Hello here (if ever some people are still there  :p ).

Here we are for improving our atmosphere model. Last post produced our  
first blue sky using Rayleigh scattering media and following physics. Now  
let's play a bit with some parameters. This will help us to see how the  
model works, and to validiate the principle.

A) Playing with some parameters
===============================

1) Elevation
------------

parameters unchanged. We get FirstBlueSky_-5_-1-1_5_20_70_deg.png. Not so  
bad, considering we only changed the elevation of the sun. The principle  
seems to work. We do have the redenning of the light, and we get something  
'acceptable' when the sun is below the horizon. Obviously, these are not  
perfect colors, but I recall, this is what we obtain considering only pure  
atmosphere, with only Rayleigh scattering, no other particles, no other  
effect or property. Not so bad on my opinion.

2) Altitude
-----------

pitch upwards. And we set altitude to different values: 2*m, 1000*m,  
4000*m. We get FirstBlueSky_2_1000_4000_m.png. Visually and with a color  
picker, we can see how the brightness decreases with altitude. This  
behaviour was expected because at high altitude, the light encounters less  
molecules (less distance + less density), there is less scattering, and we  
see less blue and more black from the outer space (considered as 100%  
black).

3) Density power factor
-----------------------
As seen previously, the density of the atmophere can be expressed by: d =  
Dmax*exp(-k*h/T), where h is the atitude and T the thickness of the  
atmosphere. In TerraPOV, the factor k os split into a default value  
TP_BASE_RAYLEIGH_POWER that expresses 'normality'and valued to 6.7 for a  
50km thick atmosphere, and a multiplying factor TP_RAYLEIGH_FACTOR,  
defaulted to 1.

As we increase TP_RAYLEIGH_FACTOR, the density curve will sharpen and  
become more spiky. The transition near the horizon will also be sharper.  
The overall number of particles will also decrease and there will be less  
scattering, yielding less reddening at low elevations. The sky will darken.

As we decrease TP_RAYLEIGH_FACTOR, the density curve will flatten. The  
transition near the horizon will also be smoother. The overall number of  
particles will also increase and there will be more scattering, yielding  
more reddening at low elevations. The sky will lighten.

The picture RelativeDensity.png shows curves of relative density for  
TP_RAYLEIGH_FACTOR = 0.5, 0.75, 1, 1.25, 1.5. The number of tarversed  
particles is proportinal to the surface under the curve until considered  
altitude.

REM: We can also control Dmax and compensate loss or gain of scattering,  
but it's another aspect. And it sems we do not - at first - need to change  
it.


FirstBlusSky_rayleigh_factor_0.5_0.75_1_1.25_1.5_elev45.png.

FirstBlusSky_rayleigh_factor_0.5_0.75_1_1.25_1.5_elev2.png.

The general behaviour seems to work fine, already with a basic model for  
atmosphere (pure atmosphere with only air molecules and only rayleigh  
scattering). It will soon be time to control the color values (Hue,  
Saturation, Light) and compare with photographs. This will help us  
determine correct values, and also reveal the limitations of this simple  
model. The model will then be augmented with additional medias to get  
closer to what we observe in nature.

To be continued in next post: TerraPOV - Sky system - Atmosphere - Part4,  
A Better model (cont'd)


Bruno

PS: Congratulations if you have read me until this point !!!

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Preview of image 'firstbluesky_-5_-1_1_5_20_70_deg.png'

Preview of image 'firstbluesky_2_1000_4000_m.png'

Preview of image 'relativedensity.png'

Preview of image 'firstbluesky_rayleigh_factor_0.5_0.75_1_1.25_1.5_elev45.png'

Preview of image 'firstbluesky_rayleigh_factor_0.5_0.75_1_1.25_1.5_elev2.png'