Cumulative effect of randomly moving, randomly positioned threads on the position-averaged emergent intensity for the Lyman α line in a multi-thread model. The solid blue line shows the emergent intensity after N threads. The number of threads is indicated at the top right of each panel. The dashed green line shows the line profile of the (N + 1)th thread in the LOS. The intensity is given in 10⁵ erg s^-1 cm^-2 sr^-1 Å^-1. The distance from line centre Δλ is given in Å. The radiation emergent from each thread is computed with the p4 model.