#if 0
gamma-lookup.c 0.1.4 (Sat 19 Dec 1998)
by Adam M. Costello <amc @ cs.berkeley.edu>

This is public domain example code for computing gamma
correction lookup tables using integer arithmetic.

#endif
#if __STDC__ != 1
#error This code relies on ANSI C conformance.
#endif


#include <limits.h>
#include <math.h>


/* In this program a type named uintN denotes the  */
/* smallest unsigned type we can find that handles */
/* at least all values 0 through (2^N)-1.          */

typedef unsigned char uint8;

#if UCHAR_MAX >= 0xffff
  typedef unsigned char uint16;
#else
  typedef unsigned short uint16;
#endif

#if UCHAR_MAX >= 0xffffffff
  typedef unsigned char uint32;
#elif USHRT_MAX >= 0xffffffff
  typedef unsigned short uint32;
#elif UINT_MAX >= 0xffffffff
  typedef unsigned int uint32;
#else
  typedef unsigned long uint32;
#endif


/*********************/
/* 16-bit arithmetic */

void
precompute16(uint16 L[511])

/* Precomputes the log table (this requires floating point). */

{
  int j;
  double f;

  /* L[j] will hold an integer representation of -log(j / */
  /* 510.0).  Knowing that L[1] (the largest) is 0xfe00   */
  /* will help avoid overflow later, so we set the scale  */
  /* factor accordingly.                                  */

  f = 0xfe00 / log(1 / 510.0);

  for (j = 1;  j <= 510;  ++j)
    L[j] = log(j / 510.0) * f + 0.5;
}


void
gamma16(uint16 L[511], uint8 G[256], uint16 g)

/* Makes a 256-entry gamma correction lookup table G[] with */
/* exponent g/pow(2,14), where g must not exceed 0xffff.    */

{
  int i, j;
  uint16 x, y, xhi, ghi, xlo, glo;

  j = 1;
  G[0] = 0;

  for (i = 1;  i <= 255;  ++i) {
    x = L[i << 1];
    xhi = x >> 8;
    ghi = g >> 8;
    y = xhi * ghi;

    if (y > 0x3f80) {
      /* We could have overflowed later. */
      /* But now we know y << 2 > L[1].  */
      G[i] = 0;
      continue;
    }

    xlo = x & 0xff;
    glo = g & 0xff;
    y = (y << 2) + ((xhi * glo) >> 6) + ((xlo * ghi) >> 6);
    while (L[j] > y) ++j;
    G[i] = j >> 1;
  }
}


/*********************/
/* 32-bit arithmetic */

void
precompute32(uint32 L[511])

/* Precomputes the log table (this requires floating point). */

{
  int j;
  double f;

  /* L[j] will hold an integer representation of -log(j /   */
  /* 510.0).  Knowing that L[1] (the largest) is 0x3fffffff */
  /* will help avoid overflow later, so we set the scale    */
  /* factor accordingly.                                    */

  f = 0x3fffffff / log(1 / 510.0);

  for (j = 1;  j <= 510;  ++j)
    L[j] = log(j / 510.0) * f + 0.5;
}


void
gamma32(uint32 L[511], uint8 G[256], uint16 g)

/* Makes a 256-entry gamma correction lookup table G[] with */
/* exponent g/pow(2,14), where g must not exceed 0xffff.    */

{
  int i, j;
  uint32 x, y;

  j = 1;
  G[0] = 0;

  for (i = 1;  i <= 255;  ++i) {
    x = L[i << 1];
    y = (x >> 14) * g;
    while (L[j] > y) ++j;
    G[i] = j >> 1;
  }
}


/**********************************************/
/* Floating-point arithmetic (for comparison) */

void
gamma_fp(uint8 G[256], double g)

/* Makes a 256-entry gamma correction */
/* lookup table G[i] with exponent g. */

{
  int i;

  G[0] = 0;

  for (i = 1;  i <= 255;  ++i)
    G[i] = pow(i/255.0, g) * 255 + 0.5;
}


/*****************/
/* Accuracy test */

/* Compare gamma tables against the gamma_fp */
/* version for various values of g.          */

#include <stdio.h>

void
compare(uint8 G[256], double g)
{
  uint8 Gfloat[256];
  int i;

  printf("exponent = %f\n", g);
  gamma_fp(Gfloat,g);

  for (i = 0;  i <= 255;  ++i) {
    if (Gfloat[i] != G[i])
      printf("i = %3d, Gfloat = %3d, G = %3d\n",
             i, Gfloat[i], G[i]);
  }
}

main()
{
  double g[7] = { 2.2, 1.7, 1.45, 1.0, 1/1.45, 1/1.7, 1/2.2 };
  int k;
  uint16 L16[511];
  uint32 L32[511];
  uint8 G[256];

  precompute16(L16);
  precompute32(L32);

  printf("16-bit:\n");

  for (k = 0;  k < 7;  ++k) {
    gamma16(L16, G, (uint16)(g[k] * (1 << 14) + 0.5));
    compare(G, g[k]);
  }

  printf("\n32-bit:\n");

  for (k = 0;  k < 7;  ++k) {
    gamma32(L32, G, (uint16)(g[k] * (1 << 14) + 0.5));
    compare(G, g[k]);
  }
}