gammaCorrectionTables.c

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/*
 * Jitter offset table
 *
 *
 * ***** BEGIN GPL LICENSE BLOCK *****
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * as published by the Free Software Foundation; either version 2
 * of the License, or (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 *
 * The Original Code is Copyright (C) 2001-2002 by NaN Holding BV.
 * All rights reserved.
 *
 * The Original Code is: all of this file.
 *
 * Contributor(s): none yet.
 *
 * ***** END GPL LICENSE BLOCK *****
 */

#include "gammaCorrectionTables.h"
#include <stdlib.h>
#include <math.h>

/* WARNING; optimized, cannot be used to do gamma(invgamma()) and expect    */
/* result remain identical (ton)                                            */   

/* gamma is only used here for correcting adding colors or alpha */
#define RE_DEFAULT_GAMMA 2.0

/* This 400 is sort of based on the number of intensity levels needed for    */
/* the typical dynamic range of a medium, in this case CRTs. (Foley)         */
/* (Actually, it says the number should be between 400 and 535.)             */
#define RE_GAMMA_TABLE_SIZE 400

/* These indicate the status of the gamma lookup table --------------------- */

static float gamma_range_table[RE_GAMMA_TABLE_SIZE + 1];
static float gamfactor_table[RE_GAMMA_TABLE_SIZE];
static float inv_gamma_range_table[RE_GAMMA_TABLE_SIZE + 1];
static float inv_gamfactor_table[RE_GAMMA_TABLE_SIZE];
static float color_domain_table[RE_GAMMA_TABLE_SIZE + 1];
static float color_step;
static float inv_color_step;
static float valid_gamma;
static float valid_inv_gamma;

/* ------------------------------------------------------------------------- */

float gammaCorrect(float c)
{
    int i;
    float res = 0.0;
    
    i = floor(c * inv_color_step);
    /* Clip to range [0,1]: outside, just do the complete calculation.       */
    /* We may have some performance problems here. Stretching up the LUT     */
    /* may help solve that, by exchanging LUT size for the interpolation.    */
    /* Negative colors are explicitly handled.                              */
    if (i < 0) res = -pow(abs(c), valid_gamma);
    else if (i >= RE_GAMMA_TABLE_SIZE ) res = pow(c, valid_gamma);
    else res = gamma_range_table[i] + 
               ( (c - color_domain_table[i]) * gamfactor_table[i]); 
    
    return res;
} /* end of float gammaCorrect(float col) */

/* ------------------------------------------------------------------------- */

float invGammaCorrect(float col)
{
    int i;
    float res = 0.0;

    i = floor(col*inv_color_step);
    /* Negative colors are explicitly handled.                              */
    if (i < 0) res = -pow(abs(col), valid_inv_gamma);
    else if (i >= RE_GAMMA_TABLE_SIZE) res = pow(col, valid_inv_gamma);
    else res = inv_gamma_range_table[i] + 
               ( (col - color_domain_table[i]) * inv_gamfactor_table[i]);
               
    return res;
} /* end of float invGammaCorrect(float col) */


/* ------------------------------------------------------------------------- */

void makeGammaTables(float gamma)
{
    /* we need two tables: one forward, one backward */
    int i;

    valid_gamma        = gamma;
    valid_inv_gamma    = 1.0f / gamma;
    color_step        = 1.0 / RE_GAMMA_TABLE_SIZE;
    inv_color_step    = (float) RE_GAMMA_TABLE_SIZE; 

    /* We could squeeze out the two range tables to gain some memory.        */ 
    for (i = 0; i < RE_GAMMA_TABLE_SIZE; i++) {
        color_domain_table[i]   = i * color_step;
        gamma_range_table[i]     = pow(color_domain_table[i],
                                        valid_gamma);
        inv_gamma_range_table[i] = pow(color_domain_table[i],
                                        valid_inv_gamma);
    }

    /* The end of the table should match 1.0 carefully. In order to avoid    */
    /* rounding errors, we just set this explicitly. The last segment may    */
    /* have a different length than the other segments, but our              */
    /* interpolation is insensitive to that.                                 */
    color_domain_table[RE_GAMMA_TABLE_SIZE]   = 1.0;
    gamma_range_table[RE_GAMMA_TABLE_SIZE]     = 1.0;
    inv_gamma_range_table[RE_GAMMA_TABLE_SIZE] = 1.0;

    /* To speed up calculations, we make these calc factor tables. They are  */
    /* multiplication factors used in scaling the interpolation.             */
    for (i = 0; i < RE_GAMMA_TABLE_SIZE; i++ ) {
        gamfactor_table[i] = inv_color_step
            * (gamma_range_table[i + 1] - gamma_range_table[i]) ;
        inv_gamfactor_table[i] = inv_color_step
            * (inv_gamma_range_table[i + 1] - inv_gamma_range_table[i]) ;
    }

} /* end of void makeGammaTables(float gamma) */



/* ------------------------------------------------------------------------- */

/* eof */