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src/Source/LibPNG/png.c  view on Meta::CPAN

   if (r <= 2147483647. && r >= -2147483648.)
      return (png_fixed_point)r;
#else
   /* This may overflow because the range of png_fixed_point isn't symmetric,
    * but this API is only used for the product of file and screen gamma so it
    * doesn't matter that the smallest number it can produce is 1/21474, not
    * 1/100000
    */
   png_fixed_point res = png_product2(a, b);

   if (res != 0)
      return png_reciprocal(res);
#endif

   return 0; /* overflow */
}
#endif /* READ_GAMMA */

#ifdef PNG_READ_GAMMA_SUPPORTED /* gamma table code */
#ifndef PNG_FLOATING_ARITHMETIC_SUPPORTED
/* Fixed point gamma.
 *
 * The code to calculate the tables used below can be found in the shell script
 * contrib/tools/intgamma.sh
 *
 * To calculate gamma this code implements fast log() and exp() calls using only
 * fixed point arithmetic.  This code has sufficient precision for either 8-bit
 * or 16-bit sample values.
 *
 * The tables used here were calculated using simple 'bc' programs, but C double
 * precision floating point arithmetic would work fine.
 *
 * 8-bit log table
 *   This is a table of -log(value/255)/log(2) for 'value' in the range 128 to
 *   255, so it's the base 2 logarithm of a normalized 8-bit floating point
 *   mantissa.  The numbers are 32-bit fractions.
 */
static const png_uint_32
png_8bit_l2[128] =
{
   4270715492U, 4222494797U, 4174646467U, 4127164793U, 4080044201U, 4033279239U,
   3986864580U, 3940795015U, 3895065449U, 3849670902U, 3804606499U, 3759867474U,
   3715449162U, 3671346997U, 3627556511U, 3584073329U, 3540893168U, 3498011834U,
   3455425220U, 3413129301U, 3371120137U, 3329393864U, 3287946700U, 3246774933U,
   3205874930U, 3165243125U, 3124876025U, 3084770202U, 3044922296U, 3005329011U,
   2965987113U, 2926893432U, 2888044853U, 2849438323U, 2811070844U, 2772939474U,
   2735041326U, 2697373562U, 2659933400U, 2622718104U, 2585724991U, 2548951424U,
   2512394810U, 2476052606U, 2439922311U, 2404001468U, 2368287663U, 2332778523U,
   2297471715U, 2262364947U, 2227455964U, 2192742551U, 2158222529U, 2123893754U,
   2089754119U, 2055801552U, 2022034013U, 1988449497U, 1955046031U, 1921821672U,
   1888774511U, 1855902668U, 1823204291U, 1790677560U, 1758320682U, 1726131893U,
   1694109454U, 1662251657U, 1630556815U, 1599023271U, 1567649391U, 1536433567U,
   1505374214U, 1474469770U, 1443718700U, 1413119487U, 1382670639U, 1352370686U,
   1322218179U, 1292211689U, 1262349810U, 1232631153U, 1203054352U, 1173618059U,
   1144320946U, 1115161701U, 1086139034U, 1057251672U, 1028498358U, 999877854U,
   971388940U, 943030410U, 914801076U, 886699767U, 858725327U, 830876614U,
   803152505U, 775551890U, 748073672U, 720716771U, 693480120U, 666362667U,
   639363374U, 612481215U, 585715177U, 559064263U, 532527486U, 506103872U,
   479792461U, 453592303U, 427502463U, 401522014U, 375650043U, 349885648U,
   324227938U, 298676034U, 273229066U, 247886176U, 222646516U, 197509248U,
   172473545U, 147538590U, 122703574U, 97967701U, 73330182U, 48790236U,
   24347096U, 0U

#if 0
   /* The following are the values for 16-bit tables - these work fine for the
    * 8-bit conversions but produce very slightly larger errors in the 16-bit
    * log (about 1.2 as opposed to 0.7 absolute error in the final value).  To
    * use these all the shifts below must be adjusted appropriately.
    */
   65166, 64430, 63700, 62976, 62257, 61543, 60835, 60132, 59434, 58741, 58054,
   57371, 56693, 56020, 55352, 54689, 54030, 53375, 52726, 52080, 51439, 50803,
   50170, 49542, 48918, 48298, 47682, 47070, 46462, 45858, 45257, 44661, 44068,
   43479, 42894, 42312, 41733, 41159, 40587, 40020, 39455, 38894, 38336, 37782,
   37230, 36682, 36137, 35595, 35057, 34521, 33988, 33459, 32932, 32408, 31887,
   31369, 30854, 30341, 29832, 29325, 28820, 28319, 27820, 27324, 26830, 26339,
   25850, 25364, 24880, 24399, 23920, 23444, 22970, 22499, 22029, 21562, 21098,
   20636, 20175, 19718, 19262, 18808, 18357, 17908, 17461, 17016, 16573, 16132,
   15694, 15257, 14822, 14390, 13959, 13530, 13103, 12678, 12255, 11834, 11415,
   10997, 10582, 10168, 9756, 9346, 8937, 8531, 8126, 7723, 7321, 6921, 6523,
   6127, 5732, 5339, 4947, 4557, 4169, 3782, 3397, 3014, 2632, 2251, 1872, 1495,
   1119, 744, 372
#endif
};

static png_int_32
png_log8bit(unsigned int x)
{
   unsigned int lg2 = 0;
   /* Each time 'x' is multiplied by 2, 1 must be subtracted off the final log,
    * because the log is actually negate that means adding 1.  The final
    * returned value thus has the range 0 (for 255 input) to 7.994 (for 1
    * input), return -1 for the overflow (log 0) case, - so the result is
    * always at most 19 bits.
    */
   if ((x &= 0xff) == 0)
      return -1;

   if ((x & 0xf0) == 0)
      lg2  = 4, x <<= 4;

   if ((x & 0xc0) == 0)
      lg2 += 2, x <<= 2;

   if ((x & 0x80) == 0)
      lg2 += 1, x <<= 1;

   /* result is at most 19 bits, so this cast is safe: */
   return (png_int_32)((lg2 << 16) + ((png_8bit_l2[x-128]+32768)>>16));
}

/* The above gives exact (to 16 binary places) log2 values for 8-bit images,
 * for 16-bit images we use the most significant 8 bits of the 16-bit value to
 * get an approximation then multiply the approximation by a correction factor
 * determined by the remaining up to 8 bits.  This requires an additional step
 * in the 16-bit case.
 *
 * We want log2(value/65535), we have log2(v'/255), where:
 *
 *    value = v' * 256 + v''
 *          = v' * f

src/Source/LibPNG/png.c  view on Meta::CPAN

   if ((x & 0xf000) == 0)
      lg2 += 4, x <<= 4;

   if ((x & 0xc000) == 0)
      lg2 += 2, x <<= 2;

   if ((x & 0x8000) == 0)
      lg2 += 1, x <<= 1;

   /* Calculate the base logarithm from the top 8 bits as a 28-bit fractional
    * value.
    */
   lg2 <<= 28;
   lg2 += (png_8bit_l2[(x>>8)-128]+8) >> 4;

   /* Now we need to interpolate the factor, this requires a division by the top
    * 8 bits.  Do this with maximum precision.
    */
   x = ((x << 16) + (x >> 9)) / (x >> 8);

   /* Since we divided by the top 8 bits of 'x' there will be a '1' at 1<<24,
    * the value at 1<<16 (ignoring this) will be 0 or 1; this gives us exactly
    * 16 bits to interpolate to get the low bits of the result.  Round the
    * answer.  Note that the end point values are scaled by 64 to retain overall
    * precision and that 'lg2' is current scaled by an extra 12 bits, so adjust
    * the overall scaling by 6-12.  Round at every step.
    */
   x -= 1U << 24;

   if (x <= 65536U) /* <= '257' */
      lg2 += ((23591U * (65536U-x)) + (1U << (16+6-12-1))) >> (16+6-12);

   else
      lg2 -= ((23499U * (x-65536U)) + (1U << (16+6-12-1))) >> (16+6-12);

   /* Safe, because the result can't have more than 20 bits: */
   return (png_int_32)((lg2 + 2048) >> 12);
}
#endif /* 16BIT */

/* The 'exp()' case must invert the above, taking a 20-bit fixed point
 * logarithmic value and returning a 16 or 8-bit number as appropriate.  In
 * each case only the low 16 bits are relevant - the fraction - since the
 * integer bits (the top 4) simply determine a shift.
 *
 * The worst case is the 16-bit distinction between 65535 and 65534. This
 * requires perhaps spurious accuracy in the decoding of the logarithm to
 * distinguish log2(65535/65534.5) - 10^-5 or 17 bits.  There is little chance
 * of getting this accuracy in practice.
 *
 * To deal with this the following exp() function works out the exponent of the
 * frational part of the logarithm by using an accurate 32-bit value from the
 * top four fractional bits then multiplying in the remaining bits.
 */
static const png_uint_32
png_32bit_exp[16] =
{
   /* NOTE: the first entry is deliberately set to the maximum 32-bit value. */
   4294967295U, 4112874773U, 3938502376U, 3771522796U, 3611622603U, 3458501653U,
   3311872529U, 3171459999U, 3037000500U, 2908241642U, 2784941738U, 2666869345U,
   2553802834U, 2445529972U, 2341847524U, 2242560872U
};

/* Adjustment table; provided to explain the numbers in the code below. */
#if 0
for (i=11;i>=0;--i){ print i, " ", (1 - e(-(2^i)/65536*l(2))) * 2^(32-i), "\n"}
   11 44937.64284865548751208448
   10 45180.98734845585101160448
    9 45303.31936980687359311872
    8 45364.65110595323018870784
    7 45395.35850361789624614912
    6 45410.72259715102037508096
    5 45418.40724413220722311168
    4 45422.25021786898173001728
    3 45424.17186732298419044352
    2 45425.13273269940811464704
    1 45425.61317555035558641664
    0 45425.85339951654943850496
#endif

static png_uint_32
png_exp(png_fixed_point x)
{
   if (x > 0 && x <= 0xfffff) /* Else overflow or zero (underflow) */
   {
      /* Obtain a 4-bit approximation */
      png_uint_32 e = png_32bit_exp[(x >> 12) & 0xf];

      /* Incorporate the low 12 bits - these decrease the returned value by
       * multiplying by a number less than 1 if the bit is set.  The multiplier
       * is determined by the above table and the shift. Notice that the values
       * converge on 45426 and this is used to allow linear interpolation of the
       * low bits.
       */
      if (x & 0x800)
         e -= (((e >> 16) * 44938U) +  16U) >> 5;

      if (x & 0x400)
         e -= (((e >> 16) * 45181U) +  32U) >> 6;

      if (x & 0x200)
         e -= (((e >> 16) * 45303U) +  64U) >> 7;

      if (x & 0x100)
         e -= (((e >> 16) * 45365U) + 128U) >> 8;

      if (x & 0x080)
         e -= (((e >> 16) * 45395U) + 256U) >> 9;

      if (x & 0x040)
         e -= (((e >> 16) * 45410U) + 512U) >> 10;

      /* And handle the low 6 bits in a single block. */
      e -= (((e >> 16) * 355U * (x & 0x3fU)) + 256U) >> 9;

      /* Handle the upper bits of x. */
      e >>= x >> 16;
      return e;
   }

   /* Check for overflow */
   if (x <= 0)
      return png_32bit_exp[0];

   /* Else underflow */
   return 0;
}

static png_byte
png_exp8bit(png_fixed_point lg2)
{
   /* Get a 32-bit value: */
   png_uint_32 x = png_exp(lg2);