/*

* jidctflt.c

*

* Copyright (C) 1994-1998, Thomas G. Lane.

* Modified 2010 by Guido Vollbeding.

* This file is part of the Independent JPEG Group's software.

* For conditions of distribution and use, see the accompanying README file.

*

* This file contains a floating-point implementation of the

* inverse DCT (Discrete Cosine Transform). In the IJG code, this routine

* must also perform dequantization of the input coefficients.

*

* This implementation should be more accurate than either of the integer

* IDCT implementations. However, it may not give the same results on all

* machines because of differences in roundoff behavior. Speed will depend

* on the hardware's floating point capacity.

*

* A 2-D IDCT can be done by 1-D IDCT on each column followed by 1-D IDCT

* on each row (or vice versa, but it's more convenient to emit a row at

* a time). Direct algorithms are also available, but they are much more

* complex and seem not to be any faster when reduced to code.

*

* This implementation is based on Arai, Agui, and Nakajima's algorithm for

* scaled DCT. Their original paper (Trans. IEICE E-71(11):1095) is in

* Japanese, but the algorithm is described in the Pennebaker & Mitchell

* JPEG textbook (see REFERENCES section in file README). The following code

* is based directly on figure 4-8 in P&M.

* While an 8-point DCT cannot be done in less than 11 multiplies, it is

* possible to arrange the computation so that many of the multiplies are

* simple scalings of the final outputs. These multiplies can then be

* folded into the multiplications or divisions by the JPEG quantization

* table entries. The AA&N method leaves only 5 multiplies and 29 adds

* to be done in the DCT itself.

* The primary disadvantage of this method is that with a fixed-point

* implementation, accuracy is lost due to imprecise representation of the

* scaled quantization values. However, that problem does not arise if

* we use floating point arithmetic.

*/

#define JPEG_INTERNALS

#include "jinclude.h"

#include "jpeglib.h"

#include "jdct.h" /* Private declarations for DCT subsystem */

#ifdef DCT_FLOAT_SUPPORTED

/*

* This module is specialized to the case DCTSIZE = 8.

*/

#if DCTSIZE != 8

Sorry, this code only copes with 8x8 DCTs. /* deliberate syntax err */

#endif

/* Dequantize a coefficient by multiplying it by the multiplier-table

* entry; produce a float result.

*/

#define DEQUANTIZE(coef,quantval) (((FAST_FLOAT) (coef)) * (quantval))

/*

* Perform dequantization and inverse DCT on one block of coefficients.

*/

GLOBAL(void)

jpeg_idct_float (j_decompress_ptr cinfo, jpeg_component_info * compptr,

JCOEFPTR coef_block,

JSAMPARRAY output_buf, JDIMENSION output_col)

{

FAST_FLOAT tmp0, tmp1, tmp2, tmp3, tmp4, tmp5, tmp6, tmp7;

FAST_FLOAT tmp10, tmp11, tmp12, tmp13;

FAST_FLOAT z5, z10, z11, z12, z13;

JCOEFPTR inptr;

FLOAT_MULT_TYPE * quantptr;

FAST_FLOAT * wsptr;

JSAMPROW outptr;

JSAMPLE *range_limit = cinfo->sample_range_limit;

int ctr;

FAST_FLOAT workspace[DCTSIZE2]; /* buffers data between passes */

/* Pass 1: process columns from input, store into work array. */

inptr = coef_block;

quantptr = (FLOAT_MULT_TYPE *) compptr->dct_table;

wsptr = workspace;

for (ctr = DCTSIZE; ctr > 0; ctr--) {

/* Due to quantization, we will usually find that many of the input

* coefficients are zero, especially the AC terms. We can exploit this

* by short-circuiting the IDCT calculation for any column in which all

* the AC terms are zero. In that case each output is equal to the

* DC coefficient (with scale factor as needed).

* With typical images and quantization tables, half or more of the

* column DCT calculations can be simplified this way.

*/

if (inptr[DCTSIZE*1] == 0 && inptr[DCTSIZE*2] == 0 &&

inptr[DCTSIZE*3] == 0 && inptr[DCTSIZE*4] == 0 &&

inptr[DCTSIZE*5] == 0 && inptr[DCTSIZE*6] == 0 &&

inptr[DCTSIZE*7] == 0) {

/* AC terms all zero */

FAST_FLOAT dcval = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]);

wsptr[DCTSIZE*0] = dcval;

wsptr[DCTSIZE*1] = dcval;

wsptr[DCTSIZE*2] = dcval;

wsptr[DCTSIZE*3] = dcval;

wsptr[DCTSIZE*4] = dcval;

wsptr[DCTSIZE*5] = dcval;

wsptr[DCTSIZE*6] = dcval;

wsptr[DCTSIZE*7] = dcval;

inptr++; /* advance pointers to next column */

quantptr++;

wsptr++;

continue;

}

/* Even part */

tmp0 = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]);

tmp1 = DEQUANTIZE(inptr[DCTSIZE*2], quantptr[DCTSIZE*2]);

tmp2 = DEQUANTIZE(inptr[DCTSIZE*4], quantptr[DCTSIZE*4]);

tmp3 = DEQUANTIZE(inptr[DCTSIZE*6], quantptr[DCTSIZE*6]);

tmp10 = tmp0 + tmp2; /* phase 3 */

tmp11 = tmp0 - tmp2;

tmp13 = tmp1 + tmp3; /* phases 5-3 */

tmp12 = (tmp1 - tmp3) * ((FAST_FLOAT) 1.414213562) - tmp13; /* 2*c4 */

tmp0 = tmp10 + tmp13; /* phase 2 */

tmp3 = tmp10 - tmp13;

tmp1 = tmp11 + tmp12;

tmp2 = tmp11 - tmp12;

/* Odd part */

tmp4 = DEQUANTIZE(inptr[DCTSIZE*1], quantptr[DCTSIZE*1]);

tmp5 = DEQUANTIZE(inptr[DCTSIZE*3], quantptr[DCTSIZE*3]);

tmp6 = DEQUANTIZE(inptr[DCTSIZE*5], quantptr[DCTSIZE*5]);

tmp7 = DEQUANTIZE(inptr[DCTSIZE*7], quantptr[DCTSIZE*7]);

z13 = tmp6 + tmp5; /* phase 6 */

z10 = tmp6 - tmp5;

z11 = tmp4 + tmp7;

z12 = tmp4 - tmp7;

tmp7 = z11 + z13; /* phase 5 */

tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562); /* 2*c4 */

z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */

tmp10 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */

tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 2*(c2+c6) */

tmp6 = tmp12 - tmp7; /* phase 2 */

tmp5 = tmp11 - tmp6;

tmp4 = tmp10 - tmp5;

wsptr[DCTSIZE*0] = tmp0 + tmp7;

wsptr[DCTSIZE*7] = tmp0 - tmp7;

wsptr[DCTSIZE*1] = tmp1 + tmp6;

wsptr[DCTSIZE*6] = tmp1 - tmp6;

wsptr[DCTSIZE*2] = tmp2 + tmp5;

wsptr[DCTSIZE*5] = tmp2 - tmp5;

wsptr[DCTSIZE*3] = tmp3 + tmp4;

wsptr[DCTSIZE*4] = tmp3 - tmp4;

inptr++; /* advance pointers to next column */

quantptr++;

wsptr++;

}

/* Pass 2: process rows from work array, store into output array. */

wsptr = workspace;

for (ctr = 0; ctr < DCTSIZE; ctr++) {

outptr = output_buf[ctr] + output_col;

/* Rows of zeroes can be exploited in the same way as we did with columns.

* However, the column calculation has created many nonzero AC terms, so

* the simplification applies less often (typically 5% to 10% of the time).

* And testing floats for zero is relatively expensive, so we don't bother.

*/

/* Even part */

/* Apply signed->unsigned and prepare float->int conversion */

z5 = wsptr[0] + ((FAST_FLOAT) CENTERJSAMPLE + (FAST_FLOAT) 0.5);

tmp10 = z5 + wsptr[4];

tmp11 = z5 - wsptr[4];

tmp13 = wsptr[2] + wsptr[6];

tmp12 = (wsptr[2] - wsptr[6]) * ((FAST_FLOAT) 1.414213562) - tmp13;

tmp0 = tmp10 + tmp13;

tmp3 = tmp10 - tmp13;

tmp1 = tmp11 + tmp12;

tmp2 = tmp11 - tmp12;

/* Odd part */

z13 = wsptr[5] + wsptr[3];

z10 = wsptr[5] - wsptr[3];

z11 = wsptr[1] + wsptr[7];

z12 = wsptr[1] - wsptr[7];

tmp7 = z11 + z13;

tmp11 = (z11 - z13) * ((FAST_FLOAT) 1.414213562);

z5 = (z10 + z12) * ((FAST_FLOAT) 1.847759065); /* 2*c2 */

tmp10 = z5 - z12 * ((FAST_FLOAT) 1.082392200); /* 2*(c2-c6) */

tmp12 = z5 - z10 * ((FAST_FLOAT) 2.613125930); /* 2*(c2+c6) */

tmp6 = tmp12 - tmp7;

tmp5 = tmp11 - tmp6;

tmp4 = tmp10 - tmp5;

/* Final output stage: float->int conversion and range-limit */

outptr[0] = range_limit[((int) (tmp0 + tmp7)) & RANGE_MASK];

outptr[7] = range_limit[((int) (tmp0 - tmp7)) & RANGE_MASK];

outptr[1] = range_limit[((int) (tmp1 + tmp6)) & RANGE_MASK];

outptr[6] = range_limit[((int) (tmp1 - tmp6)) & RANGE_MASK];

outptr[2] = range_limit[((int) (tmp2 + tmp5)) & RANGE_MASK];

outptr[5] = range_limit[((int) (tmp2 - tmp5)) & RANGE_MASK];

outptr[3] = range_limit[((int) (tmp3 + tmp4)) & RANGE_MASK];

outptr[4] = range_limit[((int) (tmp3 - tmp4)) & RANGE_MASK];

wsptr += DCTSIZE; /* advance pointer to next row */

}

}

#endif /* DCT_FLOAT_SUPPORTED */