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Atmospheric light scattering: Difference between revisions
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* Most dust and water vapour is found in the lowest convective layer of the atmosphere, i.e. beneath the lowest cloud layer, since this layer has actual contact with the surface as a source of water vapour and dust, but there is no effective transport of dust across the lowest inversion layer. Thus, most situations are approximated well by a low visibility layer close to the ground with a high visibility layer above. This neglects situations in which a second optically thick layer may be in the scene at higher altitudes. | * Most dust and water vapour is found in the lowest convective layer of the atmosphere, i.e. beneath the lowest cloud layer, since this layer has actual contact with the surface as a source of water vapour and dust, but there is no effective transport of dust across the lowest inversion layer. Thus, most situations are approximated well by a low visibility layer close to the ground with a high visibility layer above. This neglects situations in which a second optically thick layer may be in the scene at higher altitudes. | ||
* Compared to diffuse scattering, the effect of Rayleigh and Mie scattering is much less pronounced, as these apply to optically thin media with hardly any light attenuation. This neglects phenomena like the [http://en.wikipedia.org/wiki/Blue_moon#Visibly_blue_moon Blue Moon] which are caused by almost pure Rayleigh scattering in the absence of diffuse scattering. | * Compared to diffuse scattering, the effect of Rayleigh and Mie scattering is much less pronounced, as these apply to optically thin media with hardly any light attenuation. Thus, one can to a good approximation base the whole fog fading with distance on diffuse scattering. This neglects phenomena like the [http://en.wikipedia.org/wiki/Blue_moon#Visibly_blue_moon Blue Moon] which are caused by almost pure Rayleigh scattering in the absence of diffuse scattering. | ||
== Basic atmosphere model elements == | |||
Translated into a rendering problem, one can identify the following relevant elements | |||
* The skydome simulates the scattering in an optically thin atmosphere in the absence of haze layers. As such, it takes into account Rayleigh and Mie scattering with the parameters adjusted to account for the water vapour and dust distribution above the current aircraft altitude. The current skydome shader is based on [http://http.developer.nvidia.com/GPUGems2/gpugems2_chapter16.html work by Sean O'Neil] and is described there (in case you're interested in O'Neil's article - the reason why he is able to do what he describes in <i> Eliminating One Dimension</i> is that for realistic rendering distances one can neglect the full curvature of earth and Taylor-expand the expressions in the curvature, his result can then be derived analytically). Extra diffuse high-altitude layers can just be 'painted' onto the skydome. | |||
* A ground haze layer of given thickness and ground visibility takes care of simulating ground fog banks and visibility in the lowest convection layer. This needs to enter the computations of both the skydome and the terrain shaders. | |||
* An aloft layer simulates diffuse scattering above the ground haze layer. Since the visual impression for anything but large distances is dominated by the ground haze layer, the aloft layer visibility can be modelled as a function of aircraft altitude without creating an unrealistic impression. The aloft layer never obscures the skydome, as its visibility range is supposed to account for the atmosphere looking down, but never looking up where the visibility is much better. | |||
* The really optically thick clouds are drawn by the weather system as separate models into the scene. | |||