1,360
edits
Atmospheric light scattering: Difference between revisions
Jump to navigation
Jump to search
→Atmospheric haze
| Line 20: | Line 20: | ||
== Atmospheric haze == | == Atmospheric haze == | ||
What makes the problem complicated to solve in practice is that the only thing that can be calculated reliably is the density of air molecules in the atmosphere as a function of altitude, but there are only one ingredient in the light scattering problem. Dust or water vapour are at least equally important, but their distribution in the atmosphere cannot be cast into a simple form - it is in general a full 4-dim function of space and time, equal to the evolution of the weather itself. | What makes the problem complicated to solve in practice is that the only thing that can be calculated reliably is the density of air molecules in the atmosphere as a function of altitude, but there are only one ingredient in the light scattering problem. Dust or water vapour are at least equally important, but their distribution in the atmosphere cannot be cast into a simple form - it is in general a full 4-dim function of space and time, equal to the evolution of the weather itself. The information about the distribution of haze must then come from the weather system. | ||
Getting a semi-realistic haze distribution is important for rendering a scene. A normal haze distribution in the atmosphere can not be characterized by a single value of the visibility range - the visibility range depends on position and view direction. Imagine you are 10 km high and the forward visibility is an (unrealistically small) 10 km. The visibility range looking down will typically be a lot less since the atmosphere gets denser as we go down in altitude, and hence there is a lot more light scattering. Looking up on the other hand the visibility will be much more than 10 km since the density decreases. Or imagine a second case with a 1 km thick fog layer with 500 m visibility on the ground. Above the layer, the visibility can be 50 km. However, it will be impossible to see the ground beneath the fog, only mountains reaching above the fog layer will be visible. Setting a global visibility in the scene to a value of either 50 km or 500 m will never result in this behaviour. Thus, once we want to render anything resembling realistic haze distributions, visibility along any ray must be a property of the whole scene rather than the position of the aircraft. | |||
The haze problem can be approximated by observing the following points: | |||
* The vertical structure of the atmosphere changes usually much faster than the horizontal structure. A fog bank may be 500 m thick, but it is unlikely to be just 500 m wide. Most realistic haze layers are almost constant across a range O(10) km, i.e. to a good first approximation one can model only the vertical structure of the atmosphere and have the atmosphere horizontally constant in each frame and change the whole horizontal structure of the atmosphere per frame dependent on current position. This doesn't take into account that a haze layer may be seen from above to have a finite extension. | |||
* 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. | |||