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Atmospheric light scattering: Difference between revisions
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A surprisingly large fraction of whatever we get to see from an airplane is light scattered somewhere in the atmosphere. This includes the obvious phenomena like the blue color of the sky and the golden-red sunrise and sunset light, but also any form of haze and fog, for instance the effect that faraway objects loose their colors and fade into blue-white. In a typical situation, around 70% of the color values of the scene outside the cockpit are not determined by the color of the scenery textures but by sunlight and haze colors. Having a detailed model of atmospheric light scattering is therefore important for a realistic visual experience in a flight simulation. | A surprisingly large fraction of whatever we get to see from an [[Aircraft|airplane]] is '''light scattered''' somewhere '''in the atmosphere'''. This includes the obvious phenomena like the blue color of the sky and the golden-red sunrise and sunset light, but also any form of haze and fog, for instance the effect that faraway objects loose their colors and fade into blue-white. In a typical situation, around 70% of the color values of the scene outside the cockpit are not determined by the color of the scenery textures but by sunlight and haze colors. Having a detailed model of atmospheric light scattering is therefore important for a realistic visual experience in a flight simulation. | ||
However, atmospheric light scattering physics cannot actually be solved in real time. Imagine looking into the sky. The light you see could have been scattered into that ray at any distance along the ray, but part of the light which has been scattered in has already been scattered out again if the in-scattering point is too far away. Even for a single ray, the problem thus requires two nested integrals to determine the observed light as the correct balance between averaged in-scattering vs. out-scattering given the density of scattering centers in the atmosphere along the ray. Allowing for multiple scattering leads to even more nested integrals. Any integral however is numerically tough to solve, and much more difficult to solve in real time. | However, atmospheric light scattering physics cannot actually be solved in real time. Imagine looking into the sky. The light you see could have been scattered into that ray at any distance along the ray, but part of the light which has been scattered in has already been scattered out again if the in-scattering point is too far away. Even for a single ray, the problem thus requires two nested integrals to determine the observed light as the correct balance between averaged in-scattering vs. out-scattering given the density of scattering centers in the atmosphere along the ray. Allowing for multiple scattering leads to even more nested integrals. Any integral however is numerically tough to solve, and much more difficult to solve in real time. | ||
The aim of this project is to create a set of shaders which approximate the problem in a suitable way by using for instance analytical solutions for the light scattering physics under certain assumptions or parametrized versions of the true solution such that all essential physics determining the visual appearance of the scene is captured. This effort is by its nature closely linked to the weather system which determines how the atmospheric conditions are while the light scattering code determines how this translates into a visual impression. | The aim of this project is to create a set of [[shaders]] which approximate the problem in a suitable way by using for instance analytical solutions for the light scattering physics under certain assumptions or parametrized versions of the true solution such that all essential physics determining the visual appearance of the scene is captured. This effort is by its nature closely linked to the weather system which determines how the atmospheric conditions are while the light scattering code determines how this translates into a visual impression. | ||
== Light scattering basics == | == Light scattering basics == | ||
The basic processes how light scatters in the atmosphere are [http://en.wikipedia.org/wiki/Rayleigh_scattering | The basic processes how light scatters in the atmosphere are [http://en.wikipedia.org/wiki/Rayleigh_scattering '''Rayleigh scattering'''] and [http://en.wikipedia.org/wiki/Mie_scattering '''Mie scattering''']. Rayleigh scattering occurs on scattering centers which are much smaller than the wavelength of light (typically the air molecules). In this limit, the outgoing light is scattered into every direction with equal likelihood (isotrope scattering), but the probability to scatter depends on the wavelength of the light - the shorter wavelengths (blue, violet) scatter more strongly. This is the cause for the color of a clear sky - there is much more diffuse Rayleigh scattering for blue light happening in the upper atmosphere than for red light, and as a result we see all the light that gets scattered out of the direct path from sun to eye as a diffuse blue glow - the sky. The same phenomenon causes the red color of sunrises - since the sun is close to the horizon, the path the light has to travel through the dense parts of the atmosphere is long and so by the time the light reaches the eye all blue light has been scattered out and only the red light remains. | ||
Mie scattering in contrast occurs for much larger particles (water droplets for instance). In this limit, the scattering is of equal strength for all wavelength (i.e. pure Mie-scattered light is white), but the scattering is strongly directional - the scattered light prefers to go close to its original direction. Mie scattering thus tends to create bright white halos around light sources. This is illustrated in the following screenshots of a sky decomposed into the Rayleigh and Mie scattering channels: | Mie scattering in contrast occurs for much larger particles (water droplets for instance). In this limit, the scattering is of equal strength for all wavelength (i.e. pure Mie-scattered light is white), but the scattering is strongly directional - the scattered light prefers to go close to its original direction. Mie scattering thus tends to create bright white halos around light sources. This is illustrated in the following screenshots of a sky decomposed into the Rayleigh and Mie scattering channels: | ||
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As long as the light scattering effect is weak, a medium is called optically thin. The relevant measure is the ratio of the light attenuation length divided by the size of the medium which must be smaller than one, and the defining characteristic of an optically thin medium is that you can look through. This is certainly true for the upper atmosphere where visibility ranges are easily several hundred kilometers whereas the thickest part of the atmosphere is just about 30 km vertical size. Thus, a dark blue sky is actually the blackness of space, seen through the light blue-white glow of Rayleigh scattering. | As long as the light scattering effect is weak, a medium is called optically thin. The relevant measure is the ratio of the light attenuation length divided by the size of the medium which must be smaller than one, and the defining characteristic of an optically thin medium is that you can look through. This is certainly true for the upper atmosphere where visibility ranges are easily several hundred kilometers whereas the thickest part of the atmosphere is just about 30 km vertical size. Thus, a dark blue sky is actually the blackness of space, seen through the light blue-white glow of Rayleigh scattering. | ||
As clouds demonstrate quite drastically, water droplets can easily make the atmosphere optically thick. In this case, light is scattered multiple times before reaching the eye, and most information on what the basic scattering process was like is lost. Dense fog looks like a uniform grey, which means there is no color information left, and no directional information where the light originally came from. We may call this regime | As clouds demonstrate quite drastically, water droplets can easily make the atmosphere optically thick. In this case, light is scattered multiple times before reaching the eye, and most information on what the basic scattering process was like is lost. Dense fog looks like a uniform grey, which means there is no color information left, and no directional information where the light originally came from. We may call this regime '''diffuse scattering'''. | ||
Actually, it is not quite true that diffuse scattering retains no color information. A sunrise beneath an overcast cloud cover looks blue-grey rather than red, thus there are subtle color changes of the incoming light as it filters through an optically thick layer. | Actually, it is not quite true that diffuse scattering retains no color information. A sunrise beneath an overcast cloud cover looks blue-grey rather than red, thus there are subtle color changes of the incoming light as it filters through an optically thick layer. | ||
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Translated into a rendering problem, one can identify the following relevant 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 | * 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 ''Eliminating One Dimension'' 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 simulates diffuse scattering only needs to enter the computations of both the skydome and the terrain shaders. | * 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 simulates diffuse scattering only needs to enter the computations of both the skydome and the terrain shaders. | ||
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== Perception == | == Perception == | ||
One crucial thing to remember when dealing with light attenuation is that what we see is | One crucial thing to remember when dealing with light attenuation is that what we see is '''not''' physical light intensity. To give an example, the light intensity beneath an overcast sky at dawn is easily a factor 2500 less than the light intensity in the bright noon sun. However, dividing an rgb vector of (1,1,1) (white light) by 2500 and using the result for lighting the overcast dawn results in a pitch black scene. The reason is that the eye adapts to different light intensities and that in essence perception weighs intensities over a wide range not linear but logarithmically, a phenomenon known as the [http://en.wikipedia.org/wiki/Weber-Fechner_Law Weber-Fechner law]. | ||
Another perception effect is that contrasts are dynamically adjusted. Usually the brightest object in the visual field is assigned the color white, the darkest the color black, and all other shades are assigned between. This means that the raw intensity range in the scene has to be compressed (e.g. by exposure filtering) into a narrower range by dimming the highest intensities and enhancing the lowest intensities. The following screenshots compare the raw intensity results with the perception-filtered results: | Another perception effect is that contrasts are dynamically adjusted. Usually the brightest object in the visual field is assigned the color white, the darkest the color black, and all other shades are assigned between. This means that the raw intensity range in the scene has to be compressed (e.g. by exposure filtering) into a narrower range by dimming the highest intensities and enhancing the lowest intensities. The following screenshots compare the raw intensity results with the perception-filtered results: | ||
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Thus, light penetrates the thin upper atmosphere, as it filters through, Rayleigh and some Mie scattering create the blue sky. Dependent on the model for the amount of high haze and water vapour to be specified by the weather system, this is handled by the skydome shader. | Thus, light penetrates the thin upper atmosphere, as it filters through, Rayleigh and some Mie scattering create the blue sky. Dependent on the model for the amount of high haze and water vapour to be specified by the weather system, this is handled by the skydome shader. | ||
As the light reaches the first significant cloud layer, light intensity is much reduced. As clouds are drawn outside the terrain and skydome shading codes, this can not be explicitly computed by the shader, neither is it computationally feasible to compute the shadow cast by each cloudlet by ray tracing in real time. Thus, the relevant parameters (< | As the light reaches the first significant cloud layer, light intensity is much reduced. As clouds are drawn outside the terrain and skydome shading codes, this can not be explicitly computed by the shader, neither is it computationally feasible to compute the shadow cast by each cloudlet by ray tracing in real time. Thus, the relevant parameters (<tt>'''rendering/scene/scattering'''</tt> for the light intensity reduction at the position of the aircraft and <tt>'''/environment/surface/scattering'''</tt> for the light reduction on the ground) must be modelled by the weather system (which knows the cloud layer position) and passed to the shaders. The visual difference between shaded terrain and unshaded terrain is illustrated by the following two screenshots: | ||
[[File:Cloudshade01.jpg|400px|No terrain shading by clouds]] [[File:Cloudshade02.jpg|400px|Terrain shading by clouds]] | [[File:Cloudshade01.jpg|400px|No terrain shading by clouds]] [[File:Cloudshade02.jpg|400px|Terrain shading by clouds]] | ||
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This, however, is tricky, because fog does not really have a position, rather what the eye sees is the integrated effect of fog along a ray, and so the brightness of fog is really a weighted integral of fog brightness along the whole ray. This, however, can be approximated by taking the brightness at one attenuation length as a proxy (incidentially, this is the reason that two scattering parameters for the effect of clouds are passed - the light reduction at the aircraft position is a better proxy for fog shadowing due to clouds, the exact position-dependent light reduction can be used for the ground which has a definite position). | This, however, is tricky, because fog does not really have a position, rather what the eye sees is the integrated effect of fog along a ray, and so the brightness of fog is really a weighted integral of fog brightness along the whole ray. This, however, can be approximated by taking the brightness at one attenuation length as a proxy (incidentially, this is the reason that two scattering parameters for the effect of clouds are passed - the light reduction at the aircraft position is a better proxy for fog shadowing due to clouds, the exact position-dependent light reduction can be used for the ground which has a definite position). | ||
The altitude-dependent light reduction due to the clouds is the first instance of the | The altitude-dependent light reduction due to the clouds is the first instance of the '''lightfield''' technique, i.e. that the sunlight is represented as a series of functions r(x,y,z), g(x,y,z), b(x,y,z) in which the individual color channels are functions of vertex position in the scene. | ||
== Light scattering at dawn / dusk == | == Light scattering at dawn / dusk == | ||
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At daytime, this creates a whiteout of a cloud when the sun is right behind it. If other, more dense clouds are also in the scene, this creates an impressive play of light and shadow: | At daytime, this creates a whiteout of a cloud when the sun is right behind it. If other, more dense clouds are also in the scene, this creates an impressive play of light and shadow: | ||
[[File:Cloud_mie03.jpg|500px|center|Mie scattering during the day]] | |||
[[File:Cloud_mie03.jpg|500px|Mie scattering during the day]] | |||
Interestingly enough, cloud dominated by Mie scattering appear a relatively dark grey (as if they were in shadow) when seen from the side, even when they are fully illuminated. This is caused by the low probability of light scattered to large angles - since all light is focused forward, the side of the cloud becomes dark. | Interestingly enough, cloud dominated by Mie scattering appear a relatively dark grey (as if they were in shadow) when seen from the side, even when they are fully illuminated. This is caused by the low probability of light scattered to large angles - since all light is focused forward, the side of the cloud becomes dark. | ||