itemData: data relative to an item. All items (particles) require at least 6 values: p1x, p1y, p1z, p2x, p2y, p2z with p1 is the particle start position, p2-p1 is the particle initial velocity vector. Auxiliary values might be required depending on the particle object attributes. See the
particle object type combined with attributes for more information. If nil, the particle object is emptied of all its items
