Physical Sky – how to achieve any look you want

The Physical Sky settings still remain a bit of a mystery to many users and most likely they change parameters more or less blindly, hoping for the best. This post will hopefully make things a lot clearer, especially which parameters are the most important and so should be changed first, before “fine-tuning” the look with the rest of the parameters. It also helps if you imagine the atmosphere model as one big SSS sphere with tiny particles in it which reflect and spread the light, giving different hues and intensities depending on their amount/size/direction in which they reflect the incoming light (“asymmetry”).

First, I’ll change the time and rotation settings so I have the sun disk straight in front of the camera and set the time to just before sunset. Most of the parameters become more sensitive this way since the sunlight has to travel much farther through the atmosphere, hence this will help us see better what each parameter does – their effect is “multiplied”. Note that I used a little bit of scattering on all examples to give a more realistic result of the sundisk. Click any image to enlarge it and to read its caption.

The most important Sky parameters

I’ll list them in order of visual importance, meaning, how much they affect the look, not necessarily how often they are used:

Intensity

This one is the most straightforward to understand – think of it as an exposure control for the skydome only. In the below image, I changed it from the default of 1 to only 0.1. Notice how the skydome simply gets darker, while the sun itself remains bright (the Sun’s intensity is controlled separately from the “Sun Power” parameter under the Sun section). Note that a darker skydome will also send less light into the scene.

This parameter can be a handy control for the balance between the light coming from the skydome vs the sun, but I tend not to use it because with Multilight turned on, you get two intensity sliders for skydome and sun, which essentially do the same thing. So I prefer to change the balance between skydome light and sunlight using ML instead.

Turbidity

I tend to think of this as the smog. All the molecules of impurities suspended in the air which reflect and spread the sunlight. This controls their amount. The size and how they spread the light is controlled by two other parameters. It’s a very sensitive parameter and its effect is that it will first of all darken everything, including the sunlight. It also adds a coloration to the scene, its hue is dependent on the size of the smog molecules (controlled by the “Wavelenght Exponent” parameter). If it’s a sunset situation, it will make a sort of “dirty red” sunset (good for archviz in LA, Athens or Paris!), if it’s more a daylight situation it will make it look like there’s a sandstorm in the area. Sky becomes more desaturated which can give a nice “cool” look.

The below image also triples the default Turbidity but shows the effect during the day. Notice in the 0.12 turbidity image (top row of images) how the contrast is less harsh and also the shadows are less blue.

Wavelength exponent

Ok, ‘size of the particles of stuff in the air’ sounds much less impressive but that’s what this parameter does and it affects which wavelengths of light are scattered and which ones are absorbed – thus producing different hues in the sky. Of course the higher you raise the Turbidity (the amount of stuff you have suspended in the atmosphere) the more effect this parameter will have.

I wouldn’t go over 10 or so for the max. The bigger you make the particles, the more they will absorb blue wavelengths first giving you much more orange looking sunsets and increasing even more gives you a green looking sky. If you make it smaller you’ll see a desaturated effect – essentially all the wavelengths are being absorbed equally so you still get the darkening from the Turbidity, but a more monochromatic look.

And below Wavelength exp. of 0.1 compared with the default of 1.2

Reflectance

This controls the amount of light the particles in the atmosphere reflect back. So you can see that if you make these particles absorb almost all the light (0% reflectance) it will have quite an impact on the sky and lighting. I never set it below the default of 80%. I only raise it for giving more even lighting, and raise it especially at dusk/dawn times to make the sky brighter which gives a nice separation between the orange sky/smog and blue sky. I don’t raise it above 95%, I suspect raising it more will give more noisy renders.

Asymmetry

Although this parameter is rarely used it has quite an impact on the look of the sky and so also the scenes illumination so I think it’s important to see what it does. It affects the sky both at sunset and day settings.

This is also a setting that affects the particles in the atmosphere and controls the direction of the scattered light when it hits one of those particles. If positive, it scatters most of the light back towards the light source (in this case the sun) and so creates a bright halo around the sun. Negative values means most of the light is scattered in the opposite direction of the light source and so the bright halo appears in the opposite direction of the sun, as seen clearly in the right most image above.

I rarely change it below 0.3 or so, as it makes the behavior unrealistic but it does help produce a very even lighting, especially when used with higher turbidity and low wavelength exponent = monochromatic, less contrasty lighting even in a day setting with sunlight.

Also, avoid setting this to extreme values in either positive or negative as it increases the noise in the render. Don’t go over 0.8 really 🙂

This is it for the list of the most influential parameters. Intensity, Turbidity, Wavelength exponent, Reflectance, Asymmetry. It’s really only 4 of these since Intensity is changed through ML in most cases.

Briefly about the other parameters

I won’t make a screenshot of what the rest of the parameters do because they have mostly a limited influence, especially during day light situations. I’ll mention more about them in the sky formulas after this paragraph.

Planet reflectance – The more light the planet surface sends back into the atmosphere, the more diffused and less saturated the sky will be. It has more effect during the day for obvious reasons – at dusk/dawn, there’s a lot less light hitting the planet surface directly. During day settings it will brighten the sky. It can help to achieve a very “clean” bright blue sky if you also increase the Ozone parameter.

Ozone – in short: “makes sky more blue”. Has more influence the closer you get to dusk/dawn times, in which cases you wouldn’t go over 0.6 or so. I tend not to go over 1 in any situation as it adds an unrealistically strong blue tint to the light.

Water – how much humidity there is in the air and visually it tends to add blue/violet to the sky, especially during dusk/dawn times. Least sensitive of the parameters, can go to over 100 for some very violet/blue looking sunsets.

Sky formulas

So with this new-found knowledge lets try to recreate some real skies.

Deep blue sky

Following these examples.

So I know if we have such a clean blue sky there should be very small amounts of pollutants in the air, so the first thing I do is lower the Turbidity a lot. From 0.04 to 0.01. Lower than that and it doesn’t make a difference any more. Since the Turbidity is now so low, the other two parameters that are connected to it (Wavelength exp. and Reflectance) don’t really matter anymore so I don’t need to change them.

Blue sky, I need more Ozone. But I tend not to raise this over 1 because it makes everything in the scene have a too heavy blue cast, it makes the light look unnatural. So I raise a second setting, the Water, which remember also tends to add a blue/violet hue to the sky itself but it doesn’t change the light itself as much as Ozone (unless you really go crazy with the setting). Water setting is the least sensitive so I raised it up to 100 until I got an even bluer sky, without making the light itself look unnaturally blue.

Lastly, to make the sky even a darker blue, I lowered the Planet Reflectance parameter to just 5%, which makes the sky look less faded and more a deep blue.

Settings: Turbidity 0.01, Planet Refl. 5%, Ozone 1, Water 100. White balance (in Render options) 5500K.

Of course, any time you want a more saturated sky, keep in mind the output from Maxwell is using linear tone mapping so you have to add some saturation to the render in post. And adjust the white balance since the default 6500K will now give you a blue looking render. Always process your render, just as you would your RAW files from your camera. At the very least, adjust the levels to get your white/black points correct, and add some saturation.

Deep blue and orange sunset

A more saturated sunset means a bit more turbidity so I double it from the default to 0.08. To make the particles absorb more blue wavelengths and thus make a more orange horizon, I raise the Wavelength exponent a bit from 1.2 to 2. This makes a pretty nice looking horizon, and now to make the rest of the sky bluer and perhaps also a bit brighter since it looks duller (because we doubled the turbidity).

I raise the Ozone from 0.4 to 0.6. Ozone becomes much more sensitive at dusk/dawn times plus I don’t want to raise it too much since it starts making an unnaturally strong, even blue. Instead, to add even more of a blueish tint, I raise the Water parameter from the default of 2 to 10. This adds a nice blue/violet look instead of just a monochrome blue that Ozone adds.

The sky still looks a bit dull so we can give a general “boost” to all the hues by raising the Reflectance. I raised it in this case from 80% to 95%. You can see below this makes quite a difference. By making every particle reflect back more light (including all the water we added), it makes a nicer separation between the orange horizon and blue sky.

This looks pretty good but there’s a final tweak we can add, using the Asymmetry setting, to boost back a bit the light coming from the Sun. So I changed the default 0.7 asymmetry to 0.78. You can of course add even more but remember to keep it under 0.85 or so.

Settings: Turbitity: 0.08, Wavelength exp. 2, Reflectance 95%, Asymmetry 0.78, Ozone 0.6, Water 10

Faded low contrast

This can be a pretty nice look to have for archviz where you want a more even light with minimal color cast. First I double the turbidity to 0.08, not more because I just want to have a slightly more faded looking sky. Next to desaturate it further, I lower the Wavelength exponent to 0.5. So remember the lower this is, the more all wavelengths of light are scattered equally, so it has a desaturating effect on the light and sky.

To take away a bit from the greenish tint that the increased turbidity has added, I raise the Water parameter to about 20 and this makes for a more neutral color cast.

Finally, since the sky now looks a bit faded and dull, I increased the Planet reflectance and this sends out more light back into the sky, making it brighter, and also helps to diminish the contrast in the light from the sky.

Settings: Turbidity 0.08, Wavelength exp. 0.5, Planet reflectance 65%, Water 20.

Greenish sky

The basic principle is simple – you play with Turbidity and the Wavelength exponent parameters. The more turbidity you add, the more effect the Wavelength exponent will have and thus you get a more saturated sky. The higher the Wavelength exponent, the greener the sky will be.

Four preset files

Here are 4 sky files for you. OrangeBlueSunset, CoolFaded, DeepBlue, EvenMonochrome. Place them in your Maxwell/skies folder.

Final notes

  • Avoid using 0 or 100% for any of the settings. I can’t say something will definitely break if you don’t but you can get some weird results if you use min/max settings for some of the parameters.
  • Remember you can save your sky presets from the UI which is very handy. You get a .sky file, which is just a simple text file.
  • There is a bug when loading .sky files “from Disk” – the settings don’t actually change. You have to put the .sky files inside the “skies” folder of the Maxwell install folder, restart Studio or your main 3D app, and these presets will then show up in the Preset list. Choosing one here will correctly change the settings.

Leave a Reply