Have the palette code be in its own file.

Change-Id: I099a342effedd9f451c94d00a14aead27079e6cc

Android.mk

@@ -164,6 +164,7 @@ utils_dec_srcs := \
     src/utils/color_cache_utils.c \
     src/utils/filters_utils.c \
     src/utils/huffman_utils.c \
+    src/utils/palette.c \
     src/utils/quant_levels_dec_utils.c \
     src/utils/random_utils.c \
     src/utils/rescaler_utils.c \

Makefile.vc

@@ -336,6 +336,7 @@ UTILS_DEC_OBJS = \
     $(DIROBJ)\utils\color_cache_utils.obj \
     $(DIROBJ)\utils\filters_utils.obj \
     $(DIROBJ)\utils\huffman_utils.obj \
+    $(DIROBJ)\utils\palette.obj \
     $(DIROBJ)\utils\quant_levels_dec_utils.obj \
     $(DIROBJ)\utils\rescaler_utils.obj \
     $(DIROBJ)\utils\random_utils.obj \

build.gradle

@@ -173,6 +173,7 @@ model {
             include "color_cache_utils.c"
             include "filters_utils.c"
             include "huffman_utils.c"
+            include "palette.c"
             include "quant_levels_dec_utils.c"
             include "random_utils.c"
             include "rescaler_utils.c"

makefile.unix

@@ -276,6 +276,7 @@ UTILS_DEC_OBJS = \
     src/utils/color_cache_utils.o \
     src/utils/filters_utils.o \
     src/utils/huffman_utils.o \
+    src/utils/palette.o \
     src/utils/quant_levels_dec_utils.o \
     src/utils/random_utils.o \
     src/utils/rescaler_utils.o \
@@ -343,6 +344,7 @@ HDRS = \
     src/utils/filters_utils.h \
     src/utils/huffman_utils.h \
     src/utils/huffman_encode_utils.h \
+    src/utils/palette.h \
     src/utils/quant_levels_utils.h \
     src/utils/quant_levels_dec_utils.h \
     src/utils/random_utils.h \

src/dsp/lossless_common.h

@@ -16,9 +16,9 @@
 #ifndef WEBP_DSP_LOSSLESS_COMMON_H_
 #define WEBP_DSP_LOSSLESS_COMMON_H_
 
-#include "src/webp/types.h"
+#include "src/dsp/cpu.h"
-
 #include "src/utils/utils.h"
+#include "src/webp/types.h"
 
 #ifdef __cplusplus
 extern "C" {

src/enc/picture_enc.c

@@ -12,10 +12,10 @@
 // Author: Skal (pascal.massimino@gmail.com)
 
 #include <assert.h>
+#include <limits.h>
 #include <stdlib.h>
 
 #include "src/enc/vp8i_enc.h"
-#include "src/dsp/dsp.h"
 #include "src/utils/utils.h"
 
 //------------------------------------------------------------------------------

src/enc/vp8l_enc.c

@@ -23,6 +23,7 @@
 #include "src/enc/vp8li_enc.h"
 #include "src/utils/bit_writer_utils.h"
 #include "src/utils/huffman_encode_utils.h"
+#include "src/utils/palette.h"
 #include "src/utils/utils.h"
 #include "src/webp/encode.h"
 #include "src/webp/format_constants.h"
@@ -30,298 +31,6 @@
 // Maximum number of histogram images (sub-blocks).
 #define MAX_HUFF_IMAGE_SIZE       2600
 
-// Palette reordering for smaller sum of deltas (and for smaller storage).
-
-static int PaletteCompareColorsForQsort(const void* p1, const void* p2) {
-  const uint32_t a = WebPMemToUint32((uint8_t*)p1);
-  const uint32_t b = WebPMemToUint32((uint8_t*)p2);
-  assert(a != b);
-  return (a < b) ? -1 : 1;
-}
-
-static WEBP_INLINE uint32_t PaletteComponentDistance(uint32_t v) {
-  return (v <= 128) ? v : (256 - v);
-}
-
-// Computes a value that is related to the entropy created by the
-// palette entry diff.
-//
-// Note that the last & 0xff is a no-operation in the next statement, but
-// removed by most compilers and is here only for regularity of the code.
-static WEBP_INLINE uint32_t PaletteColorDistance(uint32_t col1, uint32_t col2) {
-  const uint32_t diff = VP8LSubPixels(col1, col2);
-  const int kMoreWeightForRGBThanForAlpha = 9;
-  uint32_t score;
-  score =  PaletteComponentDistance((diff >>  0) & 0xff);
-  score += PaletteComponentDistance((diff >>  8) & 0xff);
-  score += PaletteComponentDistance((diff >> 16) & 0xff);
-  score *= kMoreWeightForRGBThanForAlpha;
-  score += PaletteComponentDistance((diff >> 24) & 0xff);
-  return score;
-}
-
-static WEBP_INLINE void SwapColor(uint32_t* const col1, uint32_t* const col2) {
-  const uint32_t tmp = *col1;
-  *col1 = *col2;
-  *col2 = tmp;
-}
-
-static WEBP_INLINE int SearchColorNoIdx(const uint32_t sorted[], uint32_t color,
-                                        int num_colors) {
-  int low = 0, hi = num_colors;
-  if (sorted[low] == color) return low;  // loop invariant: sorted[low] != color
-  while (1) {
-    const int mid = (low + hi) >> 1;
-    if (sorted[mid] == color) {
-      return mid;
-    } else if (sorted[mid] < color) {
-      low = mid;
-    } else {
-      hi = mid;
-    }
-  }
-  assert(0);
-  return 0;
-}
-
-// The palette has been sorted by alpha. This function checks if the other
-// components of the palette have a monotonic development with regards to
-// position in the palette. If all have monotonic development, there is
-// no benefit to re-organize them greedily. A monotonic development
-// would be spotted in green-only situations (like lossy alpha) or gray-scale
-// images.
-static int PaletteHasNonMonotonousDeltas(const uint32_t* const palette,
-                                         int num_colors) {
-  uint32_t predict = 0x000000;
-  int i;
-  uint8_t sign_found = 0x00;
-  for (i = 0; i < num_colors; ++i) {
-    const uint32_t diff = VP8LSubPixels(palette[i], predict);
-    const uint8_t rd = (diff >> 16) & 0xff;
-    const uint8_t gd = (diff >>  8) & 0xff;
-    const uint8_t bd = (diff >>  0) & 0xff;
-    if (rd != 0x00) {
-      sign_found |= (rd < 0x80) ? 1 : 2;
-    }
-    if (gd != 0x00) {
-      sign_found |= (gd < 0x80) ? 8 : 16;
-    }
-    if (bd != 0x00) {
-      sign_found |= (bd < 0x80) ? 64 : 128;
-    }
-    predict = palette[i];
-  }
-  return (sign_found & (sign_found << 1)) != 0;  // two consequent signs.
-}
-
-static void PaletteSortMinimizeDeltas(const uint32_t* const palette_sorted,
-                                      int num_colors, uint32_t* const palette) {
-  uint32_t predict = 0x00000000;
-  int i, k;
-  memcpy(palette, palette_sorted, num_colors * sizeof(*palette));
-  if (!PaletteHasNonMonotonousDeltas(palette_sorted, num_colors)) return;
-  // Find greedily always the closest color of the predicted color to minimize
-  // deltas in the palette. This reduces storage needs since the
-  // palette is stored with delta encoding.
-  for (i = 0; i < num_colors; ++i) {
-    int best_ix = i;
-    uint32_t best_score = ~0U;
-    for (k = i; k < num_colors; ++k) {
-      const uint32_t cur_score = PaletteColorDistance(palette[k], predict);
-      if (best_score > cur_score) {
-        best_score = cur_score;
-        best_ix = k;
-      }
-    }
-    SwapColor(&palette[best_ix], &palette[i]);
-    predict = palette[i];
-  }
-}
-
-// Sort palette in increasing order and prepare an inverse mapping array.
-static void PrepareMapToPalette(const uint32_t palette[], uint32_t num_colors,
-                                uint32_t sorted[], uint32_t idx_map[]) {
-  uint32_t i;
-  memcpy(sorted, palette, num_colors * sizeof(*sorted));
-  qsort(sorted, num_colors, sizeof(*sorted), PaletteCompareColorsForQsort);
-  for (i = 0; i < num_colors; ++i) {
-    idx_map[SearchColorNoIdx(sorted, palette[i], num_colors)] = i;
-  }
-}
-
-// -----------------------------------------------------------------------------
-// Modified Zeng method from "A Survey on Palette Reordering
-// Methods for Improving the Compression of Color-Indexed Images" by Armando J.
-// Pinho and Antonio J. R. Neves.
-
-// Finds the biggest cooccurrence in the matrix.
-static void CoOccurrenceFindMax(const uint32_t* const cooccurrence,
-                                uint32_t num_colors, uint8_t* const c1,
-                                uint8_t* const c2) {
-  // Find the index that is most frequently located adjacent to other
-  // (different) indexes.
-  uint32_t best_sum = 0u;
-  uint32_t i, j, best_cooccurrence;
-  *c1 = 0u;
-  for (i = 0; i < num_colors; ++i) {
-    uint32_t sum = 0;
-    for (j = 0; j < num_colors; ++j) sum += cooccurrence[i * num_colors + j];
-    if (sum > best_sum) {
-      best_sum = sum;
-      *c1 = i;
-    }
-  }
-  // Find the index that is most frequently found adjacent to *c1.
-  *c2 = 0u;
-  best_cooccurrence = 0u;
-  for (i = 0; i < num_colors; ++i) {
-    if (cooccurrence[*c1 * num_colors + i] > best_cooccurrence) {
-      best_cooccurrence = cooccurrence[*c1 * num_colors + i];
-      *c2 = i;
-    }
-  }
-  assert(*c1 != *c2);
-}
-
-// Builds the cooccurrence matrix
-static int CoOccurrenceBuild(const WebPPicture* const pic,
-                             const uint32_t* const palette, uint32_t num_colors,
-                             uint32_t* cooccurrence) {
-  uint32_t *lines, *line_top, *line_current, *line_tmp;
-  int x, y;
-  const uint32_t* src = pic->argb;
-  uint32_t prev_pix = ~src[0];
-  uint32_t prev_idx = 0u;
-  uint32_t idx_map[MAX_PALETTE_SIZE] = {0};
-  uint32_t palette_sorted[MAX_PALETTE_SIZE];
-  lines = (uint32_t*)WebPSafeMalloc(2 * pic->width, sizeof(*lines));
-  if (lines == NULL) {
-    return WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
-  }
-  line_top = &lines[0];
-  line_current = &lines[pic->width];
-  PrepareMapToPalette(palette, num_colors, palette_sorted, idx_map);
-  for (y = 0; y < pic->height; ++y) {
-    for (x = 0; x < pic->width; ++x) {
-      const uint32_t pix = src[x];
-      if (pix != prev_pix) {
-        prev_idx = idx_map[SearchColorNoIdx(palette_sorted, pix, num_colors)];
-        prev_pix = pix;
-      }
-      line_current[x] = prev_idx;
-      // 4-connectivity is what works best as mentioned in "On the relation
-      // between Memon's and the modified Zeng's palette reordering methods".
-      if (x > 0 && prev_idx != line_current[x - 1]) {
-        const uint32_t left_idx = line_current[x - 1];
-        ++cooccurrence[prev_idx * num_colors + left_idx];
-        ++cooccurrence[left_idx * num_colors + prev_idx];
-      }
-      if (y > 0 && prev_idx != line_top[x]) {
-        const uint32_t top_idx = line_top[x];
-        ++cooccurrence[prev_idx * num_colors + top_idx];
-        ++cooccurrence[top_idx * num_colors + prev_idx];
-      }
-    }
-    line_tmp = line_top;
-    line_top = line_current;
-    line_current = line_tmp;
-    src += pic->argb_stride;
-  }
-  WebPSafeFree(lines);
-  return 1;
-}
-
-struct Sum {
-  uint8_t index;
-  uint32_t sum;
-};
-
-// Implements the modified Zeng method from "A Survey on Palette Reordering
-// Methods for Improving the Compression of Color-Indexed Images" by Armando J.
-// Pinho and Antonio J. R. Neves.
-static int PaletteSortModifiedZeng(
-    const WebPPicture* const pic, const uint32_t* const palette_sorted,
-    uint32_t num_colors, uint32_t* const palette) {
-  uint32_t i, j, ind;
-  uint8_t remapping[MAX_PALETTE_SIZE];
-  uint32_t* cooccurrence;
-  struct Sum sums[MAX_PALETTE_SIZE];
-  uint32_t first, last;
-  uint32_t num_sums;
-  // TODO(vrabaud) check whether one color images should use palette or not.
-  if (num_colors <= 1) return 1;
-  // Build the co-occurrence matrix.
-  cooccurrence =
-      (uint32_t*)WebPSafeCalloc(num_colors * num_colors, sizeof(*cooccurrence));
-  if (cooccurrence == NULL) {
-    return WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
-  }
-  if (!CoOccurrenceBuild(pic, palette_sorted, num_colors, cooccurrence)) {
-    WebPSafeFree(cooccurrence);
-    return 0;
-  }
-
-  // Initialize the mapping list with the two best indices.
-  CoOccurrenceFindMax(cooccurrence, num_colors, &remapping[0], &remapping[1]);
-
-  // We need to append and prepend to the list of remapping. To this end, we
-  // actually define the next start/end of the list as indices in a vector (with
-  // a wrap around when the end is reached).
-  first = 0;
-  last = 1;
-  num_sums = num_colors - 2;  // -2 because we know the first two values
-  if (num_sums > 0) {
-    // Initialize the sums with the first two remappings and find the best one
-    struct Sum* best_sum = &sums[0];
-    best_sum->index = 0u;
-    best_sum->sum = 0u;
-    for (i = 0, j = 0; i < num_colors; ++i) {
-      if (i == remapping[0] || i == remapping[1]) continue;
-      sums[j].index = i;
-      sums[j].sum = cooccurrence[i * num_colors + remapping[0]] +
-                    cooccurrence[i * num_colors + remapping[1]];
-      if (sums[j].sum > best_sum->sum) best_sum = &sums[j];
-      ++j;
-    }
-
-    while (num_sums > 0) {
-      const uint8_t best_index = best_sum->index;
-      // Compute delta to know if we need to prepend or append the best index.
-      int32_t delta = 0;
-      const int32_t n = num_colors - num_sums;
-      for (ind = first, j = 0; (ind + j) % num_colors != last + 1; ++j) {
-        const uint16_t l_j = remapping[(ind + j) % num_colors];
-        delta += (n - 1 - 2 * (int32_t)j) *
-                 (int32_t)cooccurrence[best_index * num_colors + l_j];
-      }
-      if (delta > 0) {
-        first = (first == 0) ? num_colors - 1 : first - 1;
-        remapping[first] = best_index;
-      } else {
-        ++last;
-        remapping[last] = best_index;
-      }
-      // Remove best_sum from sums.
-      *best_sum = sums[num_sums - 1];
-      --num_sums;
-      // Update all the sums and find the best one.
-      best_sum = &sums[0];
-      for (i = 0; i < num_sums; ++i) {
-        sums[i].sum += cooccurrence[best_index * num_colors + sums[i].index];
-        if (sums[i].sum > best_sum->sum) best_sum = &sums[i];
-      }
-    }
-  }
-  assert((last + 1) % num_colors == first);
-  WebPSafeFree(cooccurrence);
-
-  // Re-map the palette.
-  for (i = 0; i < num_colors; ++i) {
-    palette[i] = palette_sorted[remapping[(first + i) % num_colors]];
-  }
-  return 1;
-}
-
 // -----------------------------------------------------------------------------
 // Palette
 
@@ -336,13 +45,6 @@ typedef enum {
   kNumEntropyIx = 6
 } EntropyIx;
 
-typedef enum {
-  kSortedDefault = 0,
-  kMinimizeDelta = 1,
-  kModifiedZeng = 2,
-  kUnusedPalette = 3,
-} PaletteSorting;
-
 typedef enum {
   kHistoAlpha = 0,
   kHistoAlphaPred,
@@ -586,13 +288,10 @@ static int EncoderAnalyze(VP8LEncoder* const enc,
   assert(pic != NULL && pic->argb != NULL);
 
   // Check whether a palette is possible.
-  enc->palette_size_ = WebPGetColorPalette(pic, enc->palette_sorted_);
+  enc->palette_size_ = GetColorPalette(pic, enc->palette_sorted_);
   use_palette = (enc->palette_size_ <= MAX_PALETTE_SIZE);
   if (!use_palette) {
     enc->palette_size_ = 0;
-  } else {
-    qsort(enc->palette_sorted_, enc->palette_size_,
-          sizeof(*enc->palette_sorted_), PaletteCompareColorsForQsort);
   }
 
   // Empirical bit sizes.
@@ -1855,7 +1554,8 @@ static int EncodeStreamHook(void* input, void* data2) {
       } else {
         assert(crunch_configs[idx].palette_sorting_type_ == kModifiedZeng);
         if (!PaletteSortModifiedZeng(enc->pic_, enc->palette_sorted_,
-                                      enc->palette_size_, enc->palette_)) {
+                                     enc->palette_size_, enc->palette_)) {
+          WebPEncodingSetError(enc->pic_, VP8_ENC_ERROR_OUT_OF_MEMORY);
           goto Error;
         }
       }

src/utils/Makefile.am

@@ -26,6 +26,8 @@ COMMON_SOURCES += filters_utils.c
 COMMON_SOURCES += filters_utils.h
 COMMON_SOURCES += huffman_utils.c
 COMMON_SOURCES += huffman_utils.h
+COMMON_SOURCES += palette.c
+COMMON_SOURCES += palette.h
 COMMON_SOURCES += quant_levels_dec_utils.c
 COMMON_SOURCES += quant_levels_dec_utils.h
 COMMON_SOURCES += rescaler_utils.c

src/utils/palette.c

@@ -0,0 +1,377 @@
+// Copyright 2023 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.
+// -----------------------------------------------------------------------------
+//
+// Utilities for palette analysis.
+//
+// Author: Vincent Rabaud (vrabaud@google.com)
+
+#include "src/utils/palette.h"
+
+#include <assert.h>
+#include <stdlib.h>
+
+#include "src/dsp/lossless_common.h"
+#include "src/utils/color_cache_utils.h"
+#include "src/utils/utils.h"
+#include "src/webp/encode.h"
+#include "src/webp/format_constants.h"
+
+// -----------------------------------------------------------------------------
+
+// Palette reordering for smaller sum of deltas (and for smaller storage).
+
+static int PaletteCompareColorsForQsort(const void* p1, const void* p2) {
+  const uint32_t a = WebPMemToUint32((uint8_t*)p1);
+  const uint32_t b = WebPMemToUint32((uint8_t*)p2);
+  assert(a != b);
+  return (a < b) ? -1 : 1;
+}
+
+static WEBP_INLINE uint32_t PaletteComponentDistance(uint32_t v) {
+  return (v <= 128) ? v : (256 - v);
+}
+
+// Computes a value that is related to the entropy created by the
+// palette entry diff.
+//
+// Note that the last & 0xff is a no-operation in the next statement, but
+// removed by most compilers and is here only for regularity of the code.
+static WEBP_INLINE uint32_t PaletteColorDistance(uint32_t col1, uint32_t col2) {
+  const uint32_t diff = VP8LSubPixels(col1, col2);
+  const int kMoreWeightForRGBThanForAlpha = 9;
+  uint32_t score;
+  score = PaletteComponentDistance((diff >> 0) & 0xff);
+  score += PaletteComponentDistance((diff >> 8) & 0xff);
+  score += PaletteComponentDistance((diff >> 16) & 0xff);
+  score *= kMoreWeightForRGBThanForAlpha;
+  score += PaletteComponentDistance((diff >> 24) & 0xff);
+  return score;
+}
+
+static WEBP_INLINE void SwapColor(uint32_t* const col1, uint32_t* const col2) {
+  const uint32_t tmp = *col1;
+  *col1 = *col2;
+  *col2 = tmp;
+}
+
+int SearchColorNoIdx(const uint32_t sorted[], uint32_t color, int num_colors) {
+  int low = 0, hi = num_colors;
+  if (sorted[low] == color) return low;  // loop invariant: sorted[low] != color
+  while (1) {
+    const int mid = (low + hi) >> 1;
+    if (sorted[mid] == color) {
+      return mid;
+    } else if (sorted[mid] < color) {
+      low = mid;
+    } else {
+      hi = mid;
+    }
+  }
+  assert(0);
+  return 0;
+}
+
+void PrepareMapToPalette(const uint32_t palette[], uint32_t num_colors,
+                         uint32_t sorted[], uint32_t idx_map[]) {
+  uint32_t i;
+  memcpy(sorted, palette, num_colors * sizeof(*sorted));
+  qsort(sorted, num_colors, sizeof(*sorted), PaletteCompareColorsForQsort);
+  for (i = 0; i < num_colors; ++i) {
+    idx_map[SearchColorNoIdx(sorted, palette[i], num_colors)] = i;
+  }
+}
+
+//------------------------------------------------------------------------------
+
+#define COLOR_HASH_SIZE (MAX_PALETTE_SIZE * 4)
+#define COLOR_HASH_RIGHT_SHIFT 22  // 32 - log2(COLOR_HASH_SIZE).
+
+int GetColorPalette(const WebPPicture* const pic, uint32_t* const palette) {
+  int i;
+  int x, y;
+  int num_colors = 0;
+  uint8_t in_use[COLOR_HASH_SIZE] = {0};
+  uint32_t colors[COLOR_HASH_SIZE];
+  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]
+  assert(pic != NULL);
+  assert(pic->use_argb);
+
+  for (y = 0; y < height; ++y) {
+    for (x = 0; x < width; ++x) {
+      int key;
+      if (argb[x] == last_pix) {
+        continue;
+      }
+      last_pix = argb[x];
+      key = VP8LHashPix(last_pix, COLOR_HASH_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 MAX_PALETTE_SIZE + 1;  // Exact count not needed.
+          }
+          break;
+        } else if (colors[key] == last_pix) {
+          break;  // The color is already there.
+        } else {
+          // Some other color sits here, so do linear conflict resolution.
+          ++key;
+          key &= (COLOR_HASH_SIZE - 1);  // Key mask.
+        }
+      }
+    }
+    argb += pic->argb_stride;
+  }
+
+  if (palette != NULL) {  // Fill the colors into palette.
+    num_colors = 0;
+    for (i = 0; i < COLOR_HASH_SIZE; ++i) {
+      if (in_use[i]) {
+        palette[num_colors] = colors[i];
+        ++num_colors;
+      }
+    }
+    qsort(palette, num_colors, sizeof(*palette), PaletteCompareColorsForQsort);
+  }
+  return num_colors;
+}
+
+#undef COLOR_HASH_SIZE
+#undef COLOR_HASH_RIGHT_SHIFT
+
+// -----------------------------------------------------------------------------
+
+// The palette has been sorted by alpha. This function checks if the other
+// components of the palette have a monotonic development with regards to
+// position in the palette. If all have monotonic development, there is
+// no benefit to re-organize them greedily. A monotonic development
+// would be spotted in green-only situations (like lossy alpha) or gray-scale
+// images.
+static int PaletteHasNonMonotonousDeltas(const uint32_t* const palette,
+                                         int num_colors) {
+  uint32_t predict = 0x000000;
+  int i;
+  uint8_t sign_found = 0x00;
+  for (i = 0; i < num_colors; ++i) {
+    const uint32_t diff = VP8LSubPixels(palette[i], predict);
+    const uint8_t rd = (diff >> 16) & 0xff;
+    const uint8_t gd = (diff >> 8) & 0xff;
+    const uint8_t bd = (diff >> 0) & 0xff;
+    if (rd != 0x00) {
+      sign_found |= (rd < 0x80) ? 1 : 2;
+    }
+    if (gd != 0x00) {
+      sign_found |= (gd < 0x80) ? 8 : 16;
+    }
+    if (bd != 0x00) {
+      sign_found |= (bd < 0x80) ? 64 : 128;
+    }
+    predict = palette[i];
+  }
+  return (sign_found & (sign_found << 1)) != 0;  // two consequent signs.
+}
+
+void PaletteSortMinimizeDeltas(const uint32_t* const palette_sorted,
+                               int num_colors, uint32_t* const palette) {
+  uint32_t predict = 0x00000000;
+  int i, k;
+  memcpy(palette, palette_sorted, num_colors * sizeof(*palette));
+  if (!PaletteHasNonMonotonousDeltas(palette_sorted, num_colors)) return;
+  // Find greedily always the closest color of the predicted color to minimize
+  // deltas in the palette. This reduces storage needs since the
+  // palette is stored with delta encoding.
+  for (i = 0; i < num_colors; ++i) {
+    int best_ix = i;
+    uint32_t best_score = ~0U;
+    for (k = i; k < num_colors; ++k) {
+      const uint32_t cur_score = PaletteColorDistance(palette[k], predict);
+      if (best_score > cur_score) {
+        best_score = cur_score;
+        best_ix = k;
+      }
+    }
+    SwapColor(&palette[best_ix], &palette[i]);
+    predict = palette[i];
+  }
+}
+
+// -----------------------------------------------------------------------------
+// Modified Zeng method from "A Survey on Palette Reordering
+// Methods for Improving the Compression of Color-Indexed Images" by Armando J.
+// Pinho and Antonio J. R. Neves.
+
+// Finds the biggest cooccurrence in the matrix.
+static void CoOccurrenceFindMax(const uint32_t* const cooccurrence,
+                                uint32_t num_colors, uint8_t* const c1,
+                                uint8_t* const c2) {
+  // Find the index that is most frequently located adjacent to other
+  // (different) indexes.
+  uint32_t best_sum = 0u;
+  uint32_t i, j, best_cooccurrence;
+  *c1 = 0u;
+  for (i = 0; i < num_colors; ++i) {
+    uint32_t sum = 0;
+    for (j = 0; j < num_colors; ++j) sum += cooccurrence[i * num_colors + j];
+    if (sum > best_sum) {
+      best_sum = sum;
+      *c1 = i;
+    }
+  }
+  // Find the index that is most frequently found adjacent to *c1.
+  *c2 = 0u;
+  best_cooccurrence = 0u;
+  for (i = 0; i < num_colors; ++i) {
+    if (cooccurrence[*c1 * num_colors + i] > best_cooccurrence) {
+      best_cooccurrence = cooccurrence[*c1 * num_colors + i];
+      *c2 = i;
+    }
+  }
+  assert(*c1 != *c2);
+}
+
+// Builds the cooccurrence matrix
+static int CoOccurrenceBuild(const WebPPicture* const pic,
+                             const uint32_t* const palette, uint32_t num_colors,
+                             uint32_t* cooccurrence) {
+  uint32_t *lines, *line_top, *line_current, *line_tmp;
+  int x, y;
+  const uint32_t* src = pic->argb;
+  uint32_t prev_pix = ~src[0];
+  uint32_t prev_idx = 0u;
+  uint32_t idx_map[MAX_PALETTE_SIZE] = {0};
+  uint32_t palette_sorted[MAX_PALETTE_SIZE];
+  lines = (uint32_t*)WebPSafeMalloc(2 * pic->width, sizeof(*lines));
+  if (lines == NULL) {
+    return 0;
+  }
+  line_top = &lines[0];
+  line_current = &lines[pic->width];
+  PrepareMapToPalette(palette, num_colors, palette_sorted, idx_map);
+  for (y = 0; y < pic->height; ++y) {
+    for (x = 0; x < pic->width; ++x) {
+      const uint32_t pix = src[x];
+      if (pix != prev_pix) {
+        prev_idx = idx_map[SearchColorNoIdx(palette_sorted, pix, num_colors)];
+        prev_pix = pix;
+      }
+      line_current[x] = prev_idx;
+      // 4-connectivity is what works best as mentioned in "On the relation
+      // between Memon's and the modified Zeng's palette reordering methods".
+      if (x > 0 && prev_idx != line_current[x - 1]) {
+        const uint32_t left_idx = line_current[x - 1];
+        ++cooccurrence[prev_idx * num_colors + left_idx];
+        ++cooccurrence[left_idx * num_colors + prev_idx];
+      }
+      if (y > 0 && prev_idx != line_top[x]) {
+        const uint32_t top_idx = line_top[x];
+        ++cooccurrence[prev_idx * num_colors + top_idx];
+        ++cooccurrence[top_idx * num_colors + prev_idx];
+      }
+    }
+    line_tmp = line_top;
+    line_top = line_current;
+    line_current = line_tmp;
+    src += pic->argb_stride;
+  }
+  WebPSafeFree(lines);
+  return 1;
+}
+
+struct Sum {
+  uint8_t index;
+  uint32_t sum;
+};
+
+int PaletteSortModifiedZeng(const WebPPicture* const pic,
+                            const uint32_t* const palette_in,
+                            uint32_t num_colors, uint32_t* const palette) {
+  uint32_t i, j, ind;
+  uint8_t remapping[MAX_PALETTE_SIZE];
+  uint32_t* cooccurrence;
+  struct Sum sums[MAX_PALETTE_SIZE];
+  uint32_t first, last;
+  uint32_t num_sums;
+  // TODO(vrabaud) check whether one color images should use palette or not.
+  if (num_colors <= 1) return 1;
+  // Build the co-occurrence matrix.
+  cooccurrence =
+      (uint32_t*)WebPSafeCalloc(num_colors * num_colors, sizeof(*cooccurrence));
+  if (cooccurrence == NULL) {
+    return 0;
+  }
+  if (!CoOccurrenceBuild(pic, palette_in, num_colors, cooccurrence)) {
+    WebPSafeFree(cooccurrence);
+    return 0;
+  }
+
+  // Initialize the mapping list with the two best indices.
+  CoOccurrenceFindMax(cooccurrence, num_colors, &remapping[0], &remapping[1]);
+
+  // We need to append and prepend to the list of remapping. To this end, we
+  // actually define the next start/end of the list as indices in a vector (with
+  // a wrap around when the end is reached).
+  first = 0;
+  last = 1;
+  num_sums = num_colors - 2;  // -2 because we know the first two values
+  if (num_sums > 0) {
+    // Initialize the sums with the first two remappings and find the best one
+    struct Sum* best_sum = &sums[0];
+    best_sum->index = 0u;
+    best_sum->sum = 0u;
+    for (i = 0, j = 0; i < num_colors; ++i) {
+      if (i == remapping[0] || i == remapping[1]) continue;
+      sums[j].index = i;
+      sums[j].sum = cooccurrence[i * num_colors + remapping[0]] +
+                    cooccurrence[i * num_colors + remapping[1]];
+      if (sums[j].sum > best_sum->sum) best_sum = &sums[j];
+      ++j;
+    }
+
+    while (num_sums > 0) {
+      const uint8_t best_index = best_sum->index;
+      // Compute delta to know if we need to prepend or append the best index.
+      int32_t delta = 0;
+      const int32_t n = num_colors - num_sums;
+      for (ind = first, j = 0; (ind + j) % num_colors != last + 1; ++j) {
+        const uint16_t l_j = remapping[(ind + j) % num_colors];
+        delta += (n - 1 - 2 * (int32_t)j) *
+                 (int32_t)cooccurrence[best_index * num_colors + l_j];
+      }
+      if (delta > 0) {
+        first = (first == 0) ? num_colors - 1 : first - 1;
+        remapping[first] = best_index;
+      } else {
+        ++last;
+        remapping[last] = best_index;
+      }
+      // Remove best_sum from sums.
+      *best_sum = sums[num_sums - 1];
+      --num_sums;
+      // Update all the sums and find the best one.
+      best_sum = &sums[0];
+      for (i = 0; i < num_sums; ++i) {
+        sums[i].sum += cooccurrence[best_index * num_colors + sums[i].index];
+        if (sums[i].sum > best_sum->sum) best_sum = &sums[i];
+      }
+    }
+  }
+  assert((last + 1) % num_colors == first);
+  WebPSafeFree(cooccurrence);
+
+  // Re-map the palette.
+  for (i = 0; i < num_colors; ++i) {
+    palette[i] = palette_in[remapping[(first + i) % num_colors]];
+  }
+  return 1;
+}

src/utils/palette.h

@@ -0,0 +1,62 @@
+// Copyright 2023 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.
+// -----------------------------------------------------------------------------
+//
+// Utilities for palette analysis.
+//
+// Author: Vincent Rabaud (vrabaud@google.com)
+
+#ifndef WEBP_UTILS_PALETTE_H_
+#define WEBP_UTILS_PALETTE_H_
+
+#include "src/webp/types.h"
+
+struct WebPPicture;
+
+// The different ways a palette can be sorted.
+typedef enum PaletteSorting {
+  kSortedDefault = 0,
+  kMinimizeDelta = 1,
+  kModifiedZeng = 2,
+  kUnusedPalette = 3
+} PaletteSorting;
+
+// Returns the index of 'color' in the sorted palette 'sorted' of size
+// 'num_colors'.
+int SearchColorNoIdx(const uint32_t sorted[], uint32_t color, int num_colors);
+
+// Sort palette in increasing order and prepare an inverse mapping array.
+void PrepareMapToPalette(const uint32_t palette[], uint32_t num_colors,
+                         uint32_t sorted[], uint32_t idx_map[]);
+
+// Returns count of unique colors in 'pic', assuming pic->use_argb is true.
+// If the unique color count is more than MAX_PALETTE_SIZE, returns
+// MAX_PALETTE_SIZE+1.
+// If 'palette' is not NULL and the number of unique colors is less than or
+// equal to MAX_PALETTE_SIZE, also outputs the actual unique colors into
+// 'palette' in a sorted order. Note: 'palette' is assumed to be an array
+// already allocated with at least MAX_PALETTE_SIZE elements.
+int GetColorPalette(const struct WebPPicture* const pic,
+                    uint32_t* const palette);
+
+// Sorts the color sorted palette in 'palette_sorted'/'num_colors' by
+// minimizing deltas between consecutive colors and stores it in 'palette'.
+void PaletteSortMinimizeDeltas(const uint32_t* const palette_sorted,
+                               int num_colors, uint32_t* const palette);
+
+// Implements the modified Zeng method from "A Survey on Palette Reordering
+// Methods for Improving the Compression of Color-Indexed Images" by Armando J.
+// Pinho and Antonio J. R. Neves. The palette defined by 'palette_in' and
+// 'num_colors' is sorted using information from 'pic' and output in
+// 'palette'.
+// Returns 0 on memory allocation error.
+int PaletteSortModifiedZeng(const struct WebPPicture* const pic,
+                            const uint32_t* const palette_in,
+                            uint32_t num_colors, uint32_t* const palette);
+
+#endif  // WEBP_UTILS_PALETTE_H_

src/utils/utils.c

@@ -11,13 +11,13 @@
 //
 // Author: Skal (pascal.massimino@gmail.com)
 
+#include "src/utils/utils.h"
+
 #include <stdlib.h>
 #include <string.h>  // for memcpy()
-#include "src/webp/decode.h"
+
+#include "src/utils/palette.h"
 #include "src/webp/encode.h"
-#include "src/webp/format_constants.h"  // for MAX_PALETTE_SIZE
-#include "src/utils/color_cache_utils.h"
-#include "src/utils/utils.h"
 
 // If PRINT_MEM_INFO is defined, extra info (like total memory used, number of
 // alloc/free etc) is printed. For debugging/tuning purpose only (it's slow,
@@ -252,66 +252,10 @@ void WebPCopyPixels(const WebPPicture* const src, WebPPicture* const dst) {
 
 //------------------------------------------------------------------------------
 
-#define COLOR_HASH_SIZE         (MAX_PALETTE_SIZE * 4)
-#define COLOR_HASH_RIGHT_SHIFT  22  // 32 - log2(COLOR_HASH_SIZE).
-
 int WebPGetColorPalette(const WebPPicture* const pic, uint32_t* const palette) {
-  int i;
+  return GetColorPalette(pic, palette);
-  int x, y;
-  int num_colors = 0;
-  uint8_t in_use[COLOR_HASH_SIZE] = { 0 };
-  uint32_t colors[COLOR_HASH_SIZE];
-  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]
-  assert(pic != NULL);
-  assert(pic->use_argb);
-
-  for (y = 0; y < height; ++y) {
-    for (x = 0; x < width; ++x) {
-      int key;
-      if (argb[x] == last_pix) {
-        continue;
-      }
-      last_pix = argb[x];
-      key = VP8LHashPix(last_pix, COLOR_HASH_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 MAX_PALETTE_SIZE + 1;  // Exact count not needed.
-          }
-          break;
-        } else if (colors[key] == last_pix) {
-          break;  // The color is already there.
-        } else {
-          // Some other color sits here, so do linear conflict resolution.
-          ++key;
-          key &= (COLOR_HASH_SIZE - 1);  // Key mask.
-        }
-      }
-    }
-    argb += pic->argb_stride;
-  }
-
-  if (palette != NULL) {  // Fill the colors into palette.
-    num_colors = 0;
-    for (i = 0; i < COLOR_HASH_SIZE; ++i) {
-      if (in_use[i]) {
-        palette[num_colors] = colors[i];
-        ++num_colors;
-      }
-    }
-  }
-  return num_colors;
 }
 
-#undef COLOR_HASH_SIZE
-#undef COLOR_HASH_RIGHT_SHIFT
-
 //------------------------------------------------------------------------------
 
 #if defined(WEBP_NEED_LOG_TABLE_8BIT)

src/utils/utils.h

@@ -20,9 +20,7 @@
 #endif
 
 #include <assert.h>
-#include <limits.h>
 
-#include "src/dsp/dsp.h"
 #include "src/webp/types.h"
 
 #ifdef __cplusplus
@@ -198,6 +196,7 @@ WEBP_EXTERN void WebPCopyPixels(const struct WebPPicture* const src,
 // MAX_PALETTE_SIZE, also outputs the actual unique colors into 'palette'.
 // Note: 'palette' is assumed to be an array already allocated with at least
 // MAX_PALETTE_SIZE elements.
+// TODO(vrabaud) remove whenever we can break the ABI.
 WEBP_EXTERN int WebPGetColorPalette(const struct WebPPicture* const pic,
                                     uint32_t* const palette);