430 lines
11 KiB
C
430 lines
11 KiB
C
/* makesRGB.c -- build sRGB-to-linear and linear-to-sRGB conversion tables
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*
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* COPYRIGHT: Written by John Cunningham Bowler, 2013.
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* To the extent possible under law, the author has waived all copyright and
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* related or neighboring rights to this work. This work is published from:
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* United States.
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*
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* Make a table to convert 8-bit sRGB encoding values into the closest 16-bit
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* linear value.
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*
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* Make two tables to take a linear value scaled to 255*65535 and return an
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* approximation to the 8-bit sRGB encoded value. Calculate the error in these
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* tables and display it.
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*/
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#define _C99_SOURCE 1
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#include <stdio.h>
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#include <math.h>
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#include <stdlib.h>
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/* pngpriv.h includes the definition of 'PNG_sRGB_FROM_LINEAR' which is required
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* to verify the actual code.
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*/
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#include "../../pngpriv.h"
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#include "sRGB.h"
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/* The tables are declared 'const' in pngpriv.h, so this redefines the tables to
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* be used.
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*/
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#define png_sRGB_table sRGB_table
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#define png_sRGB_base sRGB_base
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#define png_sRGB_delta sRGB_delta
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static png_uint_16 png_sRGB_table[256];
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static png_uint_16 png_sRGB_base[512];
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static png_byte png_sRGB_delta[512];
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static const unsigned int max_input = 255*65535;
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double
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fsRGB(double l)
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{
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return sRGB_from_linear(l/max_input);
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}
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double
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sRGB(unsigned int i)
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{
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return fsRGB(i);
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}
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double
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finvsRGB(unsigned int i)
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{
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return 65535 * linear_from_sRGB(i/255.);
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}
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png_uint_16
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invsRGB(unsigned int i)
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{
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unsigned int x = nearbyint(finvsRGB(i));
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if (x > 65535)
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{
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fprintf(stderr, "invsRGB(%u) overflows to %u\n", i, x);
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exit(1);
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}
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return (png_uint_16)x;
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}
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int
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main(int argc, char **argv)
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{
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unsigned int i, i16, ibase;
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double min_error = 0;
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double max_error = 0;
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double min_error16 = 0;
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double max_error16 = 0;
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double adjust;
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double adjust_lo = 0.4, adjust_hi = 0.6, adjust_mid = 0.5;
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unsigned int ec_lo = 0, ec_hi = 0, ec_mid = 0;
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unsigned int error_count = 0;
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unsigned int error_count16 = 0;
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int test_only = 0;
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if (argc > 1)
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test_only = strcmp("--test", argv[1]) == 0;
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/* Initialize the encoding table first. */
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for (i=0; i<256; ++i)
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{
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png_sRGB_table[i] = invsRGB(i);
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}
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/* Now work out the decoding tables (this is where the error comes in because
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* there are 512 set points and 512 straight lines between them.)
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*/
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for (;;)
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{
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if (ec_lo == 0)
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adjust = adjust_lo;
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else if (ec_hi == 0)
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adjust = adjust_hi;
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else if (ec_mid == 0)
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adjust = adjust_mid;
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else if (ec_mid < ec_hi)
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adjust = (adjust_mid + adjust_hi)/2;
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else if (ec_mid < ec_lo)
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adjust = (adjust_mid + adjust_lo)/2;
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else
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{
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fprintf(stderr, "not reached: %u .. %u .. %u\n", ec_lo, ec_mid, ec_hi);
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exit(1);
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}
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/* Calculate the table using the current 'adjust' */
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for (i=0; i<=511; ++i)
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{
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double lo = 255 * sRGB(i << 15);
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double hi = 255 * sRGB((i+1) << 15);
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unsigned int calc;
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calc = nearbyint((lo+adjust) * 256);
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if (calc > 65535)
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{
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fprintf(stderr, "table[%d][0]: overflow %08x (%d)\n", i, calc,
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calc);
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exit(1);
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}
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png_sRGB_base[i] = calc;
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calc = nearbyint((hi-lo) * 32);
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if (calc > 255)
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{
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fprintf(stderr, "table[%d][1]: overflow %08x (%d)\n", i, calc,
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calc);
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exit(1);
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}
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png_sRGB_delta[i] = calc;
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}
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/* Check the 16-bit linear values alone: */
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error_count16 = 0;
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for (i16=0; i16 <= 65535; ++i16)
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{
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unsigned int i = 255*i16;
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unsigned int iexact = nearbyint(255*sRGB(i));
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unsigned int icalc = PNG_sRGB_FROM_LINEAR(i);
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if (icalc != iexact)
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++error_count16;
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}
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/* Now try changing the adjustment. */
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if (ec_lo == 0)
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ec_lo = error_count16;
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else if (ec_hi == 0)
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ec_hi = error_count16;
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else if (ec_mid == 0)
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{
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ec_mid = error_count16;
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printf("/* initial error counts: %u .. %u .. %u */\n", ec_lo, ec_mid,
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ec_hi);
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}
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else if (error_count16 < ec_mid)
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{
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printf("/* adjust (mid ): %f: %u -> %u */\n", adjust, ec_mid,
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error_count16);
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ec_mid = error_count16;
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adjust_mid = adjust;
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}
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else if (adjust < adjust_mid && error_count16 < ec_lo)
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{
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printf("/* adjust (low ): %f: %u -> %u */\n", adjust, ec_lo,
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error_count16);
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ec_lo = error_count16;
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adjust_lo = adjust;
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}
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else if (adjust > adjust_mid && error_count16 < ec_hi)
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{
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printf("/* adjust (high): %f: %u -> %u */\n", adjust, ec_hi,
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error_count16);
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ec_hi = error_count16;
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adjust_hi = adjust;
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}
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else
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{
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adjust = adjust_mid;
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printf("/* adjust: %f: %u */\n", adjust, ec_mid);
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break;
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}
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}
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/* For each entry in the table try to adjust it to minimize the error count
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* in that entry. Each entry corresponds to 128 input values.
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*/
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for (ibase=0; ibase<65536; ibase+=128)
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{
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png_uint_16 base = png_sRGB_base[ibase >> 7], trybase = base, ob=base;
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png_byte delta = png_sRGB_delta[ibase >> 7], trydelta = delta, od=delta;
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unsigned int ecbase = 0, eco;
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for (;;)
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{
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png_sRGB_base[ibase >> 7] = trybase;
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png_sRGB_delta[ibase >> 7] = trydelta;
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/* Check the 16-bit linear values alone: */
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error_count16 = 0;
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for (i16=ibase; i16 < ibase+128; ++i16)
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{
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unsigned int i = 255*i16;
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unsigned int iexact = nearbyint(255*sRGB(i));
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unsigned int icalc = PNG_sRGB_FROM_LINEAR(i);
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if (icalc != iexact)
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++error_count16;
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}
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if (error_count16 == 0)
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break;
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if (ecbase == 0)
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{
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eco = ecbase = error_count16;
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++trybase; /* First test */
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}
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else if (error_count16 < ecbase)
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{
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if (trybase > base)
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{
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base = trybase;
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++trybase;
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}
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else if (trybase < base)
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{
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base = trybase;
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--trybase;
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}
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else if (trydelta > delta)
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{
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delta = trydelta;
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++trydelta;
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}
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else if (trydelta < delta)
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{
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delta = trydelta;
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--trydelta;
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}
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else
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{
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fprintf(stderr, "makesRGB: impossible\n");
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exit(1);
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}
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ecbase = error_count16;
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}
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else
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{
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if (trybase > base)
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trybase = base-1;
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else if (trybase < base)
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{
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trybase = base;
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++trydelta;
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}
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else if (trydelta > delta)
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trydelta = delta-1;
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else if (trydelta < delta)
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break; /* end of tests */
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}
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}
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png_sRGB_base[ibase >> 7] = base;
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png_sRGB_delta[ibase >> 7] = delta;
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if (base != ob || delta != od)
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{
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printf("/* table[%u]={%u,%u} -> {%u,%u} %u -> %u errors */\n",
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ibase>>7, ob, od, base, delta, eco, ecbase);
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}
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else if (0)
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printf("/* table[%u]={%u,%u} %u errors */\n", ibase>>7, ob, od,
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ecbase);
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}
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/* Only do the full (slow) test at the end: */
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min_error = -.4999;
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max_error = .4999;
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error_count = 0;
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for (i=0; i <= max_input; ++i)
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{
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unsigned int iexact = nearbyint(255*sRGB(i));
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unsigned int icalc = PNG_sRGB_FROM_LINEAR(i);
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if (icalc != iexact)
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{
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double err = 255*sRGB(i) - icalc;
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if (err > (max_error+.001) || err < (min_error-.001))
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{
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printf(
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"/* 0x%08x: exact: %3d, got: %3d [tables: %08x, %08x] (%f) */\n",
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i, iexact, icalc, png_sRGB_base[i>>15],
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png_sRGB_delta[i>>15], err);
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}
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++error_count;
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if (err > max_error)
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max_error = err;
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else if (err < min_error)
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min_error = err;
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}
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}
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/* Re-check the 16-bit cases too, including the warning if there is an error
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* bigger than 1.
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*/
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error_count16 = 0;
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max_error16 = 0;
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min_error16 = 0;
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for (i16=0; i16 <= 65535; ++i16)
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{
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unsigned int i = 255*i16;
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unsigned int iexact = nearbyint(255*sRGB(i));
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unsigned int icalc = PNG_sRGB_FROM_LINEAR(i);
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if (icalc != iexact)
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{
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double err = 255*sRGB(i) - icalc;
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++error_count16;
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if (err > max_error16)
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max_error16 = err;
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else if (err < min_error16)
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min_error16 = err;
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if (abs(icalc - iexact) > 1)
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printf(
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"/* 0x%04x: exact: %3d, got: %3d [tables: %08x, %08x] (%f) */\n",
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i16, iexact, icalc, png_sRGB_base[i>>15],
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png_sRGB_delta[i>>15], err);
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}
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}
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/* Check the round trip for each 8-bit sRGB value. */
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for (i16=0; i16 <= 255; ++i16)
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{
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unsigned int i = 255 * png_sRGB_table[i16];
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unsigned int iexact = nearbyint(255*sRGB(i));
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unsigned int icalc = PNG_sRGB_FROM_LINEAR(i);
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if (i16 != iexact)
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{
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fprintf(stderr, "8-bit rounding error: %d -> %d\n", i16, iexact);
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exit(1);
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}
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if (icalc != i16)
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{
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double finv = finvsRGB(i16);
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printf("/* 8-bit roundtrip error: %d -> %f -> %d(%f) */\n",
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i16, finv, icalc, fsRGB(255*finv));
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}
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}
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printf("/* error: %g - %g, %u (%g%%) of readings inexact */\n",
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min_error, max_error, error_count, (100.*error_count)/max_input);
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printf("/* 16-bit error: %g - %g, %u (%g%%) of readings inexact */\n",
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min_error16, max_error16, error_count16, (100.*error_count16)/65535);
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if (!test_only)
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{
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printf("const png_uint_16 png_sRGB_table[256] =\n{\n ");
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for (i=0; i<255; )
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{
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do
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{
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printf("%d,", png_sRGB_table[i++]);
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}
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while ((i & 0x7) != 0 && i<255);
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if (i<255) printf("\n ");
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}
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printf("%d\n};\n\n", png_sRGB_table[i]);
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printf("const png_uint_16 png_sRGB_base[512] =\n{\n ");
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for (i=0; i<511; )
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{
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do
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{
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printf("%d,", png_sRGB_base[i++]);
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}
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while ((i & 0x7) != 0 && i<511);
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if (i<511) printf("\n ");
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}
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printf("%d\n};\n\n", png_sRGB_base[i]);
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printf("const png_byte png_sRGB_delta[512] =\n{\n ");
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for (i=0; i<511; )
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{
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do
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{
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printf("%d,", png_sRGB_delta[i++]);
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}
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while ((i & 0xf) != 0 && i<511);
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if (i<511) printf("\n ");
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}
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printf("%d\n};\n\n", png_sRGB_delta[i]);
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}
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return 0;
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}
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