic,
+                          double* const nonpredicted_bits,
+                          double* const predicted_bits) {
+  int x, y;
+  const uint32_t* argb = pic->argb;
+  const uint32_t* last_line = NULL;
+  uint32_t last_pix = argb[0];    // so we're sure that pix_diff == 0
+
+  VP8LHistogram* nonpredicted = NULL;
+  VP8LHistogram* predicted =
+      (VP8LHistogram*)malloc(2 * sizeof(*predicted));
+  if (predicted == NULL) return 0;
+  nonpredicted = predicted + 1;
+
+  VP8LHistogramInit(predicted, 0);
+  VP8LHistogramInit(nonpredicted, 0);
+  for (y = 0; y < pic->height; ++y) {
+    for (x = 0; x < pic->width; ++x) {
+      const uint32_t pix = argb[x];
+      const uint32_t pix_diff = VP8LSubPixels(pix, last_pix);
+      if (pix_diff == 0) continue;
+      if (last_line != NULL && pix == last_line[x]) {
+        continue;
+      }
+      last_pix = pix;
+      {
+        const PixOrCopy pix_token = PixOrCopyCreateLiteral(pix);
+        const PixOrCopy pix_diff_token = PixOrCopyCreateLiteral(pix_diff);
+        VP8LHistogramAddSinglePixOrCopy(nonpredicted, &pix_token);
+        VP8LHistogramAddSinglePixOrCopy(predicted, &pix_diff_token);
+      }
+    }
+    last_line = argb;
+    argb += pic->argb_stride;
+  }
+  *nonpredicted_bits = VP8LHistogramEstimateBitsBulk(nonpredicted);
+  *predicted_bits = VP8LHistogramEstimateBitsBulk(predicted);
+  free(predicted);
+  return 1;
+}
+
+static int VP8LEncAnalyze(VP8LEncoder* const enc, WebPImageHint image_hint) {
+  const WebPPicture* const pic = enc->pic_;
+  assert(pic != NULL && pic->argb != NULL);
+
+  enc->use_palette_ = (image_hint == WEBP_HINT_GRAPH) ? 0 :
+      AnalyzeAndCreatePalette(pic, enc->palette_, &enc->palette_size_);
+  if (!enc->use_palette_) {
+    if (image_hint == WEBP_HINT_DEFAULT) {
+      double non_pred_entropy, pred_entropy;
+      if (!AnalyzeEntropy(pic, &non_pred_entropy, &pred_entropy)) {
+        return 0;
+      }
+
+      if (pred_entropy < 0.95 * non_pred_entropy) {
+        enc->use_predict_ = 1;
+        enc->use_cross_color_ = 1;
+      }
+    } else if (image_hint == WEBP_HINT_PHOTO) {
+      enc->use_predict_ = 1;
+      enc->use_cross_color_ = 1;
+    }
+  }
+  return 1;
+}
+
+static int GetHuffBitLengthsAndCodes(
+    const VP8LHistogramSet* const histogram_image,
+    HuffmanTreeCode* const huffman_codes) {
+  int i, k;
+  int ok = 1;
+  uint64_t total_length_size = 0;
+  uint8_t* mem_buf = NULL;
+  const int histogram_image_size = histogram_image->size;
+
+  // Iterate over all histograms and get the aggregate number of codes used.
+  for (i = 0; i < histogram_image_size; ++i) {
+    const VP8LHistogram* const histo = histogram_image->histograms[i];
+    HuffmanTreeCode* const codes = &huffman_codes[5 * i];
+    for (k = 0; k < 5; ++k) {
+      const int num_symbols = (k == 0) ? VP8LHistogramNumCodes(histo)
+                            : (k == 4) ? NUM_DISTANCE_CODES
+                            : 256;
+      codes[k].num_symbols = num_symbols;
+      total_length_size += num_symbols;
+    }
+  }
+
+  // Allocate and Set Huffman codes.
+  {
+    uint16_t* codes;
+    uint8_t* lengths;
+    mem_buf = (uint8_t*)WebPSafeCalloc(total_length_size,
+                                       sizeof(*lengths) + sizeof(*codes));
+    if (mem_buf == NULL) {
+      ok = 0;
+      goto End;
+    }
+    codes = (uint16_t*)mem_buf;
+    lengths = (uint8_t*)&codes[total_length_size];
+    for (i = 0; i < 5 * histogram_image_size; ++i) {
+      const int bit_length = huffman_codes[i].num_symbols;
+      huffman_codes[i].codes = codes;
+      huffman_codes[i].code_lengths = lengths;
+      codes += bit_length;
+      lengths += bit_length;
+    }
+  }
+
+  // Create Huffman trees.
+  for (i = 0; i < histogram_image_size; ++i) {
+    HuffmanTreeCode* const codes = &huffman_codes[5 * i];
+    VP8LHistogram* const histo = histogram_image->histograms[i];
+    ok = ok && VP8LCreateHuffmanTree(histo->literal_, 15, codes + 0);
+    ok = ok && VP8LCreateHuffmanTree(histo->red_, 15, codes + 1);
+    ok = ok && VP8LCreateHuffmanTree(histo->blue_, 15, codes + 2);
+    ok = ok && VP8LCreateHuffmanTree(histo->alpha_, 15, codes + 3);
+    ok = ok && VP8LCreateHuffmanTree(histo->distance_, 15, codes + 4);
+  }
+
+ End:
+  if (!ok) free(mem_buf);
+  return ok;
+}
+
+static void StoreHuffmanTreeOfHuffmanTreeToBitMask(
+    VP8LBitWriter* const bw, const uint8_t* code_length_bitdepth) {
+  // RFC 1951 will calm you down if you are worried about this funny sequence.
+  // This sequence is tuned from that, but more weighted for lower symbol count,
+  // and more spiking histograms.
+  static const uint8_t kStorageOrder[CODE_LENGTH_CODES] = {
+    17, 18, 0, 1, 2, 3, 4, 5, 16, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
+  };
+  int i;
+  // Throw away trailing zeros:
+  int codes_to_store = CODE_LENGTH_CODES;
+  for (; codes_to_store > 4; --codes_to_store) {
+    if (code_length_bitdepth[kStorageOrder[codes_to_store - 1]] != 0) {
+      break;
+    }
+  }
+  VP8LWriteBits(bw, 4, codes_to_store - 4);
+  for (i = 0; i < codes_to_store; ++i) {
+    VP8LWriteBits(bw, 3, code_length_bitdepth[kStorageOrder[i]]);
+  }
+}
+
+static void ClearHuffmanTreeIfOnlyOneSymbol(
+    HuffmanTreeCode* const huffman_code) {
+  int k;
+  int count = 0;
+  for (k = 0; k < huffman_code->num_symbols; ++k) {
+    if (huffman_code->code_lengths[k] != 0) {
+      ++count;
+      if (count > 1) return;
+    }
+  }
+  for (k = 0; k < huffman_code->num_symbols; ++k) {
+    huffman_code->code_lengths[k] = 0;
+    huffman_code->codes[k] = 0;
+  }
+}
+
+static void StoreHuffmanTreeToBitMask(
+    VP8LBitWriter* const bw,
+    const HuffmanTreeToken* const tokens, const int num_tokens,
+    const HuffmanTreeCode* const huffman_code) {
+  int i;
+  for (i = 0; i < num_tokens; ++i) {
+    const int ix = tokens[i].code;
+    const int extra_bits = tokens[i].extra_bits;
+    VP8LWriteBits(bw, huffman_code->code_lengths[ix], huffman_code->codes[ix]);
+    switch (ix) {
+      case 16:
+        VP8LWriteBits(bw, 2, extra_bits);
+        break;
+      case 17:
+        VP8LWriteBits(bw, 3, extra_bits);
+        break;
+      case 18:
+        VP8LWriteBits(bw, 7, extra_bits);
+        break;
+    }
+  }
+}
+
+static int StoreFullHuffmanCode(VP8LBitWriter* const bw,
+                                const HuffmanTreeCode* const tree) {
+  int ok = 0;
+  uint8_t code_length_bitdepth[CODE_LENGTH_CODES] = { 0 };
+  uint16_t code_length_bitdepth_symbols[CODE_LENGTH_CODES] = { 0 };
+  const int max_tokens = tree->num_symbols;
+  int num_tokens;
+  HuffmanTreeCode huffman_code;
+  HuffmanTreeToken* const tokens =
+      (HuffmanTreeToken*)WebPSafeMalloc((uint64_t)max_tokens, sizeof(*tokens));
+  if (tokens == NULL) return 0;
+
+  huffman_code.num_symbols = CODE_LENGTH_CODES;
+  huffman_code.code_lengths = code_length_bitdepth;
+  huffman_code.codes = code_length_bitdepth_symbols;
+
+  VP8LWriteBits(bw, 1, 0);
+  num_tokens = VP8LCreateCompressedHuffmanTree(tree, tokens, max_tokens);
+  {
+    int histogram[CODE_LENGTH_CODES] = { 0 };
+    int i;
+    for (i = 0; i < num_tokens; ++i) {
+      ++histogram[tokens[i].code];
+    }
+
+    if (!VP8LCreateHuffmanTree(histogram, 7, &huffman_code)) {
+      goto End;
+    }
+  }
+
+  StoreHuffmanTreeOfHuffmanTreeToBitMask(bw, code_length_bitdepth);
+  ClearHuffmanTreeIfOnlyOneSymbol(&huffman_code);
+  {
+    int trailing_zero_bits = 0;
+    int trimmed_length = num_tokens;
+    int write_trimmed_length;
+    int length;
+    int i = num_tokens;
+    while (i-- > 0) {
+      const int ix = tokens[i].code;
+      if (ix == 0 || ix == 17 || ix == 18) {
+        --trimmed_length;   // discount trailing zeros
+        trailing_zero_bits += code_length_bitdepth[ix];
+        if (ix == 17) {
+          trailing_zero_bits += 3;
+        } else if (ix == 18) {
+          trailing_zero_bits += 7;
+        }
+      } else {
+        break;
+      }
+    }
+    write_trimmed_length = (trimmed_length > 1 && trailing_zero_bits > 12);
+    length = write_trimmed_length ? trimmed_length : num_tokens;
+    VP8LWriteBits(bw, 1, write_trimmed_length);
+    if (write_trimmed_length) {
+      const int nbits = VP8LBitsLog2Ceiling(trimmed_length - 1);
+      const int nbitpairs = (nbits == 0) ? 1 : (nbits + 1) / 2;
+      VP8LWriteBits(bw, 3, nbitpairs - 1);
+      assert(trimmed_length >= 2);
+      VP8LWriteBits(bw, nbitpairs * 2, trimmed_length - 2);
+    }
+    StoreHuffmanTreeToBitMask(bw, tokens, length, &huffman_code);
+  }
+  ok = 1;
+ End:
+  free(tokens);
+  return ok;
+}
+
+static int StoreHuffmanCode(VP8LBitWriter* const bw,
+                            const HuffmanTreeCode* const huffman_code) {
+  int i;
+  int count = 0;
+  int symbols[2] = { 0, 0 };
+  const int kMaxBits = 8;
+  const int kMaxSymbol = 1 << kMaxBits;
+
+  // Check whether it's a small tree.
+  for (i = 0; i < huffman_code->num_symbols && count < 3; ++i) {
+    if (huffman_code->code_lengths[i] != 0) {
+      if (count < 2) symbols[count] = i;
+      ++count;
+    }
+  }
+
+  if (count == 0) {   // emit minimal tree for empty cases
+    // bits: small tree marker: 1, count-1: 0, large 8-bit code: 0, code: 0
+    VP8LWriteBits(bw, 4, 0x01);
+    return 1;
+  } else if (count <= 2 && symbols[0] < kMaxSymbol && symbols[1] < kMaxSymbol) {
+    VP8LWriteBits(bw, 1, 1);  // Small tree marker to encode 1 or 2 symbols.
+    VP8LWriteBits(bw, 1, count - 1);
+    if (symbols[0] <= 1) {
+      VP8LWriteBits(bw, 1, 0);  // Code bit for small (1 bit) symbol value.
+      VP8LWriteBits(bw, 1, symbols[0]);
+    } else {
+      VP8LWriteBits(bw, 1, 1);
+      VP8LWriteBits(bw, 8, symbols[0]);
+    }
+    if (count == 2) {
+      VP8LWriteBits(bw, 8, symbols[1]);
+    }
+    return 1;
+  } else {
+    return StoreFullHuffmanCode(bw, huffman_code);
+  }
+}
+
+static void WriteHuffmanCode(VP8LBitWriter* const bw,
+                             const HuffmanTreeCode* const code, int index) {
+  const int depth = code->code_lengths[index];
+  const int symbol = code->codes[index];
+  VP8LWriteBits(bw, depth, symbol);
+}
+
+static void StoreImageToBitMask(
+    VP8LBitWriter* const bw, int width, int histo_bits,
+    const VP8LBackwardRefs* const refs,
+    const uint16_t* histogram_symbols,
+    const HuffmanTreeCode* const huffman_codes) {
+  // x and y trace the position in the image.
+  int x = 0;
+  int y = 0;
+  const int histo_xsize = histo_bits ? VP8LSubSampleSize(width, histo_bits) : 1;
+  int i;
+  for (i = 0; i < refs->size; ++i) {
+    const PixOrCopy* const v = &refs->refs[i];
+    const int histogram_ix = histogram_symbols[histo_bits ?
+                                               (y >> histo_bits) * histo_xsize +
+                                               (x >> histo_bits) : 0];
+    const HuffmanTreeCode* const codes = huffman_codes + 5 * histogram_ix;
+    if (PixOrCopyIsCacheIdx(v)) {
+      const int code = PixOrCopyCacheIdx(v);
+      const int literal_ix = 256 + NUM_LENGTH_CODES + code;
+      WriteHuffmanCode(bw, codes, literal_ix);
+    } else if (PixOrCopyIsLiteral(v)) {
+      static const int order[] = { 1, 2, 0, 3 };
+      int k;
+      for (k = 0; k < 4; ++k) {
+        const int code = PixOrCopyLiteral(v, order[k]);
+        WriteHuffmanCode(bw, codes + k, code);
+      }
+    } else {
+      int bits, n_bits;
+      int code, distance;
+
+      PrefixEncode(v->len, &code, &n_bits, &bits);
+      WriteHuffmanCode(bw, codes, 256 + code);
+      VP8LWriteBits(bw, n_bits, bits);
+
+      distance = PixOrCopyDistance(v);
+      PrefixEncode(distance, &code, &n_bits, &bits);
+      WriteHuffmanCode(bw, codes + 4, code);
+      VP8LWriteBits(bw, n_bits, bits);
+    }
+    x += PixOrCopyLength(v);
+    while (x >= width) {
+      x -= width;
+      ++y;
+    }
+  }
+}
+
+// Special case of EncodeImageInternal() for cache-bits=0, histo_bits=31
+static int EncodeImageNoHuffman(VP8LBitWriter* const bw,
+                                const uint32_t* const argb,
+                                int width, int height, int quality) {
+  int i;
+  int ok = 0;
+  VP8LBackwardRefs refs;
+  HuffmanTreeCode huffman_codes[5] = { { 0, NULL, NULL } };
+  const uint16_t histogram_symbols[1] = { 0 };    // only one tree, one symbol
+  VP8LHistogramSet* const histogram_image = VP8LAllocateHistogramSet(1, 0);
+  if (histogram_image == NULL) return 0;
+
+  // Calculate backward references from ARGB image.
+  if (!VP8LGetBackwardReferences(width, height, argb, quality, 0, 1, &refs)) {
+    goto Error;
+  }
+  // Build histogram image and symbols from backward references.
+  VP8LHistogramStoreRefs(&refs, histogram_image->histograms[0]);
+
+  // Create Huffman bit lengths and codes for each histogram image.
+  assert(histogram_image->size == 1);
+  if (!GetHuffBitLengthsAndCodes(histogram_image, huffman_codes)) {
+    goto Error;
+  }
+
+  // No color cache, no Huffman image.
+  VP8LWriteBits(bw, 1, 0);
+
+  // Store Huffman codes.
+  for (i = 0; i < 5; ++i) {
+    HuffmanTreeCode* const codes = &huffman_codes[i];
+    if (!StoreHuffmanCode(bw, codes)) {
+      goto Error;
+    }
+    ClearHuffmanTreeIfOnlyOneSymbol(codes);
+  }
+
+  // Store actual literals.
+  StoreImageToBitMask(bw, width, 0, &refs, histogram_symbols, huffman_codes);
+  ok = 1;
+
+ Error:
+  free(histogram_image);
+  VP8LClearBackwardRefs(&refs);
+  free(huffman_codes[0].codes);
+  return ok;
+}
+
+static int EncodeImageInternal(VP8LBitWriter* const bw,
+                               const uint32_t* const argb,
+                               int width, int height, int quality,
+                               int cache_bits, int histogram_bits) {
+  int ok = 0;
+  const int use_2d_locality = 1;
+  const int use_color_cache = (cache_bits > 0);
+  const uint32_t histogram_image_xysize =
+      VP8LSubSampleSize(width, histogram_bits) *
+      VP8LSubSampleSize(height, histogram_bits);
+  VP8LHistogramSet* histogram_image =
+      VP8LAllocateHistogramSet(histogram_image_xysize, 0);
+  int histogram_image_size = 0;
+  size_t bit_array_size = 0;
+  HuffmanTreeCode* huffman_codes = NULL;
+  VP8LBackwardRefs refs;
+  uint16_t* const histogram_symbols =
+      (uint16_t*)WebPSafeMalloc((uint64_t)histogram_image_xysize,
+                                sizeof(*histogram_symbols));
+  assert(histogram_bits >= MIN_HUFFMAN_BITS);
+  assert(histogram_bits <= MAX_HUFFMAN_BITS);
+  if (histogram_image == NULL || histogram_symbols == NULL) goto Error;
+
+  // Calculate backward references from ARGB image.
+  if (!VP8LGetBackwardReferences(width, height, argb, quality, cache_bits,
+                                 use_2d_locality, &refs)) {
+    goto Error;
+  }
+  // Build histogram image and symbols from backward references.
+  if (!VP8LGetHistoImageSymbols(width, height, &refs,
+                                quality, histogram_bits, cache_bits,
+                                histogram_image,
+                                histogram_symbols)) {
+    goto Error;
+  }
+  // Create Huffman bit lengths and codes for each histogram image.
+  histogram_image_size = histogram_image->size;
+  bit_array_size = 5 * histogram_image_size;
+  huffman_codes = (HuffmanTreeCode*)WebPSafeCalloc(bit_array_size,
+                                                   sizeof(*huffman_codes));
+  if (huffman_codes == NULL ||
+      !GetHuffBitLengthsAndCodes(histogram_image, huffman_codes)) {
+    goto Error;
+  }
+
+  // Color Cache parameters.
+  VP8LWriteBits(bw, 1, use_color_cache);
+  if (use_color_cache) {
+    VP8LWriteBits(bw, 4, cache_bits);
+  }
+
+  // Huffman image + meta huffman.
+  {
+    const int write_histogram_image = (histogram_image_size > 1);
+    VP8LWriteBits(bw, 1, write_histogram_image);
+    if (write_histogram_image) {
+      uint32_t* const histogram_argb =
+          (uint32_t*)WebPSafeMalloc((uint64_t)histogram_image_xysize,
+                                    sizeof(*histogram_argb));
+      int max_index = 0;
+      uint32_t i;
+      if (histogram_argb == NULL) goto Error;
+      for (i = 0; i < histogram_image_xysize; ++i) {
+        const int index = histogram_symbols[i] & 0xffff;
+        histogram_argb[i] = 0xff000000 | (index << 8);
+        if (index >= max_index) {
+          max_index = index + 1;
+        }
+      }
+      histogram_image_size = max_index;
+
+      VP8LWriteBits(bw, 3, histogram_bits - 2);
+      ok = EncodeImageNoHuffman(bw, histogram_argb,
+                                VP8LSubSampleSize(width, histogram_bits),
+                                VP8LSubSampleSize(height, histogram_bits),
+                                quality);
+      free(histogram_argb);
+      if (!ok) goto Error;
+    }
+  }
+
+  // Store Huffman codes.
+  {
+    int i;
+    for (i = 0; i < 5 * histogram_image_size; ++i) {
+      HuffmanTreeCode* const codes = &huffman_codes[i];
+      if (!StoreHuffmanCode(bw, codes)) goto Error;
+      ClearHuffmanTreeIfOnlyOneSymbol(codes);
+    }
+  }
+  // Free combined histograms.
+  free(histogram_image);
+  histogram_image = NULL;
+
+  // Store actual literals.
+  StoreImageToBitMask(bw, width, histogram_bits, &refs,
+                      histogram_symbols, huffman_codes);
+  ok = 1;
+
+ Error:
+  if (!ok) free(histogram_image);
+
+  VP8LClearBackwardRefs(&refs);
+  if (huffman_codes != NULL) {
+    free(huffman_codes->codes);
+    free(huffman_codes);
+  }
+  free(histogram_symbols);
+  return ok;
+}
+
+// -----------------------------------------------------------------------------
+// Transforms
+
+// Check if it would be a good idea to subtract green from red and blue. We
+// only impact entropy in red/blue components, don't bother to look at others.
+static int EvalAndApplySubtractGreen(VP8LEncoder* const enc,
+                                     int width, int height,
+                                     VP8LBitWriter* const bw) {
+  if (!enc->use_palette_) {
+    int i;
+    const uint32_t* const argb = enc->argb_;
+    double bit_cost_before, bit_cost_after;
+    VP8LHistogram* const histo = (VP8LHistogram*)malloc(sizeof(*histo));
+    if (histo == NULL) return 0;
+
+    VP8LHistogramInit(histo, 1);
+    for (i = 0; i < width * height; ++i) {
+      const uint32_t c = argb[i];
+      ++histo->red_[(c >> 16) & 0xff];
+      ++histo->blue_[(c >> 0) & 0xff];
+    }
+    bit_cost_before = VP8LHistogramEstimateBits(histo);
+
+    VP8LHistogramInit(histo, 1);
+    for (i = 0; i < width * height; ++i) {
+      const uint32_t c = argb[i];
+      const int green = (c >> 8) & 0xff;
+      ++histo->red_[((c >> 16) - green) & 0xff];
+      ++histo->blue_[((c >> 0) - green) & 0xff];
+    }
+    bit_cost_after = VP8LHistogramEstimateBits(histo);
+    free(histo);
+
+    // Check if subtracting green yields low entropy.
+    enc->use_subtract_green_ = (bit_cost_after < bit_cost_before);
+    if (enc->use_subtract_green_) {
+      VP8LWriteBits(bw, 1, TRANSFORM_PRESENT);
+      VP8LWriteBits(bw, 2, SUBTRACT_GREEN);
+      VP8LSubtractGreenFromBlueAndRed(enc->argb_, width * height);
+    }
+  }
+  return 1;
+}
+
+static int ApplyPredictFilter(const VP8LEncoder* const enc,
+                              int width, int height, int quality,
+                              VP8LBitWriter* const bw) {
+  const int pred_bits = enc->transform_bits_;
+  const int transform_width = VP8LSubSampleSize(width, pred_bits);
+  const int transform_height = VP8LSubSampleSize(height, pred_bits);
+
+  VP8LResidualImage(width, height, pred_bits, enc->argb_, enc->argb_scratch_,
+                    enc->transform_data_);
+  VP8LWriteBits(bw, 1, TRANSFORM_PRESENT);
+  VP8LWriteBits(bw, 2, PREDICTOR_TRANSFORM);
+  assert(pred_bits >= 2);
+  VP8LWriteBits(bw, 3, pred_bits - 2);
+  if (!EncodeImageNoHuffman(bw, enc->transform_data_,
+                            transform_width, transform_height, quality)) {
+    return 0;
+  }
+  return 1;
+}
+
+static int ApplyCrossColorFilter(const VP8LEncoder* const enc,
+                                 int width, int height, int quality,
+                                 VP8LBitWriter* const bw) {
+  const int ccolor_transform_bits = enc->transform_bits_;
+  const int transform_width = VP8LSubSampleSize(width, ccolor_transform_bits);
+  const int transform_height = VP8LSubSampleSize(height, ccolor_transform_bits);
+  const int step = (quality == 0) ? 32 : 8;
+
+  VP8LColorSpaceTransform(width, height, ccolor_transform_bits, step,
+                          enc->argb_, enc->transform_data_);
+  VP8LWriteBits(bw, 1, TRANSFORM_PRESENT);
+  VP8LWriteBits(bw, 2, CROSS_COLOR_TRANSFORM);
+  assert(ccolor_transform_bits >= 2);
+  VP8LWriteBits(bw, 3, ccolor_transform_bits - 2);
+  if (!EncodeImageNoHuffman(bw, enc->transform_data_,
+                            transform_width, transform_height, quality)) {
+    return 0;
+  }
+  return 1;
+}
+
+// -----------------------------------------------------------------------------
+
+static void PutLE32(uint8_t* const data, uint32_t val) {
+  data[0] = (val >>  0) & 0xff;
+  data[1] = (val >>  8) & 0xff;
+  data[2] = (val >> 16) & 0xff;
+  data[3] = (val >> 24) & 0xff;
+}
+
+static WebPEncodingError WriteRiffHeader(const WebPPicture* const pic,
+                                         size_t riff_size, size_t vp8l_size) {
+  uint8_t riff[RIFF_HEADER_SIZE + CHUNK_HEADER_SIZE + VP8L_SIGNATURE_SIZE] = {
+    'R', 'I', 'F', 'F', 0, 0, 0, 0, 'W', 'E', 'B', 'P',
+    'V', 'P', '8', 'L', 0, 0, 0, 0, VP8L_MAGIC_BYTE,
+  };
+  PutLE32(riff + TAG_SIZE, (uint32_t)riff_size);
+  PutLE32(riff + RIFF_HEADER_SIZE + TAG_SIZE, (uint32_t)vp8l_size);
+  if (!pic->writer(riff, sizeof(riff), pic)) {
+    return VP8_ENC_ERROR_BAD_WRITE;
+  }
+  return VP8_ENC_OK;
+}
+
+static int WriteImageSize(const WebPPicture* const pic,
+                          VP8LBitWriter* const bw) {
+  const int width = pic->width - 1;
+  const int height = pic->height - 1;
+  assert(width < WEBP_MAX_DIMENSION && height < WEBP_MAX_DIMENSION);
+
+  VP8LWriteBits(bw, VP8L_IMAGE_SIZE_BITS, width);
+  VP8LWriteBits(bw, VP8L_IMAGE_SIZE_BITS, height);
+  return !bw->error_;
+}
+
+static int WriteRealAlphaAndVersion(VP8LBitWriter* const bw, int has_alpha) {
+  VP8LWriteBits(bw, 1, has_alpha);
+  VP8LWriteBits(bw, VP8L_VERSION_BITS, VP8L_VERSION);
+  return !bw->error_;
+}
+
+static WebPEncodingError WriteImage(const WebPPicture* const pic,
+                                    VP8LBitWriter* const bw,
+                                    size_t* const coded_size) {
+  WebPEncodingError err = VP8_ENC_OK;
+  const uint8_t* const webpll_data = VP8LBitWriterFinish(bw);
+  const size_t webpll_size = VP8LBitWriterNumBytes(bw);
+  const size_t vp8l_size = VP8L_SIGNATURE_SIZE + webpll_size;
+  const size_t pad = vp8l_size & 1;
+  const size_t riff_size = TAG_SIZE + CHUNK_HEADER_SIZE + vp8l_size + pad;
+
+  err = WriteRiffHeader(pic, riff_size, vp8l_size);
+  if (err != VP8_ENC_OK) goto Error;
+
+  if (!pic->writer(webpll_data, webpll_size, pic)) {
+    err = VP8_ENC_ERROR_BAD_WRITE;
+    goto Error;
+  }
+
+  if (pad) {
+    const uint8_t pad_byte[1] = { 0 };
+    if (!pic->writer(pad_byte, 1, pic)) {
+      err = VP8_ENC_ERROR_BAD_WRITE;
+      goto Error;
+    }
+  }
+  *coded_size = CHUNK_HEADER_SIZE + riff_size;
+  return VP8_ENC_OK;
+
+ Error:
+  return err;
+}
+
+// -----------------------------------------------------------------------------
+
+// Allocates the memory for argb (W x H) buffer, 2 rows of context for
+// prediction and transform data.
+static WebPEncodingError AllocateTransformBuffer(VP8LEncoder* const enc,
+                                                 int width, int height) {
+  WebPEncodingError err = VP8_ENC_OK;
+  const int tile_size = 1 << enc->transform_bits_;
+  const uint64_t image_size = width * height;
+  const uint64_t argb_scratch_size = tile_size * width + width;
+  const uint64_t transform_data_size =
+      (uint64_t)VP8LSubSampleSize(width, enc->transform_bits_) *
+      (uint64_t)VP8LSubSampleSize(height, enc->transform_bits_);
+  const uint64_t total_size =
+      image_size + argb_scratch_size + transform_data_size;
+  uint32_t* mem = (uint32_t*)WebPSafeMalloc(total_size, sizeof(*mem));
+  if (mem == NULL) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+  enc->argb_ = mem;
+  mem += image_size;
+  enc->argb_scratch_ = mem;
+  mem += argb_scratch_size;
+  enc->transform_data_ = mem;
+  enc->current_width_ = width;
+
+ Error:
+  return err;
+}
+
+// Bundles multiple (2, 4 or 8) pixels into a single pixel.
+// Returns the new xsize.
+static void BundleColorMap(const WebPPicture* const pic,
+                           int xbits, uint32_t* bundled_argb, int xs) {
+  int y;
+  const int bit_depth = 1 << (3 - xbits);
+  uint32_t code = 0;
+  const uint32_t* argb = pic->argb;
+  const int width = pic->width;
+  const int height = pic->height;
+
+  for (y = 0; y < height; ++y) {
+    int x;
+    for (x = 0; x < width; ++x) {
+      const int mask = (1 << xbits) - 1;
+      const int xsub = x & mask;
+      if (xsub == 0) {
+        code = 0;
+      }
+      // TODO(vikasa): simplify the bundling logic.
+      code |= (argb[x] & 0xff00) << (bit_depth * xsub);
+      bundled_argb[y * xs + (x >> xbits)] = 0xff000000 | code;
+    }
+    argb += pic->argb_stride;
+  }
+}
+
+// Note: Expects "enc->palette_" to be set properly.
+// Also, "enc->palette_" will be modified after this call and should not be used
+// later.
+static WebPEncodingError ApplyPalette(VP8LBitWriter* const bw,
+                                      VP8LEncoder* const enc, int quality) {
+  WebPEncodingError err = VP8_ENC_OK;
+  int i, x, y;
+  const WebPPicture* const pic = enc->pic_;
+  uint32_t* argb = pic->argb;
+  const int width = pic->width;
+  const int height = pic->height;
+  uint32_t* const palette = enc->palette_;
+  const int palette_size = enc->palette_size_;
+
+  // Replace each input pixel by corresponding palette index.
+  for (y = 0; y < height; ++y) {
+    for (x = 0; x < width; ++x) {
+      const uint32_t pix = argb[x];
+      for (i = 0; i < palette_size; ++i) {
+        if (pix == palette[i]) {
+          argb[x] = 0xff000000u | (i << 8);
+          break;
+        }
+      }
+    }
+    argb += pic->argb_stride;
+  }
+
+  // Save palette to bitstream.
+  VP8LWriteBits(bw, 1, TRANSFORM_PRESENT);
+  VP8LWriteBits(bw, 2, COLOR_INDEXING_TRANSFORM);
+  assert(palette_size >= 1);
+  VP8LWriteBits(bw, 8, palette_size - 1);
+  for (i = palette_size - 1; i >= 1; --i) {
+    palette[i] = VP8LSubPixels(palette[i], palette[i - 1]);
+  }
+  if (!EncodeImageNoHuffman(bw, palette, palette_size, 1, quality)) {
+    err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+    goto Error;
+  }
+
+  if (palette_size <= 16) {
+    // Image can be packed (multiple pixels per uint32_t).
+    int xbits = 1;
+    if (palette_size <= 2) {
+      xbits = 3;
+    } else if (palette_size <= 4) {
+      xbits = 2;
+    }
+    err = AllocateTransformBuffer(enc, VP8LSubSampleSize(width, xbits), height);
+    if (err != VP8_ENC_OK) goto Error;
+    BundleColorMap(pic, xbits, enc->argb_, enc->current_width_);
+  }
+
+ Error:
+  return err;
+}
+
+// -----------------------------------------------------------------------------
+
+static int GetHistoBits(const WebPConfig* const config,
+                        const WebPPicture* const pic) {
+  const int width = pic->width;
+  const int height = pic->height;
+  const size_t hist_size = sizeof(VP8LHistogram);
+  // Make tile size a function of encoding method (Range: 0 to 6).
+  int histo_bits = 7 - config->method;
+  while (1) {
+    const size_t huff_image_size = VP8LSubSampleSize(width, histo_bits) *
+                                   VP8LSubSampleSize(height, histo_bits) *
+                                   hist_size;
+    if (huff_image_size <= MAX_HUFF_IMAGE_SIZE) break;
+    ++histo_bits;
+  }
+  return (histo_bits < MIN_HUFFMAN_BITS) ? MIN_HUFFMAN_BITS :
+         (histo_bits > MAX_HUFFMAN_BITS) ? MAX_HUFFMAN_BITS : histo_bits;
+}
+
+static void InitEncParams(VP8LEncoder* const enc) {
+  const WebPConfig* const config = enc->config_;
+  const WebPPicture* const picture = enc->pic_;
+  const int method = config->method;
+  const float quality = config->quality;
+  enc->transform_bits_ = (method < 4) ? 5 : (method > 4) ? 3 : 4;
+  enc->histo_bits_ = GetHistoBits(config, picture);
+  enc->cache_bits_ = (quality <= 25.f) ? 0 : 7;
+}
+
+// -----------------------------------------------------------------------------
+// VP8LEncoder
+
+static VP8LEncoder* VP8LEncoderNew(const WebPConfig* const config,
+                                   const WebPPicture* const picture) {
+  VP8LEncoder* const enc = (VP8LEncoder*)calloc(1, sizeof(*enc));
+  if (enc == NULL) {
+    WebPEncodingSetError(picture, VP8_ENC_ERROR_OUT_OF_MEMORY);
+    return NULL;
+  }
+  enc->config_ = config;
+  enc->pic_ = picture;
+  return enc;
+}
+
+static void VP8LEncoderDelete(VP8LEncoder* enc) {
+  free(enc->argb_);
+  free(enc);
+}
+
+// -----------------------------------------------------------------------------
+// Main call
+
+WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
+                                   const WebPPicture* const picture,
+                                   VP8LBitWriter* const bw) {
+  WebPEncodingError err = VP8_ENC_OK;
+  const int quality = (int)config->quality;
+  const int width = picture->width;
+  const int height = picture->height;
+  VP8LEncoder* const enc = VP8LEncoderNew(config, picture);
+  const size_t byte_position = VP8LBitWriterNumBytes(bw);
+
+  if (enc == NULL) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+
+  InitEncParams(enc);
+
+  // ---------------------------------------------------------------------------
+  // Analyze image (entropy, num_palettes etc)
+
+  if (!VP8LEncAnalyze(enc, config->image_hint)) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+
+  if (enc->use_palette_) {
+    err = ApplyPalette(bw, enc, quality);
+    if (err != VP8_ENC_OK) goto Error;
+    enc->cache_bits_ = 0;
+  }
+
+  // In case image is not packed.
+  if (enc->argb_ == NULL) {
+    int y;
+    err = AllocateTransformBuffer(enc, width, height);
+    if (err != VP8_ENC_OK) goto Error;
+    for (y = 0; y < height; ++y) {
+      memcpy(enc->argb_ + y * width,
+             picture->argb + y * picture->argb_stride,
+             width * sizeof(*enc->argb_));
+    }
+    enc->current_width_ = width;
+  }
+
+  // ---------------------------------------------------------------------------
+  // Apply transforms and write transform data.
+
+  if (!EvalAndApplySubtractGreen(enc, enc->current_width_, height, bw)) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+
+  if (enc->use_predict_) {
+    if (!ApplyPredictFilter(enc, enc->current_width_, height, quality, bw)) {
+      err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+      goto Error;
+    }
+  }
+
+  if (enc->use_cross_color_) {
+    if (!ApplyCrossColorFilter(enc, enc->current_width_, height, quality, bw)) {
+      err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+      goto Error;
+    }
+  }
+
+  VP8LWriteBits(bw, 1, !TRANSFORM_PRESENT);  // No more transforms.
+
+  // ---------------------------------------------------------------------------
+  // Estimate the color cache size.
+
+  if (enc->cache_bits_ > 0) {
+    if (!VP8LCalculateEstimateForCacheSize(enc->argb_, enc->current_width_,
+                                           height, &enc->cache_bits_)) {
+      err = VP8_ENC_ERROR_INVALID_CONFIGURATION;
+      goto Error;
+    }
+  }
+
+  // ---------------------------------------------------------------------------
+  // Encode and write the transformed image.
+
+  if (!EncodeImageInternal(bw, enc->argb_, enc->current_width_, height,
+                           quality, enc->cache_bits_, enc->histo_bits_)) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+
+  if (picture->stats != NULL) {
+    WebPAuxStats* const stats = picture->stats;
+    stats->lossless_features = 0;
+    if (enc->use_predict_) stats->lossless_features |= 1;
+    if (enc->use_cross_color_) stats->lossless_features |= 2;
+    if (enc->use_subtract_green_) stats->lossless_features |= 4;
+    if (enc->use_palette_) stats->lossless_features |= 8;
+    stats->histogram_bits = enc->histo_bits_;
+    stats->transform_bits = enc->transform_bits_;
+    stats->cache_bits = enc->cache_bits_;
+    stats->palette_size = enc->palette_size_;
+    stats->lossless_size = (int)(VP8LBitWriterNumBytes(bw) - byte_position);
+  }
+
+ Error:
+  VP8LEncoderDelete(enc);
+  return err;
+}
+
+int VP8LEncodeImage(const WebPConfig* const config,
+                    const WebPPicture* const picture) {
+  int width, height;
+  int has_alpha;
+  size_t coded_size;
+  int percent = 0;
+  WebPEncodingError err = VP8_ENC_OK;
+  VP8LBitWriter bw;
+
+  if (picture == NULL) return 0;
+
+  if (config == NULL || picture->argb == NULL) {
+    err = VP8_ENC_ERROR_NULL_PARAMETER;
+    WebPEncodingSetError(picture, err);
+    return 0;
+  }
+
+  width = picture->width;
+  height = picture->height;
+  if (!VP8LBitWriterInit(&bw, (width * height) >> 1)) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+
+  if (!WebPReportProgress(picture, 1, &percent)) {
+ UserAbort:
+    err = VP8_ENC_ERROR_USER_ABORT;
+    goto Error;
+  }
+  // Reset stats (for pure lossless coding)
+  if (picture->stats != NULL) {
+    WebPAuxStats* const stats = picture->stats;
+    memset(stats, 0, sizeof(*stats));
+    stats->PSNR[0] = 99.f;
+    stats->PSNR[1] = 99.f;
+    stats->PSNR[2] = 99.f;
+    stats->PSNR[3] = 99.f;
+    stats->PSNR[4] = 99.f;
+  }
+
+  // Write image size.
+  if (!WriteImageSize(picture, &bw)) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+
+  has_alpha = WebPPictureHasTransparency(picture);
+  // Write the non-trivial Alpha flag and lossless version.
+  if (!WriteRealAlphaAndVersion(&bw, has_alpha)) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
+  }
+
+  if (!WebPReportProgress(picture, 5, &percent)) goto UserAbort;
+
+  // Encode main image stream.
+  err = VP8LEncodeStream(config, picture, &bw);
+  if (err != VP8_ENC_OK) goto Error;
+
+  // TODO(skal): have a fine-grained progress report in VP8LEncodeStream().
+  if (!WebPReportProgress(picture, 90, &percent)) goto UserAbort;
+
+  // Finish the RIFF chunk.
+  err = WriteImage(picture, &bw, &coded_size);
+  if (err != VP8_ENC_OK) goto Error;
+
+  if (!WebPReportProgress(picture, 100, &percent)) goto UserAbort;
+
+  // Save size.
+  if (picture->stats != NULL) {
+    picture->stats->coded_size += (int)coded_size;
+    picture->stats->lossless_size = (int)coded_size;
+  }
+
+  if (picture->extra_info != NULL) {
+    const int mb_w = (width + 15) >> 4;
+    const int mb_h = (height + 15) >> 4;
+    memset(picture->extra_info, 0, mb_w * mb_h * sizeof(*picture->extra_info));
+  }
+
+ Error:
+  if (bw.error_) err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+  VP8LBitWriterDestroy(&bw);
+  if (err != VP8_ENC_OK) {
+    WebPEncodingSetError(picture, err);
+    return 0;
+  }
+  return 1;
+}
+
+//------------------------------------------------------------------------------
+
+#if defined(__cplusplus) || defined(c_plusplus)
+}    // extern "C"
+#endif