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Atmospheric light scattering: Difference between revisions
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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. | 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. | 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. The following two screenshots illustrate the situation: | ||
[[File:Ground fog02.jpg|400px|Thin ground fog layer]] [[File:Ground fog01.jpg|400px|Dense ground gof layer]] | |||
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 haze problem can be approximated by observing the following points: | ||
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* 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. | * 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 == | == Basic atmosphere model elements == | ||