FFmpeg
 All Data Structures Namespaces Files Functions Variables Typedefs Enumerations Enumerator Macros Groups Pages
mjpegenc_huffman.c
Go to the documentation of this file.
1 /*
2  * MJPEG encoder
3  * Copyright (c) 2016 William Ma, Ted Ying, Jerry Jiang
4  *
5  * This file is part of FFmpeg.
6  *
7  * FFmpeg is free software; you can redistribute it and/or
8  * modify it under the terms of the GNU Lesser General Public
9  * License as published by the Free Software Foundation; either
10  * version 2.1 of the License, or (at your option) any later version.
11  *
12  * FFmpeg is distributed in the hope that it will be useful,
13  * but WITHOUT ANY WARRANTY; without even the implied warranty of
14  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15  * Lesser General Public License for more details.
16  *
17  * You should have received a copy of the GNU Lesser General Public
18  * License along with FFmpeg; if not, write to the Free Software
19  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
20  */
21 
22 #include <string.h>
23 #include <stdint.h>
24 #include <stdlib.h>
25 #include "libavutil/avassert.h"
26 #include "libavutil/common.h"
27 #include "libavutil/error.h"
28 #include "libavutil/qsort.h"
29 #include "mjpegenc_huffman.h"
30 
31 /**
32  * Comparison function for two PTables by prob
33  *
34  * @param a First PTable to compare
35  * @param b Second PTable to compare
36  * @return < 0 for less than, 0 for equals, > 0 for greater than
37  */
38 static int compare_by_prob(const void *a, const void *b)
39 {
40  PTable a_val = *(PTable *) a;
41  PTable b_val = *(PTable *) b;
42  return a_val.prob - b_val.prob;
43 }
44 
45 /**
46  * Comparison function for two HuffTables by length
47  *
48  * @param a First HuffTable to compare
49  * @param b Second HuffTable to compare
50  * @return < 0 for less than, 0 for equals, > 0 for greater than
51  */
52 static int compare_by_length(const void *a, const void *b)
53 {
54  HuffTable a_val = *(HuffTable *) a;
55  HuffTable b_val = *(HuffTable *) b;
56  return a_val.length - b_val.length;
57 }
58 
59 /**
60  * Computes the length of the Huffman encoding for each distinct input value.
61  * Uses package merge algorithm as follows:
62  * 1. start with an empty list, lets call it list(0), set i = 0
63  * 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
64  * 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
65  * 4. if there is more than 1 symbol left in the current list(i), then goto 3
66  * 5. i++
67  * 6. if i < 16 goto 2
68  * 7. select the n-1 elements in the last list with the lowest score (n = the number of symbols)
69  * 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
70  * Go to guru.multimedia.cx/small-tasks-for-ffmpeg/ for more details
71  *
72  * All probabilities should be positive integers. The output is sorted by code,
73  * not by length.
74  *
75  * @param prob_table input array of a PTable for each distinct input value
76  * @param distincts output array of a HuffTable that will be populated by this function
77  * @param size size of the prob_table array
78  * @param max_length max length of an encoding
79  */
80 void ff_mjpegenc_huffman_compute_bits(PTable *prob_table, HuffTable *distincts, int size, int max_length)
81 {
82  PackageMergerList list_a, list_b, *to = &list_a, *from = &list_b, *temp;
83 
84  int times, i, j, k;
85 
86  int nbits[257] = {0};
87 
88  int min;
89 
90  av_assert0(max_length > 0);
91 
92  to->nitems = 0;
93  from->nitems = 0;
94  to->item_idx[0] = 0;
95  from->item_idx[0] = 0;
96  AV_QSORT(prob_table, size, PTable, compare_by_prob);
97 
98  for (times = 0; times <= max_length; times++) {
99  to->nitems = 0;
100  to->item_idx[0] = 0;
101 
102  j = 0;
103  k = 0;
104 
105  if (times < max_length) {
106  i = 0;
107  }
108  while (i < size || j + 1 < from->nitems) {
109  to->nitems++;
110  to->item_idx[to->nitems] = to->item_idx[to->nitems - 1];
111  if (i < size &&
112  (j + 1 >= from->nitems ||
113  prob_table[i].prob <
114  from->probability[j] + from->probability[j + 1])) {
115  to->items[to->item_idx[to->nitems]++] = prob_table[i].value;
116  to->probability[to->nitems - 1] = prob_table[i].prob;
117  i++;
118  } else {
119  for (k = from->item_idx[j]; k < from->item_idx[j + 2]; k++) {
120  to->items[to->item_idx[to->nitems]++] = from->items[k];
121  }
122  to->probability[to->nitems - 1] =
123  from->probability[j] + from->probability[j + 1];
124  j += 2;
125  }
126  }
127  temp = to;
128  to = from;
129  from = temp;
130  }
131 
132  min = (size - 1 < from->nitems) ? size - 1 : from->nitems;
133  for (i = 0; i < from->item_idx[min]; i++) {
134  nbits[from->items[i]]++;
135  }
136  // we don't want to return the 256 bit count (it was just in here to prevent
137  // all 1s encoding)
138  j = 0;
139  for (i = 0; i < 256; i++) {
140  if (nbits[i] > 0) {
141  distincts[j].code = i;
142  distincts[j].length = nbits[i];
143  j++;
144  }
145  }
146 }
147 
149 {
150  memset(s->val_count, 0, sizeof(s->val_count));
151 }
152 
153 /**
154  * Produces a Huffman encoding with a given input
155  *
156  * @param s input to encode
157  * @param bits output array where the ith character represents how many input values have i length encoding
158  * @param val output array of input values sorted by their encoded length
159  * @param max_nval maximum number of distinct input values
160  */
162  uint8_t val[], int max_nval)
163 {
164  int i, j;
165  int nval = 0;
166  PTable val_counts[257];
167  HuffTable distincts[256];
168 
169  for (i = 0; i < 256; i++) {
170  if (s->val_count[i]) nval++;
171  }
172  av_assert0 (nval <= max_nval);
173 
174  j = 0;
175  for (i = 0; i < 256; i++) {
176  if (s->val_count[i]) {
177  val_counts[j].value = i;
178  val_counts[j].prob = s->val_count[i];
179  j++;
180  }
181  }
182  val_counts[j].value = 256;
183  val_counts[j].prob = 0;
184  ff_mjpegenc_huffman_compute_bits(val_counts, distincts, nval + 1, 16);
185  AV_QSORT(distincts, nval, HuffTable, compare_by_length);
186 
187  memset(bits, 0, sizeof(bits[0]) * 17);
188  for (i = 0; i < nval; i++) {
189  val[i] = distincts[i].code;
190  bits[distincts[i].length]++;
191  }
192 }
const char const char void * val
Definition: avisynth_c.h:771
else temp
Definition: vf_mcdeint.c:256
const char * b
Definition: vf_curves.c:113
int value
input value
Definition: magicyuvenc.c:49
#define av_assert0(cond)
assert() equivalent, that is always enabled.
Definition: avassert.h:37
uint8_t
const char * from
Definition: jacosubdec.c:65
void ff_mjpeg_encode_huffman_close(MJpegEncHuffmanContext *s, uint8_t bits[17], uint8_t val[], int max_nval)
Produces a Huffman encoding with a given input.
ptrdiff_t size
Definition: opengl_enc.c:101
int nitems
number of items in the list and probability ex. 4
Definition: magicyuvenc.c:290
const char * to
Definition: webvttdec.c:34
error code definitions
simple assert() macros that are a bit more flexible than ISO C assert().
int64_t prob
number of occurences of this value in input
Definition: magicyuvenc.c:50
int length
length of the encoding
int item_idx[515]
index range for each item in items 0, 2, 5, 9, 13
Definition: magicyuvenc.c:291
#define s(width, name)
Definition: cbs_vp9.c:257
int code
code is the input value
Used to store intermediate lists in the package merge algorithm.
Definition: magicyuvenc.c:289
void ff_mjpegenc_huffman_compute_bits(PTable *prob_table, HuffTable *distincts, int size, int max_length)
Computes the length of the Huffman encoding for each distinct input value.
Used to store optimal huffman encoding results.
Used to assign a occurrence count or "probability" to an input value.
Definition: magicyuvenc.c:48
Huffman table generation for MJPEG encoder.
int probability[514]
probability of each item 3, 8, 18, 46
Definition: magicyuvenc.c:292
static int compare_by_prob(const void *a, const void *b)
Comparison function for two PTables by prob.
common internal and external API header
static int compare_by_length(const void *a, const void *b)
Comparison function for two HuffTables by length.
int items[257 *16]
chain of all individual values that make up items A, B, A, B, C, A, B, C, D, C, D, D, E
Definition: magicyuvenc.c:293
void ff_mjpeg_encode_huffman_init(MJpegEncHuffmanContext *s)
float min
#define AV_QSORT(p, num, type, cmp)
Quicksort This sort is fast, and fully inplace but not stable and it is possible to construct input t...
Definition: qsort.h:33
for(j=16;j >0;--j)