On a SPOT-3 visible image over the South China Sea continental shelf, two contrasting signatures of internal solitary waves (ISWs) are observed: darker stripes leading brighter stripes in the lower part, and brighter stripes leading darker stripes in the upper part. The authors suggest that the contrasting patterns are caused by ISWs of different polarities. A packet of ISWs is observed in the process of converting polarity due to bottom shoaling, both in the propagation direction and in the transect direction. The evolution process can be obtained by transferring information from the spatial domain to the temporal domain. The conversion process starts with the original depression ISWs passing the 160 m isobath. The evolution process can be divided into two phases: the first is the broadening of the original depression waves and the second is the appearance of the new-born elevation waves. These two phases are verified using numerical simulation.