I use square grids in my campaign and during my last session I was trying to check how many PCs I could hit with a dragon's breath. Now since this dragon is a large creature I was thinking how many point of origins I have and I could not find a good documentation except that any intersection between squares can be considered as such, which rules out the white area inside the squares, but what if the creature is large or huge?
I tried using common sense: the dragon occupies 4 squares so since he is free to choose in which direction he wants to blow he can surely chose any of the 4 corners right? What about the other 4 non corner intersections? I guess those also make sense, right?
Then what about the central intersection of the 4 squares? Can the cone originate from the center of the dragon? It kinda make sense to me: the dragon tilts his head up and blows more down then forward, so less far, but it gets more adjacent units.
Last crazy thought, what if the dragon tilts its head back? It would be like originating from a corner, but traveling on the opposite direction.
Best Answer
The PHB specifies that the point of origin has to be the creature originating the area of effect (assuming, as you are in your example that the area of effect is a breath weapon, which has a range of 0). It also states that the point of origin itself is not included in the area of effect unless you want it to be.
The DMG states that if you're using a square grid, the point of origin can be "any intersection of squares or hexes."
A strict reading of both of these rules together would indicate that the point of origin could be an intersection of squares/hexes within the creature's own space. But since you can only choose to have the point of origin be considered outside the area of effect, that would put the creature itself within the area and therefore at risk of damage.