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/*
 * jidctint.c
 *
 * Copyright (C) 1991-1998, Thomas G. Lane.
 * 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 slow-but-accurate integer implementation of the
 * inverse DCT (Discrete Cosine Transform).  In the IJG code, this routine
 * must also perform dequantization of the input coefficients.
 *
 * 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 an algorithm described in
 *   C. Loeffler, A. Ligtenberg and G. Moschytz, "Practical Fast 1-D DCT
 *   Algorithms with 11 Multiplications", Proc. Int'l. Conf. on Acoustics,
 *   Speech, and Signal Processing 1989 (ICASSP '89), pp. 988-991.
 * The primary algorithm described there uses 11 multiplies and 29 adds.
 * We use their alternate method with 12 multiplies and 32 adds.
 * The advantage of this method is that no data path contains more than one
 * multiplication; this allows a very simple and accurate implementation in
 * scaled fixed-point arithmetic, with a minimal number of shifts.
 */

#include "GenericTypeDefs.h"


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

/*
 * The poop on this scaling stuff is as follows:
 *
 * Each 1-D IDCT step produces outputs which are a factor of sqrt(N)
 * larger than the true IDCT outputs.  The final outputs are therefore
 * a factor of N larger than desired; since N=8 this can be cured by
 * a simple right shift at the end of the algorithm.  The advantage of
 * this arrangement is that we save two multiplications per 1-D IDCT,
 * because the y0 and y4 inputs need not be divided by sqrt(N).
 *
 * We have to do addition and subtraction of the integer inputs, which
 * is no problem, and multiplication by fractional constants, which is
 * a problem to do in integer arithmetic.  We multiply all the constants
 * by CONST_SCALE and convert them to integer constants (thus retaining
 * CONST_BITS bits of precision in the constants).  After doing a
 * multiplication we have to divide the product by CONST_SCALE, with proper
 * rounding, to produce the correct output.  This division can be done
 * cheaply as a right shift of CONST_BITS bits.  We postpone shifting
 * as long as possible so that partial sums can be added together with
 * full fractional precision.
 *
 * The outputs of the first pass are scaled up by PASS1_BITS bits so that
 * they are represented to better-than-integral precision.  These outputs
 * require BITS_IN_JSAMPLE + PASS1_BITS + 3 bits; this fits in a 16-bit word
 * with the recommended scaling.  (To scale up 12-bit sample data further, an
 * intermediate INT32 array would be needed.)
 *
 * To avoid overflow of the 32-bit intermediate results in pass 2, we must
 * have BITS_IN_JSAMPLE + CONST_BITS + PASS1_BITS <= 26.  Error analysis
 * shows that the values given below are the most effective.
 */
#define DCTSIZE             8   /* The basic DCT block is 8x8 samples */
#define DCTSIZE2            64  /* DCTSIZE squared; # of elements in a block */
#define BITS_IN_JSAMPLE  8
#define NO_ZERO_ROW_TEST
#define DCTELEM LONG

#define CONST_BITS  13
#define PASS1_BITS  2

#define INT32 LONG

/* Some C compilers fail to reduce "FIX(constant)" at compile time, thus
 * causing a lot of useless floating-point operations at run time.
 * To get around this we use the following pre-calculated constants.
 * If you change CONST_BITS you may want to add appropriate values.
 * (With a reasonable C compiler, you can just rely on the FIX() macro...)
 */

#define FIX_0_298631336  ((INT32)  2446)        /* FIX(0.298631336) */
#define FIX_0_390180644  ((INT32)  3196)        /* FIX(0.390180644) */
#define FIX_0_541196100  ((INT32)  4433)        /* FIX(0.541196100) */
#define FIX_0_765366865  ((INT32)  6270)        /* FIX(0.765366865) */
#define FIX_0_899976223  ((INT32)  7373)        /* FIX(0.899976223) */
#define FIX_1_175875602  ((INT32)  9633)        /* FIX(1.175875602) */
#define FIX_1_501321110  ((INT32)  12299)       /* FIX(1.501321110) */
#define FIX_1_847759065  ((INT32)  15137)       /* FIX(1.847759065) */
#define FIX_1_961570560  ((INT32)  16069)       /* FIX(1.961570560) */
#define FIX_2_053119869  ((INT32)  16819)       /* FIX(2.053119869) */
#define FIX_2_562915447  ((INT32)  20995)       /* FIX(2.562915447) */
#define FIX_3_072711026  ((INT32)  25172)       /* FIX(3.072711026) */


/* Multiply an INT32 variable by an INT32 constant to yield an INT32 result.
 * For 8-bit samples with the recommended scaling, all the variable
 * and constant values involved are no more than 16 bits wide, so a
 * 16x16->32 bit multiply can be used instead of a full 32x32 multiply.
 * For 12-bit samples, a full 32-bit multiplication will be needed.
 */

#define DESCALE(x,n)  ((x) + ((LONG)0x01 << ((n)-1)))>>(n)
#define MULTIPLY(var,constant)  ((LONG)(var) * (constant));
#define range_limit(x) ((x)<-128)?-128:((x)>127)?127:(x)

/* Dequantize a coefficient by multiplying it by the multiplier-table
 * entry; produce an int result.  In this module, both inputs and result
 * are 16 bits or less, so either int or short multiply will work.
 */

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


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

void jpeg_idct_islow (SHORT *inbuf, WORD *quantptr)
{
  LONG tmp0, tmp1, tmp2, tmp3;
  LONG tmp10, tmp11, tmp12, tmp13;
  LONG z1, z2, z3, z4, z5;

  BYTE ctr;
  SHORT *inptr = inbuf, *outptr;
  DCTELEM *wsptr;
  DCTELEM workspace[DCTSIZE2];  /* buffers data between passes */

  wsptr = workspace;

  /* Pass 1: process columns from input, store into work array. */
  /* Note results are scaled up by sqrt(8) compared to a true IDCT; */
  /* furthermore, we scale the results by 2**PASS1_BITS. */

  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 */
      LONG dcval = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]) << PASS1_BITS;
      
      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: reverse the even part of the forward DCT. */
    /* The rotator is sqrt(2)*c(-6). */
    
    z2 = DEQUANTIZE(inptr[DCTSIZE*2], quantptr[DCTSIZE*2]);
    z3 = DEQUANTIZE(inptr[DCTSIZE*6], quantptr[DCTSIZE*6]);
    
    z1 = MULTIPLY(z2 + z3, FIX_0_541196100);
    tmp2 = z1 + MULTIPLY(z3, - FIX_1_847759065);
    tmp3 = z1 + MULTIPLY(z2, FIX_0_765366865);
    
    z2 = DEQUANTIZE(inptr[DCTSIZE*0], quantptr[DCTSIZE*0]);
    z3 = DEQUANTIZE(inptr[DCTSIZE*4], quantptr[DCTSIZE*4]);

    tmp0 = (z2 + z3) << CONST_BITS;
    tmp1 = (z2 - z3) << CONST_BITS;
    
    tmp10 = tmp0 + tmp3;
    tmp13 = tmp0 - tmp3;
    tmp11 = tmp1 + tmp2;
    tmp12 = tmp1 - tmp2;
    
    /* Odd part per figure 8; the matrix is unitary and hence its
     * transpose is its inverse.  i0..i3 are y7,y5,y3,y1 respectively.
     */
    
    tmp0 = DEQUANTIZE(inptr[DCTSIZE*7], quantptr[DCTSIZE*7]);
    tmp1 = DEQUANTIZE(inptr[DCTSIZE*5], quantptr[DCTSIZE*5]);
    tmp2 = DEQUANTIZE(inptr[DCTSIZE*3], quantptr[DCTSIZE*3]);
    tmp3 = DEQUANTIZE(inptr[DCTSIZE*1], quantptr[DCTSIZE*1]);
    
    z1 = tmp0 + tmp3;
    z2 = tmp1 + tmp2;
    z3 = tmp0 + tmp2;
    z4 = tmp1 + tmp3;
    z5 = MULTIPLY(z3 + z4, FIX_1_175875602); /* sqrt(2) * c3 */
    
    tmp0 = MULTIPLY(tmp0, FIX_0_298631336); /* sqrt(2) * (-c1+c3+c5-c7) */
    tmp1 = MULTIPLY(tmp1, FIX_2_053119869); /* sqrt(2) * ( c1+c3-c5+c7) */
    tmp2 = MULTIPLY(tmp2, FIX_3_072711026); /* sqrt(2) * ( c1+c3+c5-c7) */
    tmp3 = MULTIPLY(tmp3, FIX_1_501321110); /* sqrt(2) * ( c1+c3-c5-c7) */
    z1 = MULTIPLY(z1, - FIX_0_899976223); /* sqrt(2) * (c7-c3) */
    z2 = MULTIPLY(z2, - FIX_2_562915447); /* sqrt(2) * (-c1-c3) */
    z3 = MULTIPLY(z3, - FIX_1_961570560); /* sqrt(2) * (-c3-c5) */
    z4 = MULTIPLY(z4, - FIX_0_390180644); /* sqrt(2) * (c5-c3) */
    
    z3 += z5;
    z4 += z5;
    
    tmp0 += z1 + z3;
    tmp1 += z2 + z4;
    tmp2 += z2 + z3;
    tmp3 += z1 + z4;
    
    /* Final output stage: inputs are tmp10..tmp13, tmp0..tmp3 */
    
    wsptr[DCTSIZE*0] = (LONG) DESCALE((tmp10 + tmp3), (CONST_BITS-PASS1_BITS));
    wsptr[DCTSIZE*7] = (LONG) DESCALE((tmp10 - tmp3), (CONST_BITS-PASS1_BITS));
    wsptr[DCTSIZE*1] = (LONG) DESCALE((tmp11 + tmp2), (CONST_BITS-PASS1_BITS));
    wsptr[DCTSIZE*6] = (LONG) DESCALE((tmp11 - tmp2), (CONST_BITS-PASS1_BITS));
    wsptr[DCTSIZE*2] = (LONG) DESCALE((tmp12 + tmp1), (CONST_BITS-PASS1_BITS));
    wsptr[DCTSIZE*5] = (LONG) DESCALE((tmp12 - tmp1), (CONST_BITS-PASS1_BITS));
    wsptr[DCTSIZE*3] = (LONG) DESCALE((tmp13 + tmp0), (CONST_BITS-PASS1_BITS));
    wsptr[DCTSIZE*4] = (LONG) DESCALE((tmp13 - tmp0), (CONST_BITS-PASS1_BITS));
    
    inptr++;                    /* advance pointers to next column */
    quantptr++;
    wsptr++;
  }
  
  /* Pass 2: process rows from work array, store into output array. */
  /* Note that we must descale the results by a factor of 8 == 2**3, */
  /* and also undo the PASS1_BITS scaling. */

  wsptr = workspace;
  outptr = &inbuf[0];
  for (ctr = 0; ctr < DCTSIZE; ctr++) {
    /* 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).
     * On machines with very fast multiplication, it's possible that the
     * test takes more time than it's worth.  In that case this section
     * may be commented out.
     */
    
#ifndef NO_ZERO_ROW_TEST
    if (wsptr[1] == 0 && wsptr[2] == 0 && wsptr[3] == 0 && wsptr[4] == 0 &&
        wsptr[5] == 0 && wsptr[6] == 0 && wsptr[7] == 0) {
      /* AC terms all zero */
      JSAMPLE dcval = range_limit[(LONG) DESCALE((INT32) wsptr[0], PASS1_BITS+3)
                                  & RANGE_MASK];
      
      outptr[0] = dcval;
      outptr[1] = dcval;
      outptr[2] = dcval;
      outptr[3] = dcval;
      outptr[4] = dcval;
      outptr[5] = dcval;
      outptr[6] = dcval;
      outptr[7] = dcval;

      wsptr += DCTSIZE;         /* advance pointer to next row */
      continue;
    }
#endif
    
    /* Even part: reverse the even part of the forward DCT. */
    /* The rotator is sqrt(2)*c(-6). */
    
    z2 = (INT32) wsptr[2];
    z3 = (INT32) wsptr[6];
    
    z1 = MULTIPLY(z2 + z3, FIX_0_541196100);
    tmp2 = z1 + MULTIPLY(z3, - FIX_1_847759065);
    tmp3 = z1 + MULTIPLY(z2, FIX_0_765366865);
    
    tmp0 = ((INT32) wsptr[0] + (INT32) wsptr[4]) << CONST_BITS;
    tmp1 = ((INT32) wsptr[0] - (INT32) wsptr[4]) << CONST_BITS;
    
    tmp10 = tmp0 + tmp3;
    tmp13 = tmp0 - tmp3;
    tmp11 = tmp1 + tmp2;
    tmp12 = tmp1 - tmp2;
    
    /* Odd part per figure 8; the matrix is unitary and hence its
     * transpose is its inverse.  i0..i3 are y7,y5,y3,y1 respectively.
     */
    
    tmp0 = (INT32) wsptr[7];
    tmp1 = (INT32) wsptr[5];
    tmp2 = (INT32) wsptr[3];
    tmp3 = (INT32) wsptr[1];
    
    z1 = tmp0 + tmp3;
    z2 = tmp1 + tmp2;
    z3 = tmp0 + tmp2;
    z4 = tmp1 + tmp3;
    z5 = MULTIPLY(z3 + z4, FIX_1_175875602); /* sqrt(2) * c3 */
    
    tmp0 = MULTIPLY(tmp0, FIX_0_298631336); /* sqrt(2) * (-c1+c3+c5-c7) */
    tmp1 = MULTIPLY(tmp1, FIX_2_053119869); /* sqrt(2) * ( c1+c3-c5+c7) */
    tmp2 = MULTIPLY(tmp2, FIX_3_072711026); /* sqrt(2) * ( c1+c3+c5-c7) */
    tmp3 = MULTIPLY(tmp3, FIX_1_501321110); /* sqrt(2) * ( c1+c3-c5-c7) */
    z1 = MULTIPLY(z1, - FIX_0_899976223); /* sqrt(2) * (c7-c3) */
    z2 = MULTIPLY(z2, - FIX_2_562915447); /* sqrt(2) * (-c1-c3) */
    z3 = MULTIPLY(z3, - FIX_1_961570560); /* sqrt(2) * (-c3-c5) */
    z4 = MULTIPLY(z4, - FIX_0_390180644); /* sqrt(2) * (c5-c3) */
    
    z3 += z5;
    z4 += z5;
    
    tmp0 += z1 + z3;
    tmp1 += z2 + z4;
    tmp2 += z2 + z3;
    tmp3 += z1 + z4;
    
    /* Final output stage: inputs are tmp10..tmp13, tmp0..tmp3 */
    
    outptr[0] = (SHORT)range_limit((LONG) DESCALE(tmp10 + tmp3, CONST_BITS+PASS1_BITS+3));
    outptr[7] = (SHORT)range_limit((LONG) DESCALE(tmp10 - tmp3, CONST_BITS+PASS1_BITS+3));
    outptr[1] = (SHORT)range_limit((LONG) DESCALE(tmp11 + tmp2, CONST_BITS+PASS1_BITS+3));
    outptr[6] = (SHORT)range_limit((LONG) DESCALE(tmp11 - tmp2, CONST_BITS+PASS1_BITS+3));
    outptr[2] = (SHORT)range_limit((LONG) DESCALE(tmp12 + tmp1, CONST_BITS+PASS1_BITS+3));
    outptr[5] = (SHORT)range_limit((LONG) DESCALE(tmp12 - tmp1, CONST_BITS+PASS1_BITS+3));
    outptr[3] = (SHORT)range_limit((LONG) DESCALE(tmp13 + tmp0, CONST_BITS+PASS1_BITS+3));
    outptr[4] = (SHORT)range_limit((LONG) DESCALE(tmp13 - tmp0, CONST_BITS+PASS1_BITS+3));
    
    outptr += DCTSIZE;   /* advance pointer to next row */
    wsptr += DCTSIZE;           /* advance pointer to next row */
  }
}
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