# Add a particle at `center` with `scale`, living for `ttl` from `time_offset`
	#
	# `time_offset` is added to the creation time, it can be used to delay the
	# apparition of a particle using a positive value.
	#
	# See the doc of the precise class of `program`, or the general `ParticleProgram`
	# for information on the effect of these parameters.
	fun add(center: Point3d[Float], scale: Float, ttl: Float, time_offset: nullable Float)
	do
		var i = total_particles % n_particles
		total_particles += 1

		centers[i*3  ] = center.x
		centers[i*3+1] = center.y
		centers[i*3+2] = center.z

		ttls[i] = ttl
		scales[i] = scale

		time_offset = time_offset or else 0.0
		ots[i] = clock.total.to_f + time_offset
	end