source: trunk/libjpeg/jfdctfst.c @ 15

Last change on this file since 15 was 15, checked in by Eugene Romanenko, 15 years ago

needed libs update

File size: 7.6 KB
Line 
1/*
2 * jfdctfst.c
3 *
4 * Copyright (C) 1994-1996, 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 * forward DCT (Discrete Cosine Transform).
10 *
11 * A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
12 * on each column.  Direct algorithms are also available, but they are
13 * much more complex and seem not to be any faster when reduced to code.
14 *
15 * This implementation is based on Arai, Agui, and Nakajima's algorithm for
16 * scaled DCT.  Their original paper (Trans. IEICE E-71(11):1095) is in
17 * Japanese, but the algorithm is described in the Pennebaker & Mitchell
18 * JPEG textbook (see REFERENCES section in file README).  The following code
19 * is based directly on figure 4-8 in P&M.
20 * While an 8-point DCT cannot be done in less than 11 multiplies, it is
21 * possible to arrange the computation so that many of the multiplies are
22 * simple scalings of the final outputs.  These multiplies can then be
23 * folded into the multiplications or divisions by the JPEG quantization
24 * table entries.  The AA&N method leaves only 5 multiplies and 29 adds
25 * to be done in the DCT itself.
26 * The primary disadvantage of this method is that with fixed-point math,
27 * accuracy is lost due to imprecise representation of the scaled
28 * quantization values.  The smaller the quantization table entry, the less
29 * precise the scaled value, so this implementation does worse with high-
30 * quality-setting files than with low-quality ones.
31 */
32
33#define JPEG_INTERNALS
34#include "jinclude.h"
35#include "jpeglib.h"
36#include "jdct.h"               /* Private declarations for DCT subsystem */
37
38#ifdef DCT_IFAST_SUPPORTED
39
40
41/*
42 * This module is specialized to the case DCTSIZE = 8.
43 */
44
45#if DCTSIZE != 8
46  Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */
47#endif
48
49
50/* Scaling decisions are generally the same as in the LL&M algorithm;
51 * see jfdctint.c for more details.  However, we choose to descale
52 * (right shift) multiplication products as soon as they are formed,
53 * rather than carrying additional fractional bits into subsequent additions.
54 * This compromises accuracy slightly, but it lets us save a few shifts.
55 * More importantly, 16-bit arithmetic is then adequate (for 8-bit samples)
56 * everywhere except in the multiplications proper; this saves a good deal
57 * of work on 16-bit-int machines.
58 *
59 * Again to save a few shifts, the intermediate results between pass 1 and
60 * pass 2 are not upscaled, but are represented only to integral precision.
61 *
62 * A final compromise is to represent the multiplicative constants to only
63 * 8 fractional bits, rather than 13.  This saves some shifting work on some
64 * machines, and may also reduce the cost of multiplication (since there
65 * are fewer one-bits in the constants).
66 */
67
68#define CONST_BITS  8
69
70
71/* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
72 * causing a lot of useless floating-point operations at run time.
73 * To get around this we use the following pre-calculated constants.
74 * If you change CONST_BITS you may want to add appropriate values.
75 * (With a reasonable C compiler, you can just rely on the FIX() macro...)
76 */
77
78#if CONST_BITS == 8
79#define FIX_0_382683433  ((INT32)   98)         /* FIX(0.382683433) */
80#define FIX_0_541196100  ((INT32)  139)         /* FIX(0.541196100) */
81#define FIX_0_707106781  ((INT32)  181)         /* FIX(0.707106781) */
82#define FIX_1_306562965  ((INT32)  334)         /* FIX(1.306562965) */
83#else
84#define FIX_0_382683433  FIX(0.382683433)
85#define FIX_0_541196100  FIX(0.541196100)
86#define FIX_0_707106781  FIX(0.707106781)
87#define FIX_1_306562965  FIX(1.306562965)
88#endif
89
90
91/* We can gain a little more speed, with a further compromise in accuracy,
92 * by omitting the addition in a descaling shift.  This yields an incorrectly
93 * rounded result half the time...
94 */
95
96#ifndef USE_ACCURATE_ROUNDING
97#undef DESCALE
98#define DESCALE(x,n)  RIGHT_SHIFT(x, n)
99#endif
100
101
102/* Multiply a DCTELEM variable by an INT32 constant, and immediately
103 * descale to yield a DCTELEM result.
104 */
105
106#define MULTIPLY(var,const)  ((DCTELEM) DESCALE((var) * (const), CONST_BITS))
107
108
109/*
110 * Perform the forward DCT on one block of samples.
111 */
112
113GLOBAL(void)
114jpeg_fdct_ifast (DCTELEM * data)
115{
116  DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
117  DCTELEM tmp10, tmp11, tmp12, tmp13;
118  DCTELEM z1, z2, z3, z4, z5, z11, z13;
119  DCTELEM *dataptr;
120  int ctr;
121  SHIFT_TEMPS
122
123  /* Pass 1: process rows. */
124
125  dataptr = data;
126  for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
127    tmp0 = dataptr[0] + dataptr[7];
128    tmp7 = dataptr[0] - dataptr[7];
129    tmp1 = dataptr[1] + dataptr[6];
130    tmp6 = dataptr[1] - dataptr[6];
131    tmp2 = dataptr[2] + dataptr[5];
132    tmp5 = dataptr[2] - dataptr[5];
133    tmp3 = dataptr[3] + dataptr[4];
134    tmp4 = dataptr[3] - dataptr[4];
135   
136    /* Even part */
137   
138    tmp10 = tmp0 + tmp3;        /* phase 2 */
139    tmp13 = tmp0 - tmp3;
140    tmp11 = tmp1 + tmp2;
141    tmp12 = tmp1 - tmp2;
142   
143    dataptr[0] = tmp10 + tmp11; /* phase 3 */
144    dataptr[4] = tmp10 - tmp11;
145   
146    z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
147    dataptr[2] = tmp13 + z1;    /* phase 5 */
148    dataptr[6] = tmp13 - z1;
149   
150    /* Odd part */
151
152    tmp10 = tmp4 + tmp5;        /* phase 2 */
153    tmp11 = tmp5 + tmp6;
154    tmp12 = tmp6 + tmp7;
155
156    /* The rotator is modified from fig 4-8 to avoid extra negations. */
157    z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
158    z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
159    z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
160    z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
161
162    z11 = tmp7 + z3;            /* phase 5 */
163    z13 = tmp7 - z3;
164
165    dataptr[5] = z13 + z2;      /* phase 6 */
166    dataptr[3] = z13 - z2;
167    dataptr[1] = z11 + z4;
168    dataptr[7] = z11 - z4;
169
170    dataptr += DCTSIZE;         /* advance pointer to next row */
171  }
172
173  /* Pass 2: process columns. */
174
175  dataptr = data;
176  for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
177    tmp0 = dataptr[DCTSIZE*0] + dataptr[DCTSIZE*7];
178    tmp7 = dataptr[DCTSIZE*0] - dataptr[DCTSIZE*7];
179    tmp1 = dataptr[DCTSIZE*1] + dataptr[DCTSIZE*6];
180    tmp6 = dataptr[DCTSIZE*1] - dataptr[DCTSIZE*6];
181    tmp2 = dataptr[DCTSIZE*2] + dataptr[DCTSIZE*5];
182    tmp5 = dataptr[DCTSIZE*2] - dataptr[DCTSIZE*5];
183    tmp3 = dataptr[DCTSIZE*3] + dataptr[DCTSIZE*4];
184    tmp4 = dataptr[DCTSIZE*3] - dataptr[DCTSIZE*4];
185   
186    /* Even part */
187   
188    tmp10 = tmp0 + tmp3;        /* phase 2 */
189    tmp13 = tmp0 - tmp3;
190    tmp11 = tmp1 + tmp2;
191    tmp12 = tmp1 - tmp2;
192   
193    dataptr[DCTSIZE*0] = tmp10 + tmp11; /* phase 3 */
194    dataptr[DCTSIZE*4] = tmp10 - tmp11;
195   
196    z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
197    dataptr[DCTSIZE*2] = tmp13 + z1; /* phase 5 */
198    dataptr[DCTSIZE*6] = tmp13 - z1;
199   
200    /* Odd part */
201
202    tmp10 = tmp4 + tmp5;        /* phase 2 */
203    tmp11 = tmp5 + tmp6;
204    tmp12 = tmp6 + tmp7;
205
206    /* The rotator is modified from fig 4-8 to avoid extra negations. */
207    z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
208    z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
209    z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
210    z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
211
212    z11 = tmp7 + z3;            /* phase 5 */
213    z13 = tmp7 - z3;
214
215    dataptr[DCTSIZE*5] = z13 + z2; /* phase 6 */
216    dataptr[DCTSIZE*3] = z13 - z2;
217    dataptr[DCTSIZE*1] = z11 + z4;
218    dataptr[DCTSIZE*7] = z11 - z4;
219
220    dataptr++;                  /* advance pointer to next column */
221  }
222}
223
224#endif /* DCT_IFAST_SUPPORTED */
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