[ft] Trying to understand the gblender_lookup_channel function...

[Paul Pedriana]

I'm trying to understand what the ftview demo gblender_lookup_channel 
function does. It's not obvious to me, there aren't any comments or 
documentation, there is nothing in the mailing list archive, and I did 
spend some time tracing it to try to make sense of it. The key = 
(GBlenderChanKey)blender->keys + idx; line in particular is confusing to 
me because it's casting between two unrelated structs. But in general I 
don't know what this function is trying to do, or what it means by keys, 
channels and indexes. Any help would be useful. I understand the 
concepts of color and alpha blending fine, so it's not a newbie kind of 
problem.


GBLENDER_APIDEF( unsigned char* )
gblender_lookup_channel( GBlender       blender,
                         int            background,
                         int            foreground )
{
  int              idx, idx0;
  unsigned short   backfore = (unsigned short)((foreground << 8) | 
background);
  GBlenderChanKey  key;

  idx0 = ( background + foreground*17 ) % (GBLENDER_KEY_COUNT);
  idx  = idx0;
  do
  {
    key = (GBlenderChanKey)blender->keys + idx;

    if ( key->index < 0 )
      goto NewNode;

    if ( key->backfore == backfore )
      goto Exit;

    idx = (idx+1) & (GBLENDER_KEY_COUNT-1);
  }
  while ( idx != idx0 );

NewNode:
  key->backfore   = backfore;
  key->index      = (signed short)( idx * GBLENDER_SHADE_COUNT );

  gblender_reset_channel_key( blender, key );

Exit:
  return  (unsigned char*)blender->cells + key->index;
}