wrlib/alpha_combine.c

   1 /* alpha_combine.c - Alpha channel combination, based on Gimp 1.1.24
   2  * The GIMP -- an image manipulation program
   3  * Copyright (C) 1995 Spencer Kimball and Peter Mattis
   4  *
   5  *  This library is free software; you can redistribute it and/or
   6  *  modify it under the terms of the GNU Library General Public
   7  *  License as published by the Free Software Foundation; either
   8  *  version 2 of the License, or (at your option) any later version.
   9  *
  10  *  This library is distributed in the hope that it will be useful,
  11  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  12  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  13  *  Library General Public License for more details.
  14  *
  15  *  You should have received a copy of the GNU Library General Public
  16  *  License along with this library; if not, write to the Free
  17  *  Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston,
  18  *  MA 02110-1301, USA.
  19  */
  20
  21 #include "config.h"
  22
  23 #include "wraster.h"
  24 #include "wr_i18n.h"
  25
  26
  27 void RCombineAlpha(unsigned char *d, unsigned char *s, int s_has_alpha,
  28                    int width, int height, int dwi, int swi, int opacity) {
  29         int x, y;
  30         int t, sa;
  31         int alpha;
  32         float ratio, cratio;
  33
  34         for (y=0; y<height; y++) {
  35                 for (x=0; x<width; x++) {
  36                         sa=s_has_alpha?*(s+3):255;
  37
  38                         if (opacity!=255) {
  39                                 t = sa * opacity + 0x80;
  40                                 sa = ((t>>8)+t)>>8;
  41                         }
  42
  43                         t = *(d+3) * (255-sa) + 0x80;
  44                         alpha = sa + (((t>>8)+t)>>8);
  45
  46                         if (sa==0 || alpha==0) {
  47                                 ratio = 0;
  48                                 cratio = 1.0;
  49                         } else if(sa == alpha) {
  50                                 ratio = 1.0;
  51                                 cratio = 0;
  52                         } else {
  53                                 ratio = (float)sa / alpha;
  54                                 cratio = 1.0F - ratio;
  55                         }
  56
  57                         *d = (int)*d * cratio + (int)*s * ratio;
  58                         s++; d++;
  59                         *d = (int)*d * cratio + (int)*s * ratio;
  60                         s++; d++;
  61                         *d = (int)*d * cratio + (int)*s * ratio;
  62                         s++; d++;
  63                         *d = alpha;
  64                         d++;
  65
  66                         if (s_has_alpha) s++;
  67                 }
  68                 d+=dwi;
  69                 s+=swi;
  70         }
  71 }