Reddish tint of dawn or dusk

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Old 04 April 2013   #1
Reddish tint of dawn or dusk

At about 6:20 am today, I saw on my bedroom wall the the pure red color of sun rise. It was really reddish and the shadows next to it were even more blue. I used to achieve this effect with ordinary lights by changing light and shadow colours even in scanline times.
Today I wanted to create the same reddish tint with a daylight system on mental ray.

I played with sun's height, red-blue tint value, saturation value and even color temperature values in exposure control. Yes there is eventually some redness, and very nice, too but, but it is more like a yellowish tint on the entire image - like the ambient colour is yellow so I couldn't reach the exact same effect.

The distinction between the direct illumination on surface and the shadow next to it is so vivid. One red and the other blue. Actually the same effect is noticable in noon lighting either. May be due to the Haze model, I don't know? I tried Perez and CIE, too.

Any ideas on how to improve this?
 
Old 04 April 2013   #2
Physical sky models are very general approximations really, not taking into account all the possible scattering that goes on in the atmosphere. Then there are other additional effects that happen only in the viewers 'eye' like shadows being pronounced in proximity to a lit surface or a complimentary color tint of shadows.
Another problem is that the RGB model fails to represent the natural color range of blue-purple and green-cyan, I guess that's why sky lighting looks way more realistic with renderers that are based on wavelength rather than raytracers with 'simple' RGB colors as input (no 'scientific' back up for this claim but it's something I've read very often).

I too have often noticed that most physical sky simulations tend to give the sun a more yellowish tint than what we see, or add way more cyan to the sky around the sun glow than there really is. Even dedicated applications like Vue or Terragen add some strange tints to the sky in some cases.

I guess for those extreme cases you should rather simulate this completely with an own color for the skylight and a separate sunlight, just how you did in the scanline era
 
Old 04 April 2013   #3
Thank you for the in depth contribution.
I don't mean a perfect representation of entire lighting effects was possible in old times, but a reddish light color with a blue shadow color is what I can't find in this more advanced daylight system.

I have achieved something with a combination of reddish global lighting tint and bluish red-blue tint of the non-physical tuning of the daylight system but it is so global kind of tuning and kills contrast, and also very much out of control.

May be I should use sunlight instead of daylight. I just noticed that it has light colour value, but I recall daylight to be more -kind of- photometrically realistic. Don't know where I 've read it though; may be in Help.
 
Old 04 April 2013   #4
Quote: I don't mean a perfect representation of entire lighting effects was possible in old times, but a reddish light color with a blue shadow color is what I can't find in this more advanced daylight system.


That's exactly what is so special about it, it occurs during a very narrow time window when the atmospheric scattering contributes a lot more to its reddish tint than it does for the rest of the day. I don't think it's impossible to simulate but a generalized model of the sky may miss that special condition.

As for the photometric behavior of a sunlight/directional target light - I don't think there will be any difference (not sure what that difference could be).
 
Old 04 April 2013   #5
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