Vertical callback or per pixel callback : I understand/How does Tilengine work?

[megamarc]

There are four passes, on each pass only effective items for that pass are drawn.

So the first pass of sprites only renders regular sprites, and the second pass only renders priority ones. The same for tiles: first pass draws regular tiles, second pass draws priority tiles. Items drawn last overdraw the ones drawn fist, so this is how priority is achieved.