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Diffstat (limited to 'media/libwebp/src/utils/huffman_encode_utils.c')
| -rw-r--r-- | media/libwebp/src/utils/huffman_encode_utils.c | 416 |
1 files changed, 416 insertions, 0 deletions
diff --git a/media/libwebp/src/utils/huffman_encode_utils.c b/media/libwebp/src/utils/huffman_encode_utils.c new file mode 100644 index 0000000000..585db91951 --- /dev/null +++ b/media/libwebp/src/utils/huffman_encode_utils.c @@ -0,0 +1,416 @@ +// 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. +// ----------------------------------------------------------------------------- +// +// Author: Jyrki Alakuijala (jyrki@google.com) +// +// Entropy encoding (Huffman) for webp lossless. + +#include <assert.h> +#include <stdlib.h> +#include <string.h> +#include "src/utils/huffman_encode_utils.h" +#include "src/utils/utils.h" +#include "src/webp/format_constants.h" + +// ----------------------------------------------------------------------------- +// Util function to optimize the symbol map for RLE coding + +// Heuristics for selecting the stride ranges to collapse. +static int ValuesShouldBeCollapsedToStrideAverage(int a, int b) { + return abs(a - b) < 4; +} + +// Change the population counts in a way that the consequent +// Huffman tree compression, especially its RLE-part, give smaller output. +static void OptimizeHuffmanForRle(int length, uint8_t* const good_for_rle, + uint32_t* const counts) { + // 1) Let's make the Huffman code more compatible with rle encoding. + int i; + for (; length >= 0; --length) { + if (length == 0) { + return; // All zeros. + } + if (counts[length - 1] != 0) { + // Now counts[0..length - 1] does not have trailing zeros. + break; + } + } + // 2) Let's mark all population counts that already can be encoded + // with an rle code. + { + // Let's not spoil any of the existing good rle codes. + // Mark any seq of 0's that is longer as 5 as a good_for_rle. + // Mark any seq of non-0's that is longer as 7 as a good_for_rle. + uint32_t symbol = counts[0]; + int stride = 0; + for (i = 0; i < length + 1; ++i) { + if (i == length || counts[i] != symbol) { + if ((symbol == 0 && stride >= 5) || + (symbol != 0 && stride >= 7)) { + int k; + for (k = 0; k < stride; ++k) { + good_for_rle[i - k - 1] = 1; + } + } + stride = 1; + if (i != length) { + symbol = counts[i]; + } + } else { + ++stride; + } + } + } + // 3) Let's replace those population counts that lead to more rle codes. + { + uint32_t stride = 0; + uint32_t limit = counts[0]; + uint32_t sum = 0; + for (i = 0; i < length + 1; ++i) { + if (i == length || good_for_rle[i] || + (i != 0 && good_for_rle[i - 1]) || + !ValuesShouldBeCollapsedToStrideAverage(counts[i], limit)) { + if (stride >= 4 || (stride >= 3 && sum == 0)) { + uint32_t k; + // The stride must end, collapse what we have, if we have enough (4). + uint32_t count = (sum + stride / 2) / stride; + if (count < 1) { + count = 1; + } + if (sum == 0) { + // Don't make an all zeros stride to be upgraded to ones. + count = 0; + } + for (k = 0; k < stride; ++k) { + // We don't want to change value at counts[i], + // that is already belonging to the next stride. Thus - 1. + counts[i - k - 1] = count; + } + } + stride = 0; + sum = 0; + if (i < length - 3) { + // All interesting strides have a count of at least 4, + // at least when non-zeros. + limit = (counts[i] + counts[i + 1] + + counts[i + 2] + counts[i + 3] + 2) / 4; + } else if (i < length) { + limit = counts[i]; + } else { + limit = 0; + } + } + ++stride; + if (i != length) { + sum += counts[i]; + if (stride >= 4) { + limit = (sum + stride / 2) / stride; + } + } + } + } +} + +// A comparer function for two Huffman trees: sorts first by 'total count' +// (more comes first), and then by 'value' (more comes first). +static int CompareHuffmanTrees(const void* ptr1, const void* ptr2) { + const HuffmanTree* const t1 = (const HuffmanTree*)ptr1; + const HuffmanTree* const t2 = (const HuffmanTree*)ptr2; + if (t1->total_count_ > t2->total_count_) { + return -1; + } else if (t1->total_count_ < t2->total_count_) { + return 1; + } else { + assert(t1->value_ != t2->value_); + return (t1->value_ < t2->value_) ? -1 : 1; + } +} + +static void SetBitDepths(const HuffmanTree* const tree, + const HuffmanTree* const pool, + uint8_t* const bit_depths, int level) { + if (tree->pool_index_left_ >= 0) { + SetBitDepths(&pool[tree->pool_index_left_], pool, bit_depths, level + 1); + SetBitDepths(&pool[tree->pool_index_right_], pool, bit_depths, level + 1); + } else { + bit_depths[tree->value_] = level; + } +} + +// Create an optimal Huffman tree. +// +// (data,length): population counts. +// tree_limit: maximum bit depth (inclusive) of the codes. +// bit_depths[]: how many bits are used for the symbol. +// +// Returns 0 when an error has occurred. +// +// The catch here is that the tree cannot be arbitrarily deep +// +// count_limit is the value that is to be faked as the minimum value +// and this minimum value is raised until the tree matches the +// maximum length requirement. +// +// This algorithm is not of excellent performance for very long data blocks, +// especially when population counts are longer than 2**tree_limit, but +// we are not planning to use this with extremely long blocks. +// +// See https://en.wikipedia.org/wiki/Huffman_coding +static void GenerateOptimalTree(const uint32_t* const histogram, + int histogram_size, + HuffmanTree* tree, int tree_depth_limit, + uint8_t* const bit_depths) { + uint32_t count_min; + HuffmanTree* tree_pool; + int tree_size_orig = 0; + int i; + + for (i = 0; i < histogram_size; ++i) { + if (histogram[i] != 0) { + ++tree_size_orig; + } + } + + if (tree_size_orig == 0) { // pretty optimal already! + return; + } + + tree_pool = tree + tree_size_orig; + + // For block sizes with less than 64k symbols we never need to do a + // second iteration of this loop. + // If we actually start running inside this loop a lot, we would perhaps + // be better off with the Katajainen algorithm. + assert(tree_size_orig <= (1 << (tree_depth_limit - 1))); + for (count_min = 1; ; count_min *= 2) { + int tree_size = tree_size_orig; + // We need to pack the Huffman tree in tree_depth_limit bits. + // So, we try by faking histogram entries to be at least 'count_min'. + int idx = 0; + int j; + for (j = 0; j < histogram_size; ++j) { + if (histogram[j] != 0) { + const uint32_t count = + (histogram[j] < count_min) ? count_min : histogram[j]; + tree[idx].total_count_ = count; + tree[idx].value_ = j; + tree[idx].pool_index_left_ = -1; + tree[idx].pool_index_right_ = -1; + ++idx; + } + } + + // Build the Huffman tree. + qsort(tree, tree_size, sizeof(*tree), CompareHuffmanTrees); + + if (tree_size > 1) { // Normal case. + int tree_pool_size = 0; + while (tree_size > 1) { // Finish when we have only one root. + uint32_t count; + tree_pool[tree_pool_size++] = tree[tree_size - 1]; + tree_pool[tree_pool_size++] = tree[tree_size - 2]; + count = tree_pool[tree_pool_size - 1].total_count_ + + tree_pool[tree_pool_size - 2].total_count_; + tree_size -= 2; + { + // Search for the insertion point. + int k; + for (k = 0; k < tree_size; ++k) { + if (tree[k].total_count_ <= count) { + break; + } + } + memmove(tree + (k + 1), tree + k, (tree_size - k) * sizeof(*tree)); + tree[k].total_count_ = count; + tree[k].value_ = -1; + + tree[k].pool_index_left_ = tree_pool_size - 1; + tree[k].pool_index_right_ = tree_pool_size - 2; + tree_size = tree_size + 1; + } + } + SetBitDepths(&tree[0], tree_pool, bit_depths, 0); + } else if (tree_size == 1) { // Trivial case: only one element. + bit_depths[tree[0].value_] = 1; + } + + { + // Test if this Huffman tree satisfies our 'tree_depth_limit' criteria. + int max_depth = bit_depths[0]; + for (j = 1; j < histogram_size; ++j) { + if (max_depth < bit_depths[j]) { + max_depth = bit_depths[j]; + } + } + if (max_depth <= tree_depth_limit) { + break; + } + } + } +} + +// ----------------------------------------------------------------------------- +// Coding of the Huffman tree values + +static HuffmanTreeToken* CodeRepeatedValues(int repetitions, + HuffmanTreeToken* tokens, + int value, int prev_value) { + assert(value <= MAX_ALLOWED_CODE_LENGTH); + if (value != prev_value) { + tokens->code = value; + tokens->extra_bits = 0; + ++tokens; + --repetitions; + } + while (repetitions >= 1) { + if (repetitions < 3) { + int i; + for (i = 0; i < repetitions; ++i) { + tokens->code = value; + tokens->extra_bits = 0; + ++tokens; + } + break; + } else if (repetitions < 7) { + tokens->code = 16; + tokens->extra_bits = repetitions - 3; + ++tokens; + break; + } else { + tokens->code = 16; + tokens->extra_bits = 3; + ++tokens; + repetitions -= 6; + } + } + return tokens; +} + +static HuffmanTreeToken* CodeRepeatedZeros(int repetitions, + HuffmanTreeToken* tokens) { + while (repetitions >= 1) { + if (repetitions < 3) { + int i; + for (i = 0; i < repetitions; ++i) { + tokens->code = 0; // 0-value + tokens->extra_bits = 0; + ++tokens; + } + break; + } else if (repetitions < 11) { + tokens->code = 17; + tokens->extra_bits = repetitions - 3; + ++tokens; + break; + } else if (repetitions < 139) { + tokens->code = 18; + tokens->extra_bits = repetitions - 11; + ++tokens; + break; + } else { + tokens->code = 18; + tokens->extra_bits = 0x7f; // 138 repeated 0s + ++tokens; + repetitions -= 138; + } + } + return tokens; +} + +int VP8LCreateCompressedHuffmanTree(const HuffmanTreeCode* const tree, + HuffmanTreeToken* tokens, int max_tokens) { + HuffmanTreeToken* const starting_token = tokens; + HuffmanTreeToken* const ending_token = tokens + max_tokens; + const int depth_size = tree->num_symbols; + int prev_value = 8; // 8 is the initial value for rle. + int i = 0; + assert(tokens != NULL); + while (i < depth_size) { + const int value = tree->code_lengths[i]; + int k = i + 1; + int runs; + while (k < depth_size && tree->code_lengths[k] == value) ++k; + runs = k - i; + if (value == 0) { + tokens = CodeRepeatedZeros(runs, tokens); + } else { + tokens = CodeRepeatedValues(runs, tokens, value, prev_value); + prev_value = value; + } + i += runs; + assert(tokens <= ending_token); + } + (void)ending_token; // suppress 'unused variable' warning + return (int)(tokens - starting_token); +} + +// ----------------------------------------------------------------------------- + +// Pre-reversed 4-bit values. +static const uint8_t kReversedBits[16] = { + 0x0, 0x8, 0x4, 0xc, 0x2, 0xa, 0x6, 0xe, + 0x1, 0x9, 0x5, 0xd, 0x3, 0xb, 0x7, 0xf +}; + +static uint32_t ReverseBits(int num_bits, uint32_t bits) { + uint32_t retval = 0; + int i = 0; + while (i < num_bits) { + i += 4; + retval |= kReversedBits[bits & 0xf] << (MAX_ALLOWED_CODE_LENGTH + 1 - i); + bits >>= 4; + } + retval >>= (MAX_ALLOWED_CODE_LENGTH + 1 - num_bits); + return retval; +} + +// Get the actual bit values for a tree of bit depths. +static void ConvertBitDepthsToSymbols(HuffmanTreeCode* const tree) { + // 0 bit-depth means that the symbol does not exist. + int i; + int len; + uint32_t next_code[MAX_ALLOWED_CODE_LENGTH + 1]; + int depth_count[MAX_ALLOWED_CODE_LENGTH + 1] = { 0 }; + + assert(tree != NULL); + len = tree->num_symbols; + for (i = 0; i < len; ++i) { + const int code_length = tree->code_lengths[i]; + assert(code_length <= MAX_ALLOWED_CODE_LENGTH); + ++depth_count[code_length]; + } + depth_count[0] = 0; // ignore unused symbol + next_code[0] = 0; + { + uint32_t code = 0; + for (i = 1; i <= MAX_ALLOWED_CODE_LENGTH; ++i) { + code = (code + depth_count[i - 1]) << 1; + next_code[i] = code; + } + } + for (i = 0; i < len; ++i) { + const int code_length = tree->code_lengths[i]; + tree->codes[i] = ReverseBits(code_length, next_code[code_length]++); + } +} + +// ----------------------------------------------------------------------------- +// Main entry point + +void VP8LCreateHuffmanTree(uint32_t* const histogram, int tree_depth_limit, + uint8_t* const buf_rle, HuffmanTree* const huff_tree, + HuffmanTreeCode* const huff_code) { + const int num_symbols = huff_code->num_symbols; + memset(buf_rle, 0, num_symbols * sizeof(*buf_rle)); + OptimizeHuffmanForRle(num_symbols, buf_rle, histogram); + GenerateOptimalTree(histogram, num_symbols, huff_tree, tree_depth_limit, + huff_code->code_lengths); + // Create the actual bit codes for the bit lengths. + ConvertBitDepthsToSymbols(huff_code); +} |