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 "wraster.h"
  22
  23 void RCombineAlpha(unsigned char *d, unsigned char *s, int s_has_alpha,
  24                    int width, int height, int dwi, int swi, int opacity) {
  25         int x, y;
  26         int t, sa;
  27         int alpha;
  28         float ratio, cratio;
  29
  30         for (y=0; y<height; y++) {
  31                 for (x=0; x<width; x++) {
  32                         sa=s_has_alpha?*(s+3):255;
  33
  34                         if (opacity!=255) {
  35                                 t = sa * opacity + 0x80;
  36                                 sa = ((t>>8)+t)>>8;
  37                         }
  38
  39                         t = *(d+3) * (255-sa) + 0x80;
  40                         alpha = sa + (((t>>8)+t)>>8);
  41
  42                         if (sa==0 || alpha==0) {
  43                                 ratio = 0;
  44                                 cratio = 1.0;
  45                         } else if(sa == alpha) {
  46                                 ratio = 1.0;
  47                                 cratio = 0;
  48                         } else {
  49                                 ratio = (float)sa / alpha;
  50                                 cratio = 1.0 - ratio;
  51                         }
  52
  53                         *d = (int)*d * cratio + (int)*s * ratio;
  54                         s++; d++;
  55                         *d = (int)*d * cratio + (int)*s * ratio;
  56                         s++; d++;
  57                         *d = (int)*d * cratio + (int)*s * ratio;
  58                         s++; d++;
  59                         *d = alpha;
  60                         d++;
  61
  62                         if (s_has_alpha) s++;
  63                 }
  64                 d+=dwi;
  65                 s+=swi;
  66         }
  67 }