Computer graphics systems are used in many game and simulation applications to create atmospheric effects such as fog, smoke, clouds, smog and other gaseous phenomena. These atmospheric effects are useful because they create a more realistic sense of the environment and also create the effect of objects appearing and fading at a distance.
Extensive research has been done on realistic simulation of participating media, such as fog, smoke, and clouds. The existing methods includes analytic methods, stochastic methods, numerical simulations, and pre-computation techniques. Most of these techniques focus on computing the light distribution through the gas and present various methods of simulating the light scattering from the particles of the gas. Some resolve multiple scattering of light in the gas and others consider only first order scattering (the scattering of light in the view direction) and approximate the higher order scattering by an ambient light. A majority of the techniques use ray-tracing, voxel-traversal, or other time-consuming algorithms to render the images.
In some approaches, smoke and clouds are simulated by mapping transparent textures on a polygonal object that approximates the boundary of the gas. Although the texture may simulate different densities of the gas inside the 3D boundary and compute even the light scattering inside the gas, it does not change, when viewed from different directions. Consequently, these techniques are suitable for rendering very dense gases or gasses viewed from a distance.
Other methods simplify their task by assuming constant density of the gas at a given elevation, thereby making it possible to use 3D textures to render the gas in real time. The assumption, however, prevents using the algorithm to render inhomogeneous participating media such as smoke with irregular thickness and patchy fog.
There are also methods that use pre-computation techniques in which various scene-dependent quantities are precomputed. The precomputed quantities, however, are valid only for the given static participating medium. For dynamic animation sequences with adjustable media (e.g. smoke) parameters, the preprocessing time and storage costs would be prohibitive.
With the existing techniques that do offer quality rendering, considerable simulation time is still needed to render a single image, making these approaches inappropriate for interactive applications on animated sequences. These methods may also be inadequate to addresses complex environment illumination. Rendering of smoke, for example, presents a particularly challenging problem in computer graphics because of its complicated effects on light propagation. Within a smoke volume, light undergoes absorption and scattering interactions that vary from point to point because of the spatial non-uniformity of smoke. In static participating media, the number and complexity of scattering interactions lead to a substantial expense in computation. For a dynamic medium like smoke having an intricate volumetric structure changing with time, the computational costs can be prohibitive. Even with the latest computational processing power, rendering large volume high quality images of a dynamic smoke scene can be time-consuming. For video and animation applications, if real-time rendering at a rate of at least twenty frames per second cannot be achieved, much of the rendering may need to be precomputed at a cost of losing flexibility and interactive features.
Despite the practical difficulties of rendering inhomogeneous light scattering media (e.g., smoke), such rendering nevertheless remains a popular element in many applications such as films and games. From an end user's point of view, what is needed is an ability to render in real-time complex scenes with high quality visual realism. From a designer's point of view, what is needed is affordable real-time or close to real-time control over the lighting environment and vantage point, as well as the volumetric distribution and optical properties of the smoke.
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