Line data Source code
1 : /*
2 : * jidctflt.c
3 : *
4 : * This file was part of the Independent JPEG Group's software:
5 : * Copyright (C) 1994-1998, Thomas G. Lane.
6 : * Modified 2010 by Guido Vollbeding.
7 : * libjpeg-turbo Modifications:
8 : * Copyright (C) 2014, D. R. Commander.
9 : * For conditions of distribution and use, see the accompanying README.ijg
10 : * file.
11 : *
12 : * This file contains a floating-point implementation of the
13 : * inverse DCT (Discrete Cosine Transform). In the IJG code, this routine
14 : * must also perform dequantization of the input coefficients.
15 : *
16 : * This implementation should be more accurate than either of the integer
17 : * IDCT implementations. However, it may not give the same results on all
18 : * machines because of differences in roundoff behavior. Speed will depend
19 : * on the hardware's floating point capacity.
20 : *
21 : * A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT
22 : * on each row (or vice versa, but it's more convenient to emit a row at
23 : * a time). Direct algorithms are also available, but they are much more
24 : * complex and seem not to be any faster when reduced to code.
25 : *
26 : * This implementation is based on Arai, Agui, and Nakajima's algorithm for
27 : * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
28 : * Japanese, but the algorithm is described in the Pennebaker & Mitchell
29 : * JPEG textbook (see REFERENCES section in file README.ijg). The following
30 : * code is based directly on figure 4-8 in P&M.
31 : * While an 8-point DCT cannot be done in less than 11 multiplies, it is
32 : * possible to arrange the computation so that many of the multiplies are
33 : * simple scalings of the final outputs. These multiplies can then be
34 : * folded into the multiplications or divisions by the JPEG quantization
35 : * table entries. The AA&N method leaves only 5 multiplies and 29 adds
36 : * to be done in the DCT itself.
37 : * The primary disadvantage of this method is that with a fixed-point
38 : * implementation, accuracy is lost due to imprecise representation of the
39 : * scaled quantization values. However, that problem does not arise if
40 : * we use floating point arithmetic.
41 : */
42 :
43 : #define JPEG_INTERNALS
44 : #include "jinclude.h"
45 : #include "jpeglib.h"
46 : #include "jdct.h" /* Private declarations for DCT subsystem */
47 :
48 : #ifdef DCT_FLOAT_SUPPORTED
49 :
50 :
51 : /*
52 : * This module is specialized to the case DCTSIZE = 8.
53 : */
54 :
55 : #if DCTSIZE != 8
56 : Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */
57 : #endif
58 :
59 :
60 : /* Dequantize a coefficient by multiplying it by the multiplier-table
61 : * entry; produce a float result.
62 : */
63 :
64 : #define DEQUANTIZE(coef,quantval) (((FAST_FLOAT) (coef)) * (quantval))
65 :
66 :
67 : /*
68 : * Perform dequantization and inverse DCT on one block of coefficients.
69 : */
70 :
71 : GLOBAL(void)
72 0 : jpeg_idct_float (j_decompress_ptr cinfo, jpeg_component_info *compptr,
73 : JCOEFPTR coef_block,
74 : JSAMPARRAY output_buf, JDIMENSION output_col)
75 : {
76 : FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
77 : FAST_FLOAT tmp10, tmp11, tmp12, tmp13;
78 : FAST_FLOAT z5, z10, z11, z12, z13;
79 : JCOEFPTR inptr;
80 : FLOAT_MULT_TYPE *quantptr;
81 : FAST_FLOAT *wsptr;
82 : JSAMPROW outptr;
83 0 : JSAMPLE *range_limit = cinfo->sample_range_limit;
84 : int ctr;
85 : FAST_FLOAT workspace[DCTSIZE2]; /* buffers data between passes */
86 : #define _0_125 ((FLOAT_MULT_TYPE)0.125)
87 :
88 : /* Pass 1: process columns from input, store into work array. */
89 :
90 0 : inptr = coef_block;
91 0 : quantptr = (FLOAT_MULT_TYPE *) compptr->dct_table;
92 0 : wsptr = workspace;
93 0 : for (ctr = DCTSIZE; ctr > 0; ctr--) {
94 : /* Due to quantization, we will usually find that many of the input
95 : * coefficients are zero, especially the AC terms. We can exploit this
96 : * by short-circuiting the IDCT calculation for any column in which all
97 : * the AC terms are zero. In that case each output is equal to the
98 : * DC coefficient (with scale factor as needed).
99 : * With typical images and quantization tables, half or more of the
100 : * column DCT calculations can be simplified this way.
101 : */
102 :
103 0 : if (inptr[DCTSIZE*1] == 0 && inptr[DCTSIZE*2] == 0 &&
104 0 : inptr[DCTSIZE*3] == 0 && inptr[DCTSIZE*4] == 0 &&
105 0 : inptr[DCTSIZE*5] == 0 && inptr[DCTSIZE*6] == 0 &&
106 0 : inptr[DCTSIZE*7] == 0) {
107 : /* AC terms all zero */
108 0 : FAST_FLOAT dcval = DEQUANTIZE(inptr[DCTSIZE*0],
109 : quantptr[DCTSIZE*0] * _0_125);
110 :
111 0 : wsptr[DCTSIZE*0] = dcval;
112 0 : wsptr[DCTSIZE*1] = dcval;
113 0 : wsptr[DCTSIZE*2] = dcval;
114 0 : wsptr[DCTSIZE*3] = dcval;
115 0 : wsptr[DCTSIZE*4] = dcval;
116 0 : wsptr[DCTSIZE*5] = dcval;
117 0 : wsptr[DCTSIZE*6] = dcval;
118 0 : wsptr[DCTSIZE*7] = dcval;
119 :
120 0 : inptr++; /* advance pointers to next column */
121 0 : quantptr++;
122 0 : wsptr++;
123 0 : continue;
124 : }
125 :
126 : /* Even part */
127 :
128 0 : tmp0 = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0] * _0_125);
129 0 : tmp1 = DEQUANTIZE(inptr[DCTSIZE*2], quantptr[DCTSIZE*2] * _0_125);
130 0 : tmp2 = DEQUANTIZE(inptr[DCTSIZE*4], quantptr[DCTSIZE*4] * _0_125);
131 0 : tmp3 = DEQUANTIZE(inptr[DCTSIZE*6], quantptr[DCTSIZE*6] * _0_125);
132 :
133 0 : tmp10 = tmp0 + tmp2; /* phase 3 */
134 0 : tmp11 = tmp0 - tmp2;
135 :
136 0 : tmp13 = tmp1 + tmp3; /* phases 5-3 */
137 0 : tmp12 = (tmp1 - tmp3) * ((FAST_FLOAT) 1.414213562) - tmp13; /* 2*c4 */
138 :
139 0 : tmp0 = tmp10 + tmp13; /* phase 2 */
140 0 : tmp3 = tmp10 - tmp13;
141 0 : tmp1 = tmp11 + tmp12;
142 0 : tmp2 = tmp11 - tmp12;
143 :
144 : /* Odd part */
145 :
146 0 : tmp4 = DEQUANTIZE(inptr[DCTSIZE*1], quantptr[DCTSIZE*1] * _0_125);
147 0 : tmp5 = DEQUANTIZE(inptr[DCTSIZE*3], quantptr[DCTSIZE*3] * _0_125);
148 0 : tmp6 = DEQUANTIZE(inptr[DCTSIZE*5], quantptr[DCTSIZE*5] * _0_125);
149 0 : tmp7 = DEQUANTIZE(inptr[DCTSIZE*7], quantptr[DCTSIZE*7] * _0_125);
150 :
151 0 : z13 = tmp6 + tmp5; /* phase 6 */
152 0 : z10 = tmp6 - tmp5;
153 0 : z11 = tmp4 + tmp7;
154 0 : z12 = tmp4 - tmp7;
155 :
156 0 : tmp7 = z11 + z13; /* phase 5 */
157 0 : tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); /* 2*c4 */
158 :
159 0 : z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */
160 0 : tmp10 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */
161 0 : tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 2*(c2+c6) */
162 :
163 0 : tmp6 = tmp12 - tmp7; /* phase 2 */
164 0 : tmp5 = tmp11 - tmp6;
165 0 : tmp4 = tmp10 - tmp5;
166 :
167 0 : wsptr[DCTSIZE*0] = tmp0 + tmp7;
168 0 : wsptr[DCTSIZE*7] = tmp0 - tmp7;
169 0 : wsptr[DCTSIZE*1] = tmp1 + tmp6;
170 0 : wsptr[DCTSIZE*6] = tmp1 - tmp6;
171 0 : wsptr[DCTSIZE*2] = tmp2 + tmp5;
172 0 : wsptr[DCTSIZE*5] = tmp2 - tmp5;
173 0 : wsptr[DCTSIZE*3] = tmp3 + tmp4;
174 0 : wsptr[DCTSIZE*4] = tmp3 - tmp4;
175 :
176 0 : inptr++; /* advance pointers to next column */
177 0 : quantptr++;
178 0 : wsptr++;
179 : }
180 :
181 : /* Pass 2: process rows from work array, store into output array. */
182 :
183 0 : wsptr = workspace;
184 0 : for (ctr = 0; ctr < DCTSIZE; ctr++) {
185 0 : outptr = output_buf[ctr] + output_col;
186 : /* Rows of zeroes can be exploited in the same way as we did with columns.
187 : * However, the column calculation has created many nonzero AC terms, so
188 : * the simplification applies less often (typically 5% to 10% of the time).
189 : * And testing floats for zero is relatively expensive, so we don't bother.
190 : */
191 :
192 : /* Even part */
193 :
194 : /* Apply signed->unsigned and prepare float->int conversion */
195 0 : z5 = wsptr[0] + ((FAST_FLOAT) CENTERJSAMPLE + (FAST_FLOAT) 0.5);
196 0 : tmp10 = z5 + wsptr[4];
197 0 : tmp11 = z5 - wsptr[4];
198 :
199 0 : tmp13 = wsptr[2] + wsptr[6];
200 0 : tmp12 = (wsptr[2] - wsptr[6]) * ((FAST_FLOAT) 1.414213562) - tmp13;
201 :
202 0 : tmp0 = tmp10 + tmp13;
203 0 : tmp3 = tmp10 - tmp13;
204 0 : tmp1 = tmp11 + tmp12;
205 0 : tmp2 = tmp11 - tmp12;
206 :
207 : /* Odd part */
208 :
209 0 : z13 = wsptr[5] + wsptr[3];
210 0 : z10 = wsptr[5] - wsptr[3];
211 0 : z11 = wsptr[1] + wsptr[7];
212 0 : z12 = wsptr[1] - wsptr[7];
213 :
214 0 : tmp7 = z11 + z13;
215 0 : tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562);
216 :
217 0 : z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */
218 0 : tmp10 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */
219 0 : tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 2*(c2+c6) */
220 :
221 0 : tmp6 = tmp12 - tmp7;
222 0 : tmp5 = tmp11 - tmp6;
223 0 : tmp4 = tmp10 - tmp5;
224 :
225 : /* Final output stage: float->int conversion and range-limit */
226 :
227 0 : outptr[0] = range_limit[((int) (tmp0 + tmp7)) & RANGE_MASK];
228 0 : outptr[7] = range_limit[((int) (tmp0 - tmp7)) & RANGE_MASK];
229 0 : outptr[1] = range_limit[((int) (tmp1 + tmp6)) & RANGE_MASK];
230 0 : outptr[6] = range_limit[((int) (tmp1 - tmp6)) & RANGE_MASK];
231 0 : outptr[2] = range_limit[((int) (tmp2 + tmp5)) & RANGE_MASK];
232 0 : outptr[5] = range_limit[((int) (tmp2 - tmp5)) & RANGE_MASK];
233 0 : outptr[3] = range_limit[((int) (tmp3 + tmp4)) & RANGE_MASK];
234 0 : outptr[4] = range_limit[((int) (tmp3 - tmp4)) & RANGE_MASK];
235 :
236 0 : wsptr += DCTSIZE; /* advance pointer to next row */
237 : }
238 0 : }
239 :
240 : #endif /* DCT_FLOAT_SUPPORTED */
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