Difference between primate graded and blended facial displays

I wondered if someone could elucidate the difference between graded and blended facial displays in primates, more specifically apes? I believe blended displays to arise from 2 or more prototypical parent expressions, whereas graded is the same expression at differing intensities? (as described by parr et al 2005) The reason I'm asking is because it's been proposed that the full play face is a blend of the bared teeth display and play face. However, could it be another explanation that it's the same display but at different intensity/grad? I've found there's not much known on how and why 3 laugh variants (3 types of playfaces) evolved in primates, wondering what others known on the topic, thanks.