source/libs/libpng/libpng-src/contrib/tools/intgamma.sh

   1 #!/bin/sh
   2 #
   3 # intgamma.sh
   4 #
   5 # Last changed in libpng 1.6.0 [February 14, 2013]
   6 #
   7 # COPYRIGHT: Written by John Cunningham Bowler, 2013.
   8 # To the extent possible under law, the author has waived all copyright and
   9 # related or neighboring rights to this work.  This work is published from:
  10 # United States.
  11 #
  12 # Shell script to generate png.c 8-bit and 16-bit log tables (see the code in
  13 # png.c for details).
  14 #
  15 # This script uses the "bc" arbitrary precision calculator to calculate 32-bit
  16 # fixed point values of logarithms appropriate to finding the log of an 8-bit
  17 # (0..255) value and a similar table for the exponent calculation.
  18 #
  19 # "bc" must be on the path when the script is executed, and the math library
  20 # (-lm) must be available
  21 #
  22 # function to print out a list of numbers as integers; the function truncates
  23 # the integers which must be one-per-line
  24 function print(){
  25    awk 'BEGIN{
  26       str = ""
  27    }
  28    {
  29       sub("\\.[0-9]*$", "")
  30       if ($0 == "")
  31          $0 = "0"
  32
  33       if (str == "")
  34          t = "   " $0 "U"
  35       else
  36          t = str ", " $0 "U"
  37
  38       if (length(t) >= 80) {
  39          print str ","
  40          str = "   " $0 "U"
  41       } else
  42          str = t
  43    }
  44    END{
  45       print str
  46    }'
  47 }
  48 #
  49 # The logarithm table.
  50 cat <<END
  51 /* 8-bit log table: png_8bit_l2[128]
  52  * This is a table of -log(value/255)/log(2) for 'value' in the range 128 to
  53  * 255, so it's the base 2 logarithm of a normalized 8-bit floating point
  54  * mantissa.  The numbers are 32-bit fractions.
  55  */
  56 static const png_uint_32
  57 png_8bit_l2[128] =
  58 {
  59 END
  60 #
  61 bc -lqws <<END | print
  62 f=65536*65536/l(2)
  63 for (i=128;i<256;++i) { .5 - l(i/255)*f; }
  64 END
  65 echo '};'
  66 echo
  67 #
  68 # The exponent table.
  69 cat <<END
  70 /* The 'exp()' case must invert the above, taking a 20-bit fixed point
  71  * logarithmic value and returning a 16 or 8-bit number as appropriate.  In
  72  * each case only the low 16 bits are relevant - the fraction - since the
  73  * integer bits (the top 4) simply determine a shift.
  74  *
  75  * The worst case is the 16-bit distinction between 65535 and 65534; this
  76  * requires perhaps spurious accuracy in the decoding of the logarithm to
  77  * distinguish log2(65535/65534.5) - 10^-5 or 17 bits.  There is little chance
  78  * of getting this accuracy in practice.
  79  *
  80  * To deal with this the following exp() function works out the exponent of the
  81  * frational part of the logarithm by using an accurate 32-bit value from the
  82  * top four fractional bits then multiplying in the remaining bits.
  83  */
  84 static const png_uint_32
  85 png_32bit_exp[16] =
  86 {
  87 END
  88 #
  89 bc -lqws <<END | print
  90 f=l(2)/16
  91 for (i=0;i<16;++i) {
  92    x = .5 + e(-i*f)*2^32;
  93    if (x >= 2^32) x = 2^32-1;
  94    x;
  95 }
  96 END
  97 echo '};'
  98 echo
  99 #
 100 # And the table of adjustment values.
 101 cat <<END
 102 /* Adjustment table; provided to explain the numbers in the code below. */
 103 #if 0
 104 END
 105 bc -lqws <<END | awk '{ printf "%5d %s\n", 12-NR, $0 }'
 106 for (i=11;i>=0;--i){
 107    (1 - e(-(2^i)/65536*l(2))) * 2^(32-i)
 108 }
 109 END
 110 echo '#endif'