## The Underwater Ocean Environment  
Simulating visually realistic marine environment can be challenging due to the presence of several factors affecting the illumination and motion of objects within a region. In contrast to the air medium, the underwater marine environment appears significantly more turbid and dynamic, being rich with organisms. Each organism has its own special visual characteristics and dynamics. They can be for example, semi-transparent like jellyfish or completely opaque. The scene can be highly dynamic where schools of fish pass very close to the camera. Such complex and dynamic environments need to be modelled for simulating the motion of an underwater remotely operated vehicle (ROV). This section focuses on generating the underwater ocean environment and the generation of bioluminescent plankton in the marine environment and the associated visual effects. Today the visual capabilities of game engines are exceptional. Those capabilities are motivated by a multi-billion industry and the  
Figure 13: Introducing random forces to the system. Both images show the cable state after the system is stabilized (\(t>>0\) ). The first and the second images show the cable state with and without random forces respectively. We can see the “Memory Loss” that we would expect to see in a marine-like environment with underwater currents. rapid advancements in the graphics hardware domain. In our simulation which focuses on the realism of the environment, using game engine is a natural choice. Figure 14 presents an image we took underwater in an area rich with organic matter. The predominant fog and snow like particle effects are created by plankton. This figure clearly shows the need for realistic plankton simulation both for testing computer vision algorithms and also for human training purposes. Although some visually simulated underwater environments exists in game engines, to the authors knowledge, no prior research work has been reported on graphical modelling and simulation of plankton. In this section we'll give some background information regarding relevant marine environment characteristics, we'll give an example of existing simulated underwater environment in Unreal Engine and we'll discuss in detail about the two types of plankton (zooplankton and phytoplankton) which we modelled in order to create a more convincingly underwater ocean behaviour. The marine environment supports two basic types of marine organisms. One type comprises of planktons or those organisms whose powers of locomotion are such that they are incapable of making their way against a current and thus are passively transported by currents in the sea. The word plankton comes from the Greek planktons, meaning that which is passively drifting or wandering. Depending upon whether a planktonic organism is a plant or animal, a distinction is made between phytoplankton and zooplankton. Although many planktonic species are of microscopic dimensions, the term is not synonymous with small size as some of the zooplankton include jellyfish of several meters in diameter. Nor are all plankton completely passive; most, including many of the phytoplankton, are capable of swimming. The remaining inhabitants of the pelagic environment form the nekton. These are free-swimming animals that, in contrast to plankton, are strong enough to swim against currents and are therefore independent of water movements. The category of nekton includes fish, squid, and marine mammals [31]. In addition, the marine environment also include plants like seaweed and also some natural static objects such as rocks. Man-made objects are also significant part of the sea in the form of boats structures and also (unfortunately) garbage. From an optical imaging point of view, seawater is an absorbing and scattering medium. Light energy that propagates in water is absorbed by water molecules and dissolved organic matter  
Figure 14: We took this underwater image during daylight in Pelorus Sound New Zealand to demonstrate the challenges of modelling realistic underwater imagery.