lib/art/art_alphagamma.c

   1 /* Libart_LGPL - library of basic graphic primitives
   2  * Copyright (C) 1998 Raph Levien
   3  *
   4  * This library is free software; you can redistribute it and/or
   5  * modify it under the terms of the GNU Library General Public
   6  * License as published by the Free Software Foundation; either
   7  * version 2 of the License, or (at your option) any later version.
   8  *
   9  * This library is distributed in the hope that it will be useful,
  10  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12  * Library General Public License for more details.
  13  *
  14  * You should have received a copy of the GNU Library General Public
  15  * License along with this library; if not, write to the
  16  * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  17  * Boston, MA 02111-1307, USA.
  18  */
  19
  20 /* Some functions to build alphagamma tables */
  21
  22 #include "config.h"
  23 #include "art_alphagamma.h"
  24
  25 #include <math.h>
  26
  27 /**
  28  * art_alphagamma_new: Create a new #ArtAlphaGamma.
  29  * @gamma: Gamma value.
  30  *
  31  * Create a new #ArtAlphaGamma for a specific value of @gamma. When
  32  * correctly implemented (which is generally not the case in libart),
  33  * alpha compositing with an alphagamma parameter is equivalent to
  34  * applying the gamma transformation to source images, doing the alpha
  35  * compositing (in linear intensity space), then applying the inverse
  36  * gamma transformation, bringing it back to a gamma-adjusted
  37  * intensity space.
  38  *
  39  * Return value: The newly created #ArtAlphaGamma.
  40  **/
  41 ArtAlphaGamma *
  42 art_alphagamma_new (double gamma)
  43 {
  44   int tablesize;
  45   ArtAlphaGamma *alphagamma;
  46   int i;
  47   int *table;
  48   art_u8 *invtable;
  49   double s, r_gamma;
  50
  51   tablesize = ceil (gamma * 8);
  52   if (tablesize < 10)
  53     tablesize = 10;
  54
  55   alphagamma = (ArtAlphaGamma *)art_alloc (sizeof(ArtAlphaGamma) +
  56                                            ((1 << tablesize) - 1) *
  57                                            sizeof(art_u8));
  58   alphagamma->gamma = gamma;
  59   alphagamma->invtable_size = tablesize;
  60
  61   table = alphagamma->table;
  62   for (i = 0; i < 256; i++)
  63     table[i] = (int)floor (((1 << tablesize) - 1) *
  64                            pow (i * (1.0 / 255), gamma) + 0.5);
  65
  66   invtable = alphagamma->invtable;
  67   s = 1.0 / ((1 << tablesize) - 1);
  68   r_gamma = 1.0 / gamma;
  69   for (i = 0; i < 1 << tablesize; i++)
  70     invtable[i] = (int)floor (255 * pow (i * s, r_gamma) + 0.5);
  71
  72   return alphagamma;
  73 }
  74
  75 /**
  76  * art_alphagamma_free: Free an #ArtAlphaGamma.
  77  * @alphagamma: An #ArtAlphaGamma.
  78  *
  79  * Frees the #ArtAlphaGamma.
  80  **/
  81 void
  82 art_alphagamma_free (ArtAlphaGamma *alphagamma)
  83 {
  84   art_free (alphagamma);
  85 }