/* * MJPEG encoder * Copyright (c) 2016 William Ma, Ted Ying, Jerry Jiang * * This file is part of FFmpeg. * * FFmpeg is free software; you can redistribute it and/or * modify it under the terms of the GNU Lesser General Public * License as published by the Free Software Foundation; either * version 2.1 of the License, or (at your option) any later version. * * FFmpeg is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * Lesser General Public License for more details. * * You should have received a copy of the GNU Lesser General Public * License along with FFmpeg; if not, write to the Free Software * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA */ #include <string.h> #include <stdint.h> #include <stdlib.h> #include "libavutil/avassert.h" #include "libavutil/common.h" #include "libavutil/error.h" #include "libavutil/qsort.h" #include "mjpegenc_huffman.h" /** * Comparison function for two PTables by prob * * @param a First PTable to compare * @param b Second PTable to compare * @return < 0 for less than, 0 for equals, > 0 for greater than */ static int compare_by_prob(const void *a, const void *b) { PTable a_val = *(PTable *) a; PTable b_val = *(PTable *) b; return a_val.prob - b_val.prob; } /** * Comparison function for two HuffTables by length * * @param a First HuffTable to compare * @param b Second HuffTable to compare * @return < 0 for less than, 0 for equals, > 0 for greater than */ static int compare_by_length(const void *a, const void *b) { HuffTable a_val = *(HuffTable *) a; HuffTable b_val = *(HuffTable *) b; return a_val.length - b_val.length; } /** * Computes the length of the Huffman encoding for each distinct input value. * Uses package merge algorithm as follows: * 1. start with an empty list, lets call it list(0), set i = 0 * 2. add 1 entry to list(i) for each symbol we have and give each a score equal to the probability of the respective symbol * 3. merge the 2 symbols of least score and put them in list(i+1), and remove them from list(i). The new score will be the sum of the 2 scores * 4. if there is more than 1 symbol left in the current list(i), then goto 3 * 5. i++ * 6. if i < 16 goto 2 * 7. select the n-1 elements in the last list with the lowest score (n = the number of symbols) * 8. the length of the huffman code for symbol s will be equal to the number of times the symbol occurs in the select elements * Go to guru.multimedia.cx/small-tasks-for-ffmpeg/ for more details * * All probabilities should be positive integers. The output is sorted by code, * not by length. * * @param prob_table input array of a PTable for each distinct input value * @param distincts output array of a HuffTable that will be populated by this function * @param size size of the prob_table array * @param max_length max length of an encoding */ void ff_mjpegenc_huffman_compute_bits(PTable *prob_table, HuffTable *distincts, int size, int max_length) { PackageMergerList list_a, list_b, *to = &list_a, *from = &list_b, *temp; int times, i, j, k; int nbits[257] = {0}; int min; av_assert0(max_length > 0); to->nitems = 0; from->nitems = 0; to->item_idx[0] = 0; from->item_idx[0] = 0; AV_QSORT(prob_table, size, PTable, compare_by_prob); for (times = 0; times <= max_length; times++) { to->nitems = 0; to->item_idx[0] = 0; j = 0; k = 0; if (times < max_length) { i = 0; } while (i < size || j + 1 < from->nitems) { to->nitems++; to->item_idx[to->nitems] = to->item_idx[to->nitems - 1]; if (i < size && (j + 1 >= from->nitems || prob_table[i].prob < from->probability[j] + from->probability[j + 1])) { to->items[to->item_idx[to->nitems]++] = prob_table[i].value; to->probability[to->nitems - 1] = prob_table[i].prob; i++; } else { for (k = from->item_idx[j]; k < from->item_idx[j + 2]; k++) { to->items[to->item_idx[to->nitems]++] = from->items[k]; } to->probability[to->nitems - 1] = from->probability[j] + from->probability[j + 1]; j += 2; } } temp = to; to = from; from = temp; } min = (size - 1 < from->nitems) ? size - 1 : from->nitems; for (i = 0; i < from->item_idx[min]; i++) { nbits[from->items[i]]++; } // we don't want to return the 256 bit count (it was just in here to prevent // all 1s encoding) j = 0; for (i = 0; i < 256; i++) { if (nbits[i] > 0) { distincts[j].code = i; distincts[j].length = nbits[i]; j++; } } } void ff_mjpeg_encode_huffman_init(MJpegEncHuffmanContext *s) { memset(s->val_count, 0, sizeof(s->val_count)); } /** * Produces a Huffman encoding with a given input * * @param s input to encode * @param bits output array where the ith character represents how many input values have i length encoding * @param val output array of input values sorted by their encoded length * @param max_nval maximum number of distinct input values */ void ff_mjpeg_encode_huffman_close(MJpegEncHuffmanContext *s, uint8_t bits[17], uint8_t val[], int max_nval) { int i, j; int nval = 0; PTable val_counts[257]; HuffTable distincts[256]; for (i = 0; i < 256; i++) { if (s->val_count[i]) nval++; } av_assert0 (nval <= max_nval); j = 0; for (i = 0; i < 256; i++) { if (s->val_count[i]) { val_counts[j].value = i; val_counts[j].prob = s->val_count[i]; j++; } } val_counts[j].value = 256; val_counts[j].prob = 0; ff_mjpegenc_huffman_compute_bits(val_counts, distincts, nval + 1, 16); AV_QSORT(distincts, nval, HuffTable, compare_by_length); memset(bits, 0, sizeof(bits[0]) * 17); for (i = 0; i < nval; i++) { val[i] = distincts[i].code; bits[distincts[i].length]++; } }