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jidctfst.c
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1 /*
2  * jidctfst.c
3  *
4  * Copyright (C) 1994-1998, Thomas G. Lane.
5  * Modified 2015-2017 by Guido Vollbeding.
6  * This file is part of the Independent JPEG Group's software.
7  * For conditions of distribution and use, see the accompanying README file.
8  *
9  * This file contains a fast, not so accurate integer implementation of the
10  * inverse DCT (Discrete Cosine Transform). In the IJG code, this routine
11  * must also perform dequantization of the input coefficients.
12  *
13  * A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT
14  * on each row (or vice versa, but it's more convenient to emit a row at
15  * a time). Direct algorithms are also available, but they are much more
16  * complex and seem not to be any faster when reduced to code.
17  *
18  * This implementation is based on Arai, Agui, and Nakajima's algorithm for
19  * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
20  * Japanese, but the algorithm is described in the Pennebaker & Mitchell
21  * JPEG textbook (see REFERENCES section in file README). The following code
22  * is based directly on figure 4-8 in P&M.
23  * While an 8-point DCT cannot be done in less than 11 multiplies, it is
24  * possible to arrange the computation so that many of the multiplies are
25  * simple scalings of the final outputs. These multiplies can then be
26  * folded into the multiplications or divisions by the JPEG quantization
27  * table entries. The AA&N method leaves only 5 multiplies and 29 adds
28  * to be done in the DCT itself.
29  * The primary disadvantage of this method is that with fixed-point math,
30  * accuracy is lost due to imprecise representation of the scaled
31  * quantization values. The smaller the quantization table entry, the less
32  * precise the scaled value, so this implementation does worse with high-
33  * quality-setting files than with low-quality ones.
34  */
35 
36 #define JPEG_INTERNALS
37 #include "jinclude.h"
38 #include "jpeglib.h"
39 #include "jdct.h" /* Private declarations for DCT subsystem */
40 
41 #ifdef DCT_IFAST_SUPPORTED
42 
43 
44 /*
45  * This module is specialized to the case DCTSIZE = 8.
46  */
47 
48 #if DCTSIZE != 8
49  Sorry, this code only copes with 8x8 DCT blocks. /* deliberate syntax err */
50 #endif
51 
52 
53 /* Scaling decisions are generally the same as in the LL&M algorithm;
54  * see jidctint.c for more details. However, we choose to descale
55  * (right shift) multiplication products as soon as they are formed,
56  * rather than carrying additional fractional bits into subsequent additions.
57  * This compromises accuracy slightly, but it lets us save a few shifts.
58  * More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
59  * everywhere except in the multiplications proper; this saves a good deal
60  * of work on 16-bit-int machines.
61  *
62  * The dequantized coefficients are not integers because the AA&N scaling
63  * factors have been incorporated. We represent them scaled up by PASS1_BITS,
64  * so that the first and second IDCT rounds have the same input scaling.
65  * For 8-bit JSAMPLEs, we choose IFAST_SCALE_BITS = PASS1_BITS so as to
66  * avoid a descaling shift; this compromises accuracy rather drastically
67  * for small quantization table entries, but it saves a lot of shifts.
68  * For 12-bit JSAMPLEs, there's no hope of using 16x16 multiplies anyway,
69  * so we use a much larger scaling factor to preserve accuracy.
70  *
71  * A final compromise is to represent the multiplicative constants to only
72  * 8 fractional bits, rather than 13. This saves some shifting work on some
73  * machines, and may also reduce the cost of multiplication (since there
74  * are fewer one-bits in the constants).
75  */
76 
77 #if BITS_IN_JSAMPLE == 8
78 #define CONST_BITS 8
79 #define PASS1_BITS 2
80 #else
81 #define CONST_BITS 8
82 #define PASS1_BITS 1 /* lose a little precision to avoid overflow */
83 #endif
84 
85 /* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
86  * causing a lot of useless floating-point operations at run time.
87  * To get around this we use the following pre-calculated constants.
88  * If you change CONST_BITS you may want to add appropriate values.
89  * (With a reasonable C compiler, you can just rely on the FIX() macro...)
90  */
91 
92 #if CONST_BITS == 8
93 #define FIX_1_082392200 ((INT32) 277) /* FIX(1.082392200) */
94 #define FIX_1_414213562 ((INT32) 362) /* FIX(1.414213562) */
95 #define FIX_1_847759065 ((INT32) 473) /* FIX(1.847759065) */
96 #define FIX_2_613125930 ((INT32) 669) /* FIX(2.613125930) */
97 #else
98 #define FIX_1_082392200 FIX(1.082392200)
99 #define FIX_1_414213562 FIX(1.414213562)
100 #define FIX_1_847759065 FIX(1.847759065)
101 #define FIX_2_613125930 FIX(2.613125930)
102 #endif
103 
104 
105 /* We can gain a little more speed, with a further compromise in accuracy,
106  * by omitting the addition in a descaling shift. This yields an incorrectly
107  * rounded result half the time...
108  */
109 
110 #ifndef USE_ACCURATE_ROUNDING
111 #undef DESCALE
112 #define DESCALE(x,n) RIGHT_SHIFT(x, n)
113 #endif
114 
115 
116 /* Multiply a DCTELEM variable by an INT32 constant, and immediately
117  * descale to yield a DCTELEM result.
118  */
119 
120 #define MULTIPLY(var,const) ((DCTELEM) DESCALE((var) * (const), CONST_BITS))
121 
122 
123 /* Dequantize a coefficient by multiplying it by the multiplier-table
124  * entry; produce a DCTELEM result. For 8-bit data a 16x16->16
125  * multiplication will do. For 12-bit data, the multiplier table is
126  * declared INT32, so a 32-bit multiply will be used.
127  */
128 
129 #if BITS_IN_JSAMPLE == 8
130 #define DEQUANTIZE(coef,quantval) (((IFAST_MULT_TYPE) (coef)) * (quantval))
131 #else
132 #define DEQUANTIZE(coef,quantval) \
133  DESCALE((coef)*(quantval), IFAST_SCALE_BITS-PASS1_BITS)
134 #endif
135 
136 
137 /*
138  * Perform dequantization and inverse DCT on one block of coefficients.
139  *
140  * cK represents cos(K*pi/16).
141  */
142 
143 GLOBAL(void)
144 jpeg_idct_ifast (j_decompress_ptr cinfo, jpeg_component_info * compptr,
147 {
148  DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
149  DCTELEM tmp10, tmp11, tmp12, tmp13;
150  DCTELEM z5, z10, z11, z12, z13;
151  JCOEFPTR inptr;
152  IFAST_MULT_TYPE * quantptr;
153  int * wsptr;
154  JSAMPROW outptr;
155  JSAMPLE *range_limit = IDCT_range_limit(cinfo);
156  int ctr;
157  int workspace[DCTSIZE2]; /* buffers data between passes */
158  SHIFT_TEMPS /* for DESCALE */
159  ISHIFT_TEMPS /* for IRIGHT_SHIFT */
160 
161  /* Pass 1: process columns from input, store into work array. */
162 
163  inptr = coef_block;
164  quantptr = (IFAST_MULT_TYPE *) compptr->dct_table;
165  wsptr = workspace;
166  for (ctr = DCTSIZE; ctr > 0; ctr--) {
167  /* Due to quantization, we will usually find that many of the input
168  * coefficients are zero, especially the AC terms. We can exploit this
169  * by short-circuiting the IDCT calculation for any column in which all
170  * the AC terms are zero. In that case each output is equal to the
171  * DC coefficient (with scale factor as needed).
172  * With typical images and quantization tables, half or more of the
173  * column DCT calculations can be simplified this way.
174  */
175 
176  if (inptr[DCTSIZE*1] == 0 && inptr[DCTSIZE*2] == 0 &&
177  inptr[DCTSIZE*3] == 0 && inptr[DCTSIZE*4] == 0 &&
178  inptr[DCTSIZE*5] == 0 && inptr[DCTSIZE*6] == 0 &&
179  inptr[DCTSIZE*7] == 0) {
180  /* AC terms all zero */
181  int dcval = (int) DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]);
182 
183  wsptr[DCTSIZE*0] = dcval;
184  wsptr[DCTSIZE*1] = dcval;
185  wsptr[DCTSIZE*2] = dcval;
186  wsptr[DCTSIZE*3] = dcval;
187  wsptr[DCTSIZE*4] = dcval;
188  wsptr[DCTSIZE*5] = dcval;
189  wsptr[DCTSIZE*6] = dcval;
190  wsptr[DCTSIZE*7] = dcval;
191 
192  inptr++; /* advance pointers to next column */
193  quantptr++;
194  wsptr++;
195  continue;
196  }
197 
198  /* Even part */
199 
200  tmp0 = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]);
201  tmp1 = DEQUANTIZE(inptr[DCTSIZE*2], quantptr[DCTSIZE*2]);
202  tmp2 = DEQUANTIZE(inptr[DCTSIZE*4], quantptr[DCTSIZE*4]);
203  tmp3 = DEQUANTIZE(inptr[DCTSIZE*6], quantptr[DCTSIZE*6]);
204 
205  tmp10 = tmp0 + tmp2; /* phase 3 */
206  tmp11 = tmp0 - tmp2;
207 
208  tmp13 = tmp1 + tmp3; /* phases 5-3 */
209  tmp12 = MULTIPLY(tmp1 - tmp3, FIX_1_414213562) - tmp13; /* 2*c4 */
210 
211  tmp0 = tmp10 + tmp13; /* phase 2 */
212  tmp3 = tmp10 - tmp13;
213  tmp1 = tmp11 + tmp12;
214  tmp2 = tmp11 - tmp12;
215 
216  /* Odd part */
217 
218  tmp4 = DEQUANTIZE(inptr[DCTSIZE*1], quantptr[DCTSIZE*1]);
219  tmp5 = DEQUANTIZE(inptr[DCTSIZE*3], quantptr[DCTSIZE*3]);
220  tmp6 = DEQUANTIZE(inptr[DCTSIZE*5], quantptr[DCTSIZE*5]);
221  tmp7 = DEQUANTIZE(inptr[DCTSIZE*7], quantptr[DCTSIZE*7]);
222 
223  z13 = tmp6 + tmp5; /* phase 6 */
224  z10 = tmp6 - tmp5;
225  z11 = tmp4 + tmp7;
226  z12 = tmp4 - tmp7;
227 
228  tmp7 = z11 + z13; /* phase 5 */
229  tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562); /* 2*c4 */
230 
231  z5 = MULTIPLY(z10 + z12, FIX_1_847759065); /* 2*c2 */
232  tmp10 = z5 - MULTIPLY(z12, FIX_1_082392200); /* 2*(c2-c6) */
233  tmp12 = z5 - MULTIPLY(z10, FIX_2_613125930); /* 2*(c2+c6) */
234 
235  tmp6 = tmp12 - tmp7; /* phase 2 */
236  tmp5 = tmp11 - tmp6;
237  tmp4 = tmp10 - tmp5;
238 
239  wsptr[DCTSIZE*0] = (int) (tmp0 + tmp7);
240  wsptr[DCTSIZE*7] = (int) (tmp0 - tmp7);
241  wsptr[DCTSIZE*1] = (int) (tmp1 + tmp6);
242  wsptr[DCTSIZE*6] = (int) (tmp1 - tmp6);
243  wsptr[DCTSIZE*2] = (int) (tmp2 + tmp5);
244  wsptr[DCTSIZE*5] = (int) (tmp2 - tmp5);
245  wsptr[DCTSIZE*3] = (int) (tmp3 + tmp4);
246  wsptr[DCTSIZE*4] = (int) (tmp3 - tmp4);
247 
248  inptr++; /* advance pointers to next column */
249  quantptr++;
250  wsptr++;
251  }
252 
253  /* Pass 2: process rows from work array, store into output array.
254  * Note that we must descale the results by a factor of 8 == 2**3,
255  * and also undo the PASS1_BITS scaling.
256  */
257 
258  wsptr = workspace;
259  for (ctr = 0; ctr < DCTSIZE; ctr++) {
260  outptr = output_buf[ctr] + output_col;
261 
262  /* Add range center and fudge factor for final descale and range-limit. */
263  z5 = (DCTELEM) wsptr[0] +
264  ((((DCTELEM) RANGE_CENTER) << (PASS1_BITS+3)) +
265  (1 << (PASS1_BITS+2)));
266 
267  /* Rows of zeroes can be exploited in the same way as we did with columns.
268  * However, the column calculation has created many nonzero AC terms, so
269  * the simplification applies less often (typically 5% to 10% of the time).
270  * On machines with very fast multiplication, it's possible that the
271  * test takes more time than it's worth. In that case this section
272  * may be commented out.
273  */
274 
275 #ifndef NO_ZERO_ROW_TEST
276  if (wsptr[1] == 0 && wsptr[2] == 0 && wsptr[3] == 0 && wsptr[4] == 0 &&
277  wsptr[5] == 0 && wsptr[6] == 0 && wsptr[7] == 0) {
278  /* AC terms all zero */
279  JSAMPLE dcval = range_limit[(int) IRIGHT_SHIFT(z5, PASS1_BITS+3)
280  & RANGE_MASK];
281 
282  outptr[0] = dcval;
283  outptr[1] = dcval;
284  outptr[2] = dcval;
285  outptr[3] = dcval;
286  outptr[4] = dcval;
287  outptr[5] = dcval;
288  outptr[6] = dcval;
289  outptr[7] = dcval;
290 
291  wsptr += DCTSIZE; /* advance pointer to next row */
292  continue;
293  }
294 #endif
295 
296  /* Even part */
297 
298  tmp10 = z5 + (DCTELEM) wsptr[4];
299  tmp11 = z5 - (DCTELEM) wsptr[4];
300 
301  tmp13 = (DCTELEM) wsptr[2] + (DCTELEM) wsptr[6];
302  tmp12 = MULTIPLY((DCTELEM) wsptr[2] - (DCTELEM) wsptr[6],
303  FIX_1_414213562) - tmp13; /* 2*c4 */
304 
305  tmp0 = tmp10 + tmp13;
306  tmp3 = tmp10 - tmp13;
307  tmp1 = tmp11 + tmp12;
308  tmp2 = tmp11 - tmp12;
309 
310  /* Odd part */
311 
312  z13 = (DCTELEM) wsptr[5] + (DCTELEM) wsptr[3];
313  z10 = (DCTELEM) wsptr[5] - (DCTELEM) wsptr[3];
314  z11 = (DCTELEM) wsptr[1] + (DCTELEM) wsptr[7];
315  z12 = (DCTELEM) wsptr[1] - (DCTELEM) wsptr[7];
316 
317  tmp7 = z11 + z13; /* phase 5 */
318  tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562); /* 2*c4 */
319 
320  z5 = MULTIPLY(z10 + z12, FIX_1_847759065); /* 2*c2 */
321  tmp10 = z5 - MULTIPLY(z12, FIX_1_082392200); /* 2*(c2-c6) */
322  tmp12 = z5 - MULTIPLY(z10, FIX_2_613125930); /* 2*(c2+c6) */
323 
324  tmp6 = tmp12 - tmp7; /* phase 2 */
325  tmp5 = tmp11 - tmp6;
326  tmp4 = tmp10 - tmp5;
327 
328  /* Final output stage: scale down by a factor of 8 and range-limit */
329 
330  outptr[0] = range_limit[(int) IRIGHT_SHIFT(tmp0 + tmp7, PASS1_BITS+3)
331  & RANGE_MASK];
332  outptr[7] = range_limit[(int) IRIGHT_SHIFT(tmp0 - tmp7, PASS1_BITS+3)
333  & RANGE_MASK];
334  outptr[1] = range_limit[(int) IRIGHT_SHIFT(tmp1 + tmp6, PASS1_BITS+3)
335  & RANGE_MASK];
336  outptr[6] = range_limit[(int) IRIGHT_SHIFT(tmp1 - tmp6, PASS1_BITS+3)
337  & RANGE_MASK];
338  outptr[2] = range_limit[(int) IRIGHT_SHIFT(tmp2 + tmp5, PASS1_BITS+3)
339  & RANGE_MASK];
340  outptr[5] = range_limit[(int) IRIGHT_SHIFT(tmp2 - tmp5, PASS1_BITS+3)
341  & RANGE_MASK];
342  outptr[3] = range_limit[(int) IRIGHT_SHIFT(tmp3 + tmp4, PASS1_BITS+3)
343  & RANGE_MASK];
344  outptr[4] = range_limit[(int) IRIGHT_SHIFT(tmp3 - tmp4, PASS1_BITS+3)
345  & RANGE_MASK];
346 
347  wsptr += DCTSIZE; /* advance pointer to next row */
348  }
349 }
350 
351 #endif /* DCT_IFAST_SUPPORTED */
#define IDCT_range_limit(cinfo)
Definition: jdct.h:90
char JSAMPLE
Definition: jmorecfg.h:74
JSAMPLE FAR * JSAMPROW
Definition: jpeglib.h:75
jpeg_component_info JCOEFPTR coef_block
Definition: jdct.h:238
#define RANGE_MASK
Definition: jdct.h:87
INT32 DCTELEM
Definition: jdct.h:38
#define SHIFT_TEMPS
Definition: jpegint.h:301
jpeg_component_info * compptr
Definition: jdct.h:238
jpeg_component_info JCOEFPTR JSAMPARRAY JDIMENSION output_col
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static int blocks
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#define DCTSIZE2
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INT32 IFAST_MULT_TYPE
Definition: jdct.h:71
#define for
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Definition: jpeglib.h:84
#define RANGE_CENTER
Definition: jpegint.h:273
#define IRIGHT_SHIFT(x, shft)
Definition: jcarith.c:111
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Definition: jpeglib.h:76
#define ISHIFT_TEMPS
Definition: jcarith.c:110
#define GLOBAL(type)
Definition: jmorecfg.h:291
#define DCTSIZE
Definition: jpeglib.h:50
jpeg_component_info JCOEFPTR JSAMPARRAY output_buf
Definition: jdct.h:238
unsigned int JDIMENSION
Definition: jmorecfg.h:229
Sorry
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