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