| 1 | /*
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| 2 | * jfdctflt.c
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| 3 | *
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| 4 | * Copyright (C) 1994-1996, Thomas G. Lane.
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| 5 | * This file is part of the Independent JPEG Group's software.
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| 6 | * For conditions of distribution and use, see the accompanying README file.
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| 7 | *
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| 8 | * This file contains a floating-point implementation of the
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| 9 | * forward DCT (Discrete Cosine Transform).
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| 10 | *
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| 11 | * This implementation should be more accurate than either of the integer
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| 12 | * DCT implementations. However, it may not give the same results on all
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| 13 | * machines because of differences in roundoff behavior. Speed will depend
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| 14 | * on the hardware's floating point capacity.
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| 15 | *
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| 16 | * A 2-D DCT can be done by 1-D DCT on each row followed by 1-D DCT
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| 17 | * on each column. Direct algorithms are also available, but they are
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| 18 | * much more complex and seem not to be any faster when reduced to code.
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| 19 | *
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| 20 | * This implementation is based on Arai, Agui, and Nakajima's algorithm for
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| 21 | * scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in
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| 22 | * Japanese, but the algorithm is described in the Pennebaker & Mitchell
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| 23 | * JPEG textbook (see REFERENCES section in file README). The following code
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| 24 | * is based directly on figure 4-8 in P&M.
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| 25 | * While an 8-point DCT cannot be done in less than 11 multiplies, it is
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| 26 | * possible to arrange the computation so that many of the multiplies are
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| 27 | * simple scalings of the final outputs. These multiplies can then be
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| 28 | * folded into the multiplications or divisions by the JPEG quantization
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| 29 | * table entries. The AA&N method leaves only 5 multiplies and 29 adds
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| 30 | * to be done in the DCT itself.
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| 31 | * The primary disadvantage of this method is that with a fixed-point
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| 32 | * implementation, accuracy is lost due to imprecise representation of the
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| 33 | * scaled quantization values. However, that problem does not arise if
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| 34 | * we use floating point arithmetic.
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| 35 | */
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| 36 |
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| 37 | #define JPEG_INTERNALS
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| 38 | #include "jinclude.h"
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| 39 | #include "jpeglib.h"
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| 40 | #include "jdct.h" /* Private declarations for DCT subsystem */
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| 41 |
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| 42 | #ifdef DCT_FLOAT_SUPPORTED
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| 43 |
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| 44 |
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| 45 | /*
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| 46 | * This module is specialized to the case DCTSIZE = 8.
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| 47 | */
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| 48 |
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| 49 | #if DCTSIZE != 8
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| 50 | Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */
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| 51 | #endif
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| 52 |
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| 53 |
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| 54 | /*
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| 55 | * Perform the forward DCT on one block of samples.
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| 56 | */
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| 57 |
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| 58 | GLOBAL(void)
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| 59 | jpeg_fdct_float (FAST_FLOAT * data)
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| 60 | {
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| 61 | FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;
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| 62 | FAST_FLOAT tmp10, tmp11, tmp12, tmp13;
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| 63 | FAST_FLOAT z1, z2, z3, z4, z5, z11, z13;
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| 64 | FAST_FLOAT *dataptr;
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| 65 | int ctr;
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| 66 |
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| 67 | /* Pass 1: process rows. */
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| 68 |
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| 69 | dataptr = data;
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| 70 | for (ctr = DCTSIZE-1; ctr >= 0; ctr--) {
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| 71 | tmp0 = dataptr[0] + dataptr[7];
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| 72 | tmp7 = dataptr[0] - dataptr[7];
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| 73 | tmp1 = dataptr[1] + dataptr[6];
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| 74 | tmp6 = dataptr[1] - dataptr[6];
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| 75 | tmp2 = dataptr[2] + dataptr[5];
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| 76 | tmp5 = dataptr[2] - dataptr[5];
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| 77 | tmp3 = dataptr[3] + dataptr[4];
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| 78 | tmp4 = dataptr[3] - dataptr[4];
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| 79 |
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| 80 | /* Even part */
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| 81 |
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| 82 | tmp10 = tmp0 + tmp3; /* phase 2 */
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| 83 | tmp13 = tmp0 - tmp3;
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| 84 | tmp11 = tmp1 + tmp2;
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| 85 | tmp12 = tmp1 - tmp2;
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| 86 |
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| 87 | dataptr[0] = tmp10 + tmp11; /* phase 3 */
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