|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
|
using System;using System.IO;namespace Gif.Components{ public class LZWEncoder { private static readonly int EOF = -1; private int imgW, imgH; private byte[] pixAry; private int initCodeSize; private int remaining; private int curPixel; // GIFCOMPR.C - GIF Image compression routines // // Lempel-Ziv compression based on 'compress'. GIF modifications by // David Rowley (mgardi@watdcsu.waterloo.edu) // General DEFINEs static readonly int BITS = 12; static readonly int HSIZE = 5003; // 80% occupancy // GIF Image compression - modified 'compress' // // Based on: compress.c - File compression ala IEEE Computer, June 1984. // // By Authors: Spencer W. Thomas (decvax!harpo!utah-cs!utah-gr!thomas) // Jim McKie (decvax!mcvax!jim) // Steve Davies (decvax!vax135!petsd!peora!srd) // Ken Turkowski (decvax!decwrl!turtlevax!ken) // James A. Woods (decvax!ihnp4!ames!jaw) // Joe Orost (decvax!vax135!petsd!joe) int n_bits; // number of bits/code int maxbits = BITS; // user settable max # bits/code int maxcode; // maximum code, given n_bits int maxmaxcode = 1 << BITS; // should NEVER generate this code int[] htab = new int[HSIZE];//这个是放hash的筒子,在这里面可以很快的找到1个key int[] codetab = new int[HSIZE]; int hsize = HSIZE; // for dynamic table sizing int free_ent = 0; // first unused entry // block compression parameters -- after all codes are used up, // and compression rate changes, start over. bool clear_flg = false; // Algorithm: use open addressing double hashing (no chaining) on the // prefix code / next character combination. We do a variant of Knuth's // algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime // secondary probe. Here, the modular division first probe is gives way // to a faster exclusive-or manipulation. Also do block compression with // an adaptive reset, whereby the code table is cleared when the compression // ratio decreases, but after the table fills. The variable-length output // codes are re-sized at this point, and a special CLEAR code is generated // for the decompressor. Late addition: construct the table according to // file size for noticeable speed improvement on small files. Please direct // questions about this implementation to ames!jaw. int g_init_bits; int ClearCode; int EOFCode; // output // // Output the given code. // Inputs: // code: A n_bits-bit integer. If == -1, then EOF. This assumes // that n_bits =< wordsize - 1. // Outputs: // Outputs code to the file. // Assumptions: // Chars are 8 bits long. // Algorithm: // Maintain a BITS character long buffer (so that 8 codes will // fit in it exactly). Use the VAX insv instruction to insert each // code in turn. When the buffer fills up empty it and start over. int cur_accum = 0; int cur_bits = 0; int [] masks = { 0x0000, 0x0001, 0x0003, 0x0007, 0x000F, 0x001F, 0x003F, 0x007F, 0x00FF, 0x01FF, 0x03FF, 0x07FF, 0x0FFF, 0x1FFF, 0x3FFF, 0x7FFF, 0xFFFF }; // Number of characters so far in this 'packet' int a_count; // Define the storage for the packet accumulator byte[] accum = new byte[256]; //---------------------------------------------------------------------------- public LZWEncoder(int width, int height, byte[] pixels, int color_depth) { imgW = width; imgH = height; pixAry = pixels; initCodeSize = Math.Max(2, color_depth); } // Add a character to the end of the current packet, and if it is 254 // characters, flush the packet to disk. void Add(byte c, Stream outs) { accum[a_count++] = c; if (a_count >= 254) Flush(outs); } // Clear out the hash table // table clear for block compress void ClearTable(Stream outs) { ResetCodeTable(hsize); free_ent = ClearCode + 2; clear_flg = true; Output(ClearCode, outs); } // reset code table // 全部初始化为-1 void ResetCodeTable(int hsize) { for (int i = 0; i < hsize; ++i) htab[i] = -1; } void Compress(int init_bits, Stream outs) { int fcode; int i /* = 0 */; int c; int ent; int disp; int hsize_reg; int hshift; // Set up the globals: g_init_bits - initial number of bits //原始数据的字长,在gif文件中，原始数据的字长可以为1(单色图),4(16色)，和8(256色) //开始的时候先加上1 //但是当原始数据长度为1的时候，开始为3 //因此原始长度1->3,4->5,8->9 //?为何原始数据字长为1的时候，开始长度为3呢？? //如果+1=2，只能表示四种状态，加上clearcode和endcode就用完了。所以必须扩展到3 g_init_bits = init_bits; // Set up the necessary values //是否需要加清除标志 //GIF为了提高压缩率，采用的是变长的字长(VCL)。比如说原始数据是8位，那么开始先加上1位(8+1=9) //当标号到2^9=512的时候，超过了当前长度9所能表现的最大值，此时后面的标号就必须用10位来表示 //以此类推，当标号到2^12的时候，因为最大为12,不能继续扩展了，需要在2^12=4096的位置上插入一个ClearCode,表示从这往后，从9位重新再来了 clear_flg = false; n_bits = g_init_bits; //获得n位数能表述的最大值(gif图像中开始一般为3,5,9，故maxcode一般为7,31,511) maxcode = MaxCode(n_bits); //表示从这里我重新开始构造字典字典了，以前的所有标记作废， //开始使用新的标记。这个标号集的大小多少比较合适呢？据说理论上是越大压缩率越高（我个人感觉太大了也不见得就好）， //不过处理的开销也呈指数增长 //gif规定，clearcode的值为原始数据最大字长所能表达的数值+1;比如原始数据长度为8,则clearcode=1<<(9-1)=256 ClearCode = 1 << (init_bits - 1); //结束标志为clearcode+1 EOFCode = ClearCode + 1; //这个是解除结束的 free_ent = ClearCode + 2; //清楚数量 a_count = 0; // clear packet //从图像中获得下一个像素 ent = NextPixel(); hshift = 0; for (fcode = hsize; fcode < 65536; fcode *= 2) ++hshift; //设置hash码范围 hshift = 8 - hshift; // set hash code range bound hsize_reg = hsize; //清除固定大小的hash表，用于存储标记，这个相当于字典 ResetCodeTable(hsize_reg); // clear hash table Output(ClearCode, outs); outer_loop : while ((c = NextPixel()) != EOF) { fcode = (c << maxbits) + ent; i = (c << hshift) ^ ent; // xor hashing //嘿嘿,小样,又来了,我认识你 if (htab[i] == fcode) { ent = codetab[i]; continue; } //这小子,新来的 else if (htab[i] >= 0) // non-empty slot { disp = hsize_reg - i; // secondary hash (after G. Knott) if (i == 0) disp = 1; do { if ((i -= disp) < 0) i += hsize_reg; if (htab[i] == fcode) { ent = codetab[i]; goto outer_loop; } } while (htab[i] >= 0); } Output(ent, outs); //从这里可以看出,ent就是前缀（prefix）,而当前正在处理的字符标志就是后缀（suffix） ent = c; //判断终止结束符是否超过当前位数所能表述的范围 if (free_ent < maxmaxcode) { //如果没有超 codetab[i] = free_ent++; // code -> hashtable //hash表里面建立相应索引 htab[i] = fcode; } else //说明超过了当前所能表述的范围,清空字典,重新再来 ClearTable(outs); } // Put out the final code. Output(ent, outs); Output(EOFCode, outs); } //---------------------------------------------------------------------------- public void Encode( Stream os) { os.WriteByte( Convert.ToByte( initCodeSize) ); // write "initial code size" byte //这个图像包含多少个像素 remaining = imgW * imgH; // reset navigation variables //当前处理的像素索引 curPixel = 0; Compress(initCodeSize + 1, os); // compress and write the pixel data os.WriteByte(0); // write block terminator } // Flush the packet to disk, and reset the accumulator void Flush(Stream outs) { if (a_count > 0) { outs.WriteByte( Convert.ToByte( a_count )); outs.Write(accum, 0, a_count); a_count = 0; } } /// <summary> /// 获得n位数所能表达的最大数值 /// </summary> /// <param name="n_bits">位数，一般情况下n_bits = 9</param> /// <returns>最大值,例如n_bits=8,则返回值就为2^8-1=255</returns> int MaxCode(int n_bits) { return (1 << n_bits) - 1; } //---------------------------------------------------------------------------- // Return the next pixel from the image //---------------------------------------------------------------------------- /// <summary> /// 从图像中获得下一个像素 /// </summary> /// <returns></returns> private int NextPixel() { //还剩多少个像素没有处理 //如果没有了,返回结束标志 if (remaining == 0) return EOF; //否则处理下一个,并将未处理像素数目-1 --remaining; //当前处理的像素 int temp = curPixel + 1; //如果当前处理像素在像素范围之内 if ( temp < pixAry.GetUpperBound( 0 )) { //下一个像素 byte pix = pixAry[curPixel++]; return pix & 0xff; } return 0xff; } /// <summary> /// 输出字到输出流 /// </summary> /// <param name="code">要输出的字</param> /// <param name="outs">输出流</param> void Output(int code, Stream outs) { //得到当前标志位所能表示的最大标志值 cur_accum &= masks[cur_bits]; if (cur_bits > 0) cur_accum |= (code << cur_bits); else //如果标志位为0,就将当前标号为输入流 cur_accum = code; //当前能标志的最大字长度(9-10-11-12-9-10。。。。。。。) cur_bits += n_bits; //如果当前最大长度大于8 while (cur_bits >= 8) { //向流中输出一个字节 Add((byte) (cur_accum & 0xff), outs); //将当前标号右移8位 cur_accum >>= 8; cur_bits -= 8; } // If the next entry is going to be too big for the code size, // then increase it, if possible. if (free_ent > maxcode || clear_flg) { if (clear_flg) { maxcode = MaxCode(n_bits = g_init_bits); clear_flg = false; } else { ++n_bits; if (n_bits == maxbits) maxcode = maxmaxcode; else maxcode = MaxCode(n_bits); } } if (code == EOFCode) { // At EOF, write the rest of the buffer. while (cur_bits > 0) { Add((byte) (cur_accum & 0xff), outs); cur_accum >>= 8; cur_bits -= 8; } Flush(outs); } } }} |
以上就是本文的全部内容，希望能给大家一个参考，也希望大家多多支持服务器之家。
