http://www.imagemagick.org/discourse-server/viewtopic.php?f=2&t=28946
static MagickBooleanType GetOneAuthenticPixelFromCache(Image *image,
  const ssize_t x,const ssize_t y,PixelPacket *pixel,ExceptionInfo *exception)
{
  CacheInfo
    *magick_restrict cache_info;

  const int
    id = GetOpenMPThreadId();

  PixelPacket
    *magick_restrict pixels;

  assert(image != (const Image *) NULL);
  assert(image->signature == MagickSignature);
  assert(image->cache != (Cache) NULL);
  cache_info=(CacheInfo *) image->cache;
  assert(cache_info->signature == MagickSignature);
  *pixel=image->background_color;
  assert(id < (int) cache_info->number_threads);
  pixels=GetAuthenticPixelCacheNexus(image,x,y,1UL,1UL,
    cache_info->nexus_info[id],exception);
  if (pixels == (PixelPacket *) NULL)
    return(MagickFalse);
  *pixel=(*pixels);
  return(MagickTrue);
}

static MagickBooleanType GetOneAuthenticPixelFromCache(Image *image,
  const ssize_t x,const ssize_t y,PixelPacket *pixel,ExceptionInfo *exception)
{
  CacheInfo
    *magick_restrict cache_info;

  const int
    id = GetOpenMPThreadId();

  PixelPacket
    *magick_restrict pixels;

  assert(image != (const Image *) NULL);
  assert(image->signature == MagickSignature);
  assert(image->cache != (Cache) NULL);
  cache_info=(CacheInfo *) image->cache;
  assert(cache_info->signature == MagickSignature);
  *pixel=image->background_color;
  assert(id < (int) cache_info->number_threads);
  pixels=GetAuthenticPixelCacheNexus(image,x,y,1UL,1UL,
    cache_info->nexus_info[id],exception);
  if (pixels == (PixelPacket *) NULL)
    return(MagickFalse);
  *pixel=(*pixels);
  return(MagickTrue);
}
