static int stbi__compute_transparency(stbi__png *z, stbi_uc tc[3], int out_n)
{
   stbi__context *s = z->s;
   stbi__uint32 i, pixel_count = s->img_x * s->img_y;
   stbi_uc *p = z->out;

   // compute color-based transparency, assuming we've
   // already got 255 as the alpha value in the output
   STBI_ASSERT(out_n == 2 || out_n == 4);

   if (out_n == 2) {
      for (i=0; i < pixel_count; ++i) {
         p[1] = (p[0] == tc[0] ? 0 : 255);
         p += 2;
      }
   } else {
      for (i=0; i < pixel_count; ++i) {
         if (p[0] == tc[0] && p[1] == tc[1] && p[2] == tc[2])
            p[3] = 0;
         p += 4;
      }
   }
   return 1;
}
static int stbi__compute_transparency(stbi__png *z, stbi_uc tc[3], int out_n)
{
   stbi__context *s = z->s;
   stbi__uint32 i, pixel_count = s->img_x * s->img_y;
   stbi_uc *p = z->out;

    
    
   STBI_ASSERT(out_n == 2 || out_n == 4);

   if (out_n == 2) {
      for (i=0; i < pixel_count; ++i) {
         p[1] = (p[0] == tc[0] ? 0 : 255);
         p += 2;
      }
   } else {
      for (i=0; i < pixel_count; ++i) {
         if (p[0] == tc[0] && p[1] == tc[1] && p[2] == tc[2])
            p[3] = 0;
         p += 4;
      }
   }
   return 1;
}