| 1 |
/* |
| 2 |
* jidctfst.c |
| 3 |
* |
| 4 |
* Copyright (C) 1994-1998, Thomas G. Lane. |
| 5 |
* This file is part of the Independent JPEG Group's software. |
| 6 |
* For conditions of distribution and use, see the accompanying README file. |
| 7 |
* |
| 8 |
* This file contains a fast, not so accurate integer implementation of the |
| 9 |
* inverse DCT (Discrete Cosine Transform). In the IJG code, this routine |
| 10 |
* must also perform dequantization of the input coefficients. |
| 11 |
* |
| 12 |
* A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT |
| 13 |
* on each row (or vice versa, but it's more convenient to emit a row at |
| 14 |
* a time). Direct algorithms are also available, but they are much more |
| 15 |
* complex and seem not to be any faster when reduced to code. |
| 16 |
* |
| 17 |
* This implementation is based on Arai, Agui, and Nakajima's algorithm for |
| 18 |
* scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in |
| 19 |
* Japanese, but the algorithm is described in the Pennebaker & Mitchell |
| 20 |
* JPEG textbook (see REFERENCES section in file README). The following code |
| 21 |
* is based directly on figure 4-8 in P&M. |
| 22 |
* While an 8-point DCT cannot be done in less than 11 multiplies, it is |
| 23 |
* possible to arrange the computation so that many of the multiplies are |
| 24 |
* simple scalings of the final outputs. These multiplies can then be |
| 25 |
* folded into the multiplications or divisions by the JPEG quantization |
| 26 |
* table entries. The AA&N method leaves only 5 multiplies and 29 adds |
| 27 |
* to be done in the DCT itself. |
| 28 |
* The primary disadvantage of this method is that with fixed-point math, |
| 29 |
* accuracy is lost due to imprecise representation of the scaled |
| 30 |
* quantization values. The smaller the quantization table entry, the less |
| 31 |
* precise the scaled value, so this implementation does worse with high- |
| 32 |
* quality-setting files than with low-quality ones. |
| 33 |
*/ |
| 34 |
|
| 35 |
#define JPEG_INTERNALS |
| 36 |
#include "jinclude.h" |
| 37 |
#include "jpeglib.h" |
| 38 |
#include "jdct.h" /* Private declarations for DCT subsystem */ |
| 39 |
|
| 40 |
#ifdef DCT_IFAST_SUPPORTED |
| 41 |
|
| 42 |
|
| 43 |
/* |
| 44 |
* This module is specialized to the case DCTSIZE = 8. |
| 45 |
*/ |
| 46 |
|
| 47 |
#if DCTSIZE != 8 |
| 48 |
Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */ |
| 49 |
#endif |
| 50 |
|
| 51 |
|
| 52 |
/* Scaling decisions are generally the same as in the LL&M algorithm; |
| 53 |
* see jidctint.c for more details. However, we choose to descale |
| 54 |
* (right shift) multiplication products as soon as they are formed, |
| 55 |
* rather than carrying additional fractional bits into subsequent additions. |
| 56 |
* This compromises accuracy slightly, but it lets us save a few shifts. |
| 57 |
* More importantly, 16-bit arithmetic is then adequate (for 8-bit samples) |
| 58 |
* everywhere except in the multiplications proper; this saves a good deal |
| 59 |
* of work on 16-bit-int machines. |
| 60 |
* |
| 61 |
* The dequantized coefficients are not integers because the AA&N scaling |
| 62 |
* factors have been incorporated. We represent them scaled up by PASS1_BITS, |
| 63 |
* so that the first and second IDCT rounds have the same input scaling. |
| 64 |
* For 8-bit JSAMPLEs, we choose IFAST_SCALE_BITS = PASS1_BITS so as to |
| 65 |
* avoid a descaling shift; this compromises accuracy rather drastically |
| 66 |
* for small quantization table entries, but it saves a lot of shifts. |
| 67 |
* For 12-bit JSAMPLEs, there's no hope of using 16x16 multiplies anyway, |
| 68 |
* so we use a much larger scaling factor to preserve accuracy. |
| 69 |
* |
| 70 |
* A final compromise is to represent the multiplicative constants to only |
| 71 |
* 8 fractional bits, rather than 13. This saves some shifting work on some |
| 72 |
* machines, and may also reduce the cost of multiplication (since there |
| 73 |
* are fewer one-bits in the constants). |
| 74 |
*/ |
| 75 |
|
| 76 |
#if BITS_IN_JSAMPLE == 8 |
| 77 |
#define CONST_BITS 8 |
| 78 |
#define PASS1_BITS 2 |
| 79 |
#else |
| 80 |
#define CONST_BITS 8 |
| 81 |
#define PASS1_BITS 1 /* lose a little precision to avoid overflow */ |
| 82 |
#endif |
| 83 |
|
| 84 |
/* Some C compilers fail to reduce "FIX(constant)" at compile time, thus |
| 85 |
* causing a lot of useless floating-point operations at run time. |
| 86 |
* To get around this we use the following pre-calculated constants. |
| 87 |
* If you change CONST_BITS you may want to add appropriate values. |
| 88 |
* (With a reasonable C compiler, you can just rely on the FIX() macro...) |
| 89 |
*/ |
| 90 |
|
| 91 |
#if CONST_BITS == 8 |
| 92 |
#define FIX_1_082392200 ((INT32) 277) /* FIX(1.082392200) */ |
| 93 |
#define FIX_1_414213562 ((INT32) 362) /* FIX(1.414213562) */ |
| 94 |
#define FIX_1_847759065 ((INT32) 473) /* FIX(1.847759065) */ |
| 95 |
#define FIX_2_613125930 ((INT32) 669) /* FIX(2.613125930) */ |
| 96 |
#else |
| 97 |
#define FIX_1_082392200 FIX(1.082392200) |
| 98 |
#define FIX_1_414213562 FIX(1.414213562) |
| 99 |
#define FIX_1_847759065 FIX(1.847759065) |
| 100 |
#define FIX_2_613125930 FIX(2.613125930) |
| 101 |
#endif |
| 102 |
|
| 103 |
|
| 104 |
/* We can gain a little more speed, with a further compromise in accuracy, |
| 105 |
* by omitting the addition in a descaling shift. This yields an incorrectly |
| 106 |
* rounded result half the time... |
| 107 |
*/ |
| 108 |
|
| 109 |
#ifndef USE_ACCURATE_ROUNDING |
| 110 |
#undef DESCALE |
| 111 |
#define DESCALE(x,n) RIGHT_SHIFT(x, n) |
| 112 |
#endif |
| 113 |
|
| 114 |
|
| 115 |
/* Multiply a DCTELEM variable by an INT32 constant, and immediately |
| 116 |
* descale to yield a DCTELEM result. |
| 117 |
*/ |
| 118 |
|
| 119 |
#define MULTIPLY(var,const) ((DCTELEM) DESCALE((var) * (const), CONST_BITS)) |
| 120 |
|
| 121 |
|
| 122 |
/* Dequantize a coefficient by multiplying it by the multiplier-table |
| 123 |
* entry; produce a DCTELEM result. For 8-bit data a 16x16->16 |
| 124 |
* multiplication will do. For 12-bit data, the multiplier table is |
| 125 |
* declared INT32, so a 32-bit multiply will be used. |
| 126 |
*/ |
| 127 |
|
| 128 |
#if BITS_IN_JSAMPLE == 8 |
| 129 |
#define DEQUANTIZE(coef,quantval) (((IFAST_MULT_TYPE) (coef)) * (quantval)) |
| 130 |
#else |
| 131 |
#define DEQUANTIZE(coef,quantval) \ |
| 132 |
DESCALE((coef)*(quantval), IFAST_SCALE_BITS-PASS1_BITS) |
| 133 |
#endif |
| 134 |
|
| 135 |
|
| 136 |
/* Like DESCALE, but applies to a DCTELEM and produces an int. |
| 137 |
* We assume that int right shift is unsigned if INT32 right shift is. |
| 138 |
*/ |
| 139 |
|
| 140 |
#ifdef RIGHT_SHIFT_IS_UNSIGNED |
| 141 |
#define ISHIFT_TEMPS DCTELEM ishift_temp; |
| 142 |
#if BITS_IN_JSAMPLE == 8 |
| 143 |
#define DCTELEMBITS 16 /* DCTELEM may be 16 or 32 bits */ |
| 144 |
#else |
| 145 |
#define DCTELEMBITS 32 /* DCTELEM must be 32 bits */ |
| 146 |
#endif |
| 147 |
#define IRIGHT_SHIFT(x,shft) \ |
| 148 |
((ishift_temp = (x)) < 0 ? \ |
| 149 |
(ishift_temp >> (shft)) | ((~((DCTELEM) 0)) << (DCTELEMBITS-(shft))) : \ |
| 150 |
(ishift_temp >> (shft))) |
| 151 |
#else |
| 152 |
#define ISHIFT_TEMPS |
| 153 |
#define IRIGHT_SHIFT(x,shft) ((x) >> (shft)) |
| 154 |
#endif |
| 155 |
|
| 156 |
#ifdef USE_ACCURATE_ROUNDING |
| 157 |
#define IDESCALE(x,n) ((int) IRIGHT_SHIFT((x) + (1 << ((n)-1)), n)) |
| 158 |
#else |
| 159 |
#define IDESCALE(x,n) ((int) IRIGHT_SHIFT(x, n)) |
| 160 |
#endif |
| 161 |
|
| 162 |
|
| 163 |
/* |
| 164 |
* Perform dequantization and inverse DCT on one block of coefficients. |
| 165 |
*/ |
| 166 |
|
| 167 |
GLOBAL(void) |
| 168 |
jpeg_idct_ifast (j_decompress_ptr cinfo, jpeg_component_info * compptr, |
| 169 |
JCOEFPTR coef_block, |
| 170 |
JSAMPARRAY output_buf, JDIMENSION output_col) |
| 171 |
{ |
| 172 |
DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7; |
| 173 |
DCTELEM tmp10, tmp11, tmp12, tmp13; |
| 174 |
DCTELEM z5, z10, z11, z12, z13; |
| 175 |
JCOEFPTR inptr; |
| 176 |
IFAST_MULT_TYPE * quantptr; |
| 177 |
int * wsptr; |
| 178 |
JSAMPROW outptr; |
| 179 |
JSAMPLE *range_limit = IDCT_range_limit(cinfo); |
| 180 |
int ctr; |
| 181 |
int workspace[DCTSIZE2]; /* buffers data between passes */ |
| 182 |
SHIFT_TEMPS /* for DESCALE */ |
| 183 |
ISHIFT_TEMPS /* for IDESCALE */ |
| 184 |
|
| 185 |
/* Pass 1: process columns from input, store into work array. */ |
| 186 |
|
| 187 |
inptr = coef_block; |
| 188 |
quantptr = (IFAST_MULT_TYPE *) compptr->dct_table; |
| 189 |
wsptr = workspace; |
| 190 |
for (ctr = DCTSIZE; ctr > 0; ctr--) { |
| 191 |
/* Due to quantization, we will usually find that many of the input |
| 192 |
* coefficients are zero, especially the AC terms. We can exploit this |
| 193 |
* by short-circuiting the IDCT calculation for any column in which all |
| 194 |
* the AC terms are zero. In that case each output is equal to the |
| 195 |
* DC coefficient (with scale factor as needed). |
| 196 |
* With typical images and quantization tables, half or more of the |
| 197 |
* column DCT calculations can be simplified this way. |
| 198 |
*/ |
| 199 |
|
| 200 |
if (inptr[DCTSIZE*1] == 0 && inptr[DCTSIZE*2] == 0 && |
| 201 |
inptr[DCTSIZE*3] == 0 && inptr[DCTSIZE*4] == 0 && |
| 202 |
inptr[DCTSIZE*5] == 0 && inptr[DCTSIZE*6] == 0 && |
| 203 |
inptr[DCTSIZE*7] == 0) { |
| 204 |
/* AC terms all zero */ |
| 205 |
int dcval = (int) DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]); |
| 206 |
|
| 207 |
wsptr[DCTSIZE*0] = dcval; |
| 208 |
wsptr[DCTSIZE*1] = dcval; |
| 209 |
wsptr[DCTSIZE*2] = dcval; |
| 210 |
wsptr[DCTSIZE*3] = dcval; |
| 211 |
wsptr[DCTSIZE*4] = dcval; |
| 212 |
wsptr[DCTSIZE*5] = dcval; |
| 213 |
wsptr[DCTSIZE*6] = dcval; |
| 214 |
wsptr[DCTSIZE*7] = dcval; |
| 215 |
|
| 216 |
inptr++; /* advance pointers to next column */ |
| 217 |
quantptr++; |
| 218 |
wsptr++; |
| 219 |
continue; |
| 220 |
} |
| 221 |
|
| 222 |
/* Even part */ |
| 223 |
|
| 224 |
tmp0 = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]); |
| 225 |
tmp1 = DEQUANTIZE(inptr[DCTSIZE*2], quantptr[DCTSIZE*2]); |
| 226 |
tmp2 = DEQUANTIZE(inptr[DCTSIZE*4], quantptr[DCTSIZE*4]); |
| 227 |
tmp3 = DEQUANTIZE(inptr[DCTSIZE*6], quantptr[DCTSIZE*6]); |
| 228 |
|
| 229 |
tmp10 = tmp0 + tmp2; /* phase 3 */ |
| 230 |
tmp11 = tmp0 - tmp2; |
| 231 |
|
| 232 |
tmp13 = tmp1 + tmp3; /* phases 5-3 */ |
| 233 |
tmp12 = MULTIPLY(tmp1 - tmp3, FIX_1_414213562) - tmp13; /* 2*c4 */ |
| 234 |
|
| 235 |
tmp0 = tmp10 + tmp13; /* phase 2 */ |
| 236 |
tmp3 = tmp10 - tmp13; |
| 237 |
tmp1 = tmp11 + tmp12; |
| 238 |
tmp2 = tmp11 - tmp12; |
| 239 |
|
| 240 |
/* Odd part */ |
| 241 |
|
| 242 |
tmp4 = DEQUANTIZE(inptr[DCTSIZE*1], quantptr[DCTSIZE*1]); |
| 243 |
tmp5 = DEQUANTIZE(inptr[DCTSIZE*3], quantptr[DCTSIZE*3]); |
| 244 |
tmp6 = DEQUANTIZE(inptr[DCTSIZE*5], quantptr[DCTSIZE*5]); |
| 245 |
tmp7 = DEQUANTIZE(inptr[DCTSIZE*7], quantptr[DCTSIZE*7]); |
| 246 |
|
| 247 |
z13 = tmp6 + tmp5; /* phase 6 */ |
| 248 |
z10 = tmp6 - tmp5; |
| 249 |
z11 = tmp4 + tmp7; |
| 250 |
z12 = tmp4 - tmp7; |
| 251 |
|
| 252 |
tmp7 = z11 + z13; /* phase 5 */ |
| 253 |
tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562); /* 2*c4 */ |
| 254 |
|
| 255 |
z5 = MULTIPLY(z10 + z12, FIX_1_847759065); /* 2*c2 */ |
| 256 |
tmp10 = MULTIPLY(z12, FIX_1_082392200) - z5; /* 2*(c2-c6) */ |
| 257 |
tmp12 = MULTIPLY(z10, - FIX_2_613125930) + z5; /* -2*(c2+c6) */ |
| 258 |
|
| 259 |
tmp6 = tmp12 - tmp7; /* phase 2 */ |
| 260 |
tmp5 = tmp11 - tmp6; |
| 261 |
tmp4 = tmp10 + tmp5; |
| 262 |
|
| 263 |
wsptr[DCTSIZE*0] = (int) (tmp0 + tmp7); |
| 264 |
wsptr[DCTSIZE*7] = (int) (tmp0 - tmp7); |
| 265 |
wsptr[DCTSIZE*1] = (int) (tmp1 + tmp6); |
| 266 |
wsptr[DCTSIZE*6] = (int) (tmp1 - tmp6); |
| 267 |
wsptr[DCTSIZE*2] = (int) (tmp2 + tmp5); |
| 268 |
wsptr[DCTSIZE*5] = (int) (tmp2 - tmp5); |
| 269 |
wsptr[DCTSIZE*4] = (int) (tmp3 + tmp4); |
| 270 |
wsptr[DCTSIZE*3] = (int) (tmp3 - tmp4); |
| 271 |
|
| 272 |
inptr++; /* advance pointers to next column */ |
| 273 |
quantptr++; |
| 274 |
wsptr++; |
| 275 |
} |
| 276 |
|
| 277 |
/* Pass 2: process rows from work array, store into output array. */ |
| 278 |
/* Note that we must descale the results by a factor of 8 == 2**3, */ |
| 279 |
/* and also undo the PASS1_BITS scaling. */ |
| 280 |
|
| 281 |
wsptr = workspace; |
| 282 |
for (ctr = 0; ctr < DCTSIZE; ctr++) { |
| 283 |
outptr = output_buf[ctr] + output_col; |
| 284 |
/* Rows of zeroes can be exploited in the same way as we did with columns. |
| 285 |
* However, the column calculation has created many nonzero AC terms, so |
| 286 |
* the simplification applies less often (typically 5% to 10% of the time). |
| 287 |
* On machines with very fast multiplication, it's possible that the |
| 288 |
* test takes more time than it's worth. In that case this section |
| 289 |
* may be commented out. |
| 290 |
*/ |
| 291 |
|
| 292 |
#ifndef NO_ZERO_ROW_TEST |
| 293 |
if (wsptr[1] == 0 && wsptr[2] == 0 && wsptr[3] == 0 && wsptr[4] == 0 && |
| 294 |
wsptr[5] == 0 && wsptr[6] == 0 && wsptr[7] == 0) { |
| 295 |
/* AC terms all zero */ |
| 296 |
JSAMPLE dcval = range_limit[IDESCALE(wsptr[0], PASS1_BITS+3) |
| 297 |
& RANGE_MASK]; |
| 298 |
|
| 299 |
outptr[0] = dcval; |
| 300 |
outptr[1] = dcval; |
| 301 |
outptr[2] = dcval; |
| 302 |
outptr[3] = dcval; |
| 303 |
outptr[4] = dcval; |
| 304 |
outptr[5] = dcval; |
| 305 |
outptr[6] = dcval; |
| 306 |
outptr[7] = dcval; |
| 307 |
|
| 308 |
wsptr += DCTSIZE; /* advance pointer to next row */ |
| 309 |
continue; |
| 310 |
} |
| 311 |
#endif |
| 312 |
|
| 313 |
/* Even part */ |
| 314 |
|
| 315 |
tmp10 = ((DCTELEM) wsptr[0] + (DCTELEM) wsptr[4]); |
| 316 |
tmp11 = ((DCTELEM) wsptr[0] - (DCTELEM) wsptr[4]); |
| 317 |
|
| 318 |
tmp13 = ((DCTELEM) wsptr[2] + (DCTELEM) wsptr[6]); |
| 319 |
tmp12 = MULTIPLY((DCTELEM) wsptr[2] - (DCTELEM) wsptr[6], FIX_1_414213562) |
| 320 |
- tmp13; |
| 321 |
|
| 322 |
tmp0 = tmp10 + tmp13; |
| 323 |
tmp3 = tmp10 - tmp13; |
| 324 |
tmp1 = tmp11 + tmp12; |
| 325 |
tmp2 = tmp11 - tmp12; |
| 326 |
|
| 327 |
/* Odd part */ |
| 328 |
|
| 329 |
z13 = (DCTELEM) wsptr[5] + (DCTELEM) wsptr[3]; |
| 330 |
z10 = (DCTELEM) wsptr[5] - (DCTELEM) wsptr[3]; |
| 331 |
z11 = (DCTELEM) wsptr[1] + (DCTELEM) wsptr[7]; |
| 332 |
z12 = (DCTELEM) wsptr[1] - (DCTELEM) wsptr[7]; |
| 333 |
|
| 334 |
tmp7 = z11 + z13; /* phase 5 */ |
| 335 |
tmp11 = MULTIPLY(z11 - z13, FIX_1_414213562); /* 2*c4 */ |
| 336 |
|
| 337 |
z5 = MULTIPLY(z10 + z12, FIX_1_847759065); /* 2*c2 */ |
| 338 |
tmp10 = MULTIPLY(z12, FIX_1_082392200) - z5; /* 2*(c2-c6) */ |
| 339 |
tmp12 = MULTIPLY(z10, - FIX_2_613125930) + z5; /* -2*(c2+c6) */ |
| 340 |
|
| 341 |
tmp6 = tmp12 - tmp7; /* phase 2 */ |
| 342 |
tmp5 = tmp11 - tmp6; |
| 343 |
tmp4 = tmp10 + tmp5; |
| 344 |
|
| 345 |
/* Final output stage: scale down by a factor of 8 and range-limit */ |
| 346 |
|
| 347 |
outptr[0] = range_limit[IDESCALE(tmp0 + tmp7, PASS1_BITS+3) |
| 348 |
& RANGE_MASK]; |
| 349 |
outptr[7] = range_limit[IDESCALE(tmp0 - tmp7, PASS1_BITS+3) |
| 350 |
& RANGE_MASK]; |
| 351 |
outptr[1] = range_limit[IDESCALE(tmp1 + tmp6, PASS1_BITS+3) |
| 352 |
& RANGE_MASK]; |
| 353 |
outptr[6] = range_limit[IDESCALE(tmp1 - tmp6, PASS1_BITS+3) |
| 354 |
& RANGE_MASK]; |
| 355 |
outptr[2] = range_limit[IDESCALE(tmp2 + tmp5, PASS1_BITS+3) |
| 356 |
& RANGE_MASK]; |
| 357 |
outptr[5] = range_limit[IDESCALE(tmp2 - tmp5, PASS1_BITS+3) |
| 358 |
& RANGE_MASK]; |
| 359 |
outptr[4] = range_limit[IDESCALE(tmp3 + tmp4, PASS1_BITS+3) |
| 360 |
& RANGE_MASK]; |
| 361 |
outptr[3] = range_limit[IDESCALE(tmp3 - tmp4, PASS1_BITS+3) |
| 362 |
& RANGE_MASK]; |
| 363 |
|
| 364 |
wsptr += DCTSIZE; /* advance pointer to next row */ |
| 365 |
} |
| 366 |
} |
| 367 |
|
| 368 |
#endif /* DCT_IFAST_SUPPORTED */ |
Parent Directory
| 
Revision Log