Line data Source code
1 : /*
2 : * jfdctfst.c
3 : *
4 : * This file was part of the Independent JPEG Group's software:
5 : * Copyright (C) 1994-1996, Thomas G. Lane.
6 : * libjpeg-turbo Modifications:
7 : * Copyright (C) 2015, D. R. Commander.
8 : * For conditions of distribution and use, see the accompanying README.ijg
9 : * file.
10 : *
11 : * This file contains a fast, not so accurate integer implementation of the
12 : * forward DCT (Discrete Cosine Transform).
13 : *
14 : * A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
15 : * on each column. Direct algorithms are also available, but they are
16 : * much more 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.ijg). The following
22 : * code 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 DCTs. /* deliberate syntax err */
50 : #endif
51 :
52 :
53 : /* Scaling decisions are generally the same as in the LL&M algorithm;
54 : * see jfdctint.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 : * Again to save a few shifts, the intermediate results between pass 1 and
63 : * pass 2 are not upscaled, but are represented only to integral precision.
64 : *
65 : * A final compromise is to represent the multiplicative constants to only
66 : * 8 fractional bits, rather than 13. This saves some shifting work on some
67 : * machines, and may also reduce the cost of multiplication (since there
68 : * are fewer one-bits in the constants).
69 : */
70 :
71 : #define CONST_BITS 8
72 :
73 :
74 : /* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
75 : * causing a lot of useless floating-point operations at run time.
76 : * To get around this we use the following pre-calculated constants.
77 : * If you change CONST_BITS you may want to add appropriate values.
78 : * (With a reasonable C compiler, you can just rely on the FIX() macro...)
79 : */
80 :
81 : #if CONST_BITS == 8
82 : #define FIX_0_382683433 ((JLONG) 98) /* FIX(0.382683433) */
83 : #define FIX_0_541196100 ((JLONG) 139) /* FIX(0.541196100) */
84 : #define FIX_0_707106781 ((JLONG) 181) /* FIX(0.707106781) */
85 : #define FIX_1_306562965 ((JLONG) 334) /* FIX(1.306562965) */
86 : #else
87 : #define FIX_0_382683433 FIX(0.382683433)
88 : #define FIX_0_541196100 FIX(0.541196100)
89 : #define FIX_0_707106781 FIX(0.707106781)
90 : #define FIX_1_306562965 FIX(1.306562965)
91 : #endif
92 :
93 :
94 : /* We can gain a little more speed, with a further compromise in accuracy,
95 : * by omitting the addition in a descaling shift. This yields an incorrectly
96 : * rounded result half the time...
97 : */
98 :
99 : #ifndef USE_ACCURATE_ROUNDING
100 : #undef DESCALE
101 : #define DESCALE(x,n) RIGHT_SHIFT(x, n)
102 : #endif
103 :
104 :
105 : /* Multiply a DCTELEM variable by an JLONG constant, and immediately
106 : * descale to yield a DCTELEM result.
107 : */
108 :
109 : #define MULTIPLY(var,const) ((DCTELEM) DESCALE((var) * (const), CONST_BITS))
110 :
111 :
112 : /*
113 : * Perform the forward DCT on one block of samples.
114 : */
115 :
116 : GLOBAL(void)
117 0 : jpeg_fdct_ifast (DCTELEM *data)
118 : {
119 : DCTELEM tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
120 : DCTELEM tmp10, tmp11, tmp12, tmp13;
121 : DCTELEM z1, z2, z3, z4, z5, z11, z13;
122 : DCTELEM *dataptr;
123 : int ctr;
124 : SHIFT_TEMPS
125 :
126 : /* Pass 1: process rows. */
127 :
128 0 : dataptr = data;
129 0 : for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
130 0 : tmp0 = dataptr[0] + dataptr[7];
131 0 : tmp7 = dataptr[0] - dataptr[7];
132 0 : tmp1 = dataptr[1] + dataptr[6];
133 0 : tmp6 = dataptr[1] - dataptr[6];
134 0 : tmp2 = dataptr[2] + dataptr[5];
135 0 : tmp5 = dataptr[2] - dataptr[5];
136 0 : tmp3 = dataptr[3] + dataptr[4];
137 0 : tmp4 = dataptr[3] - dataptr[4];
138 :
139 : /* Even part */
140 :
141 0 : tmp10 = tmp0 + tmp3; /* phase 2 */
142 0 : tmp13 = tmp0 - tmp3;
143 0 : tmp11 = tmp1 + tmp2;
144 0 : tmp12 = tmp1 - tmp2;
145 :
146 0 : dataptr[0] = tmp10 + tmp11; /* phase 3 */
147 0 : dataptr[4] = tmp10 - tmp11;
148 :
149 0 : z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
150 0 : dataptr[2] = tmp13 + z1; /* phase 5 */
151 0 : dataptr[6] = tmp13 - z1;
152 :
153 : /* Odd part */
154 :
155 0 : tmp10 = tmp4 + tmp5; /* phase 2 */
156 0 : tmp11 = tmp5 + tmp6;
157 0 : tmp12 = tmp6 + tmp7;
158 :
159 : /* The rotator is modified from fig 4-8 to avoid extra negations. */
160 0 : z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
161 0 : z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
162 0 : z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
163 0 : z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
164 :
165 0 : z11 = tmp7 + z3; /* phase 5 */
166 0 : z13 = tmp7 - z3;
167 :
168 0 : dataptr[5] = z13 + z2; /* phase 6 */
169 0 : dataptr[3] = z13 - z2;
170 0 : dataptr[1] = z11 + z4;
171 0 : dataptr[7] = z11 - z4;
172 :
173 0 : dataptr += DCTSIZE; /* advance pointer to next row */
174 : }
175 :
176 : /* Pass 2: process columns. */
177 :
178 0 : dataptr = data;
179 0 : for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
180 0 : tmp0 = dataptr[DCTSIZE*0] + dataptr[DCTSIZE*7];
181 0 : tmp7 = dataptr[DCTSIZE*0] - dataptr[DCTSIZE*7];
182 0 : tmp1 = dataptr[DCTSIZE*1] + dataptr[DCTSIZE*6];
183 0 : tmp6 = dataptr[DCTSIZE*1] - dataptr[DCTSIZE*6];
184 0 : tmp2 = dataptr[DCTSIZE*2] + dataptr[DCTSIZE*5];
185 0 : tmp5 = dataptr[DCTSIZE*2] - dataptr[DCTSIZE*5];
186 0 : tmp3 = dataptr[DCTSIZE*3] + dataptr[DCTSIZE*4];
187 0 : tmp4 = dataptr[DCTSIZE*3] - dataptr[DCTSIZE*4];
188 :
189 : /* Even part */
190 :
191 0 : tmp10 = tmp0 + tmp3; /* phase 2 */
192 0 : tmp13 = tmp0 - tmp3;
193 0 : tmp11 = tmp1 + tmp2;
194 0 : tmp12 = tmp1 - tmp2;
195 :
196 0 : dataptr[DCTSIZE*0] = tmp10 + tmp11; /* phase 3 */
197 0 : dataptr[DCTSIZE*4] = tmp10 - tmp11;
198 :
199 0 : z1 = MULTIPLY(tmp12 + tmp13, FIX_0_707106781); /* c4 */
200 0 : dataptr[DCTSIZE*2] = tmp13 + z1; /* phase 5 */
201 0 : dataptr[DCTSIZE*6] = tmp13 - z1;
202 :
203 : /* Odd part */
204 :
205 0 : tmp10 = tmp4 + tmp5; /* phase 2 */
206 0 : tmp11 = tmp5 + tmp6;
207 0 : tmp12 = tmp6 + tmp7;
208 :
209 : /* The rotator is modified from fig 4-8 to avoid extra negations. */
210 0 : z5 = MULTIPLY(tmp10 - tmp12, FIX_0_382683433); /* c6 */
211 0 : z2 = MULTIPLY(tmp10, FIX_0_541196100) + z5; /* c2-c6 */
212 0 : z4 = MULTIPLY(tmp12, FIX_1_306562965) + z5; /* c2+c6 */
213 0 : z3 = MULTIPLY(tmp11, FIX_0_707106781); /* c4 */
214 :
215 0 : z11 = tmp7 + z3; /* phase 5 */
216 0 : z13 = tmp7 - z3;
217 :
218 0 : dataptr[DCTSIZE*5] = z13 + z2; /* phase 6 */
219 0 : dataptr[DCTSIZE*3] = z13 - z2;
220 0 : dataptr[DCTSIZE*1] = z11 + z4;
221 0 : dataptr[DCTSIZE*7] = z11 - z4;
222 :
223 0 : dataptr++; /* advance pointer to next column */
224 : }
225 0 : }
226 :
227 : #endif /* DCT_IFAST_SUPPORTED */
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