Cool, a new codec is always good news.
Nitpick posted here because I don't see any method of feedback on the site:
Huffyuv's prediction (which Lagarith also uses) is "median(L,T,L+T-LT)". Paeth (used in e.g. PNG) is "whichever of L,T,LT is nearest to median(L,T,L+T-LT)".