How to Perceive Sky Color?

How well can you read the sky? Are you able to decode the location purely by its colors - hue and brightness? That's quite a handy skill to have. Especially if you're a 3d artist. So let's dive into mastering the color of the midday sky, today!

By COMMONPOINT / Aug 30, 2021 / 15min READ

Common Knowledge

We are sensitive to color. Skin, greenery, and the sky - these are what we care about the most. We have clear expectations of them, even though frequently, they don’t match reality. Hah, I bet you heard “make the sky bluer” from your client, even though you knew it’s not a good idea. That single topic seems plain simple on the surface, but it goes deep, and we will break it down for you. First, let’s do a little test: Which one of the gradients below “feels natural” when you think of the sky color? Would you pick one, many, or maybe none?


Well, that was a tricky question since all of them come from different places. You might want to have a rich blue sky every single time, but many factors conspire against our own will (such as dust, pollution, overexposure). Still, there’s an excellent reason to use any of them and make it feel natural. And today, you’ll learn why the sky varies from green to magenta and how people perceive it. Eventually, you’ll be able to choose a perfect hue before jumping to work and nail the colors afterward.

Clear Blue Sky

First, let's imagine a blissful place up in the wild. No clouds, no pollution, and low humidity. In those conditions, we can experience a clear, vivid blue sky. Why is it blue, though? It obviously starts with the sun. All the light is traveling through space until it reaches the earth’s atmosphere. Then, it changes direction because it hits molecules in the air. Those particles make the short-wavelength light get scattered much more than long-wavelength light. And the result is the color blue that eventually reaches our eyes. That phenomenon is referred to as Rayleigh scattering.

But there are gradients in the sky, right? Having deep blues at the top (1) and slowly getting brighter and more desaturated colors as we reach the horizon (2). The reason for that is pretty simple. As light is traveling through the atmosphere, it gets scattered; we already know that. “Point 2” lies near the horizon, where light needs to travel through an increasingly thicker layer of the atmosphere. The longer the distance traveled, the more scattering we get. The more scattering happens, the less blue color we see. And that results in brighter colors. Pretty straightforward. So can you guess why the colors get darker in the images below?

The answer still lies in the light traveling through our atmosphere. The higher we get, the shorter distance light needs to travel in general. So overall, the sky gets darker. You might notice that when you look at mountain photography. That’s a pretty handy rule to follow, isn’t it? So don’t forget to darken the sky whenever you approach renderings at a location well above sea level. Now, let’s get back to the so-called “Clear blue sky.”

Atmosphere Impact

Unfortunately, our atmosphere consists of dust, smoke, salt, pollen, and all kinds of water vapor. Those particles are pretty big, so the entire spectrum of light gets scattered during those conditions. And instead of affecting mostly a short-wavelength as earlier, we get even more scattering. That results in us experiencing less color in general. And since most dust remains close to the ground, the horizon will become more desaturated in such conditions.

This phenomenon is called Mie scattering, and it is layered on top of the Rayleigh scattering. The dustier or more humid the air is, the paler and more desaturated sky colors will appear. Anyhow, what stays in the atmosphere changes the perception of the sky in general. We can also notice slight tints near the horizon, depending on the ground activity, like in the examples below:

At the left, we can observe a brownish-yellow tint resulting from the sand in the atmosphere. On the right, the sky changes its hue due to the highly polluted sky. So to put things in a simple perspective - humidity, dust, smoke, and pollution make the sky appear more desaturated, flatter, and even slightly tinted.

Well-Informed Decision

Now, let’s get back to our example from the first chapter. Take a look at these midday skies gradients and think what impression they make on you. Try to guess where the pictures were taken. We tried to pick references with as little postprocessing as possible to focus on the naturalness of the scenes. Even though we didn’t fully cover the topic, would you try your luck anyway? Keep in mind the brightness, hues, and intensity of sky gradients.

  • 01. Desert in Namibia – this gradient might look like twilight, but these are, in reality, full sun midday conditions in a dusty environment

  • 02. Thailand – bland and slightly greenish gradient is the result of very high humidity

  • 03. Los Angeles – smog-related yellowish hue at the bottom of the gradient

  • 04. Greece – the sky is slightly underlit due to the exposition on the typical Greek white plastered houses. It is a pretty specific scenario, but we find it rather characteristic for many high-contrast cases

  • 05. Colorado – higher altitude makes the sky gradient more pronounced. A similar effect might be the result of the high albedo ground (i.e., snow)

  • 06. Baltic beach – this gradient might be more characteristic for higher latitudes, but it will likely result in a fresh-looking scene with a hint of the seaside chill.

  • 07. Australia beach – deep and the most cyan-shifted colors indicate that the photo was taken in the direction opposite to the Sun position.


To be honest, that’s a challenging game to play (especially since we set a trap in 4 and 7). So don’t worry if you didn’t score high. Still, we hope you already perceive the sky on a deeper level. Brightness, hues, gradients - let you always help to make a calculated guess. So, whenever you work for a client abroad, do a little research about the project location. Experiment with the color and try to pinpoint the nuances. Your client will definitely appreciate the effort, and you’ll become a better artist, one step at a time.

Color Approach

We now understand why the sky appears the way it does, so we can use it to our advantage. Let’s break down a few aspects to look for when working with sky color. And remember, if you ever feel lost, just search for references and use PureRef to compare. So let’s get into them one by one:


Exposition

Keep in mind that a higher altitude makes a darker sky. More humidity flattens down the whole sky. Also, don’t forget that the brightness of your sky is closely related to your scene. Setting the exposition for the white wall (in full sunshine) will bring the entire image down, and the sky itself might appear pretty dark. The opposite happens when you try to present deep shadows in a dense forest. You can feature details near the ground, but the sky will be blown out:

White Balance

The sky’s hue will vary between cyan and blue, and the color is tightly connected with the temperature of color correction (frame buffer). If we add warmth to the image, the sky will become cyan; if it gets colder, it turns even bluer. Various tints might be added to a sky color as a result of the incorrect white balance. So you always look at the highlights and decide if the image is balanced or shifted.

Camera Direction

You will have the brightest sky when you look directly at the sun (3). The darkest part of the sky will appear when the sun hits your back (1), simple. But interestingly enough, when the sun is behind the camera, the sky will be the most saturated, and the sky gradient should be pretty much vertical (1). When you rotate the sun, the gradients will become more angled, brighter, and paler (2,4).

Field Of View

The color of the sky isn't constant, and it spans from the brighter horizon to the darker zenith. This aspect will be very noticeable when seen through a wide-angle lens. In the following example, you can see more sky visible when a wider lens is used, thus creating a dramatic sky. Remember that you can always make the sky more intense (even if it’s not consistent with the lens used), just for the sake of making it visually pleasing.

FOV Change

Now, that we know what to control, let's get to the more nuanced color selection.

Color Strategy

So we covered a lot of ground on this one, yet we still have some small things to add. Someone's preferred colors may have nothing to do with the actual values. This is when your client tells you: “I don’t care if it’s real or not; just boost the saturation.” Obviously, we need to adapt to this kind of situation too, and it’s good to understand where they’re coming from. In order to establish the colors, you need to realize what is the purpose of your image. Check the examples and think about the goal of the colors, however that may seem at this point.

Remember that the sky always plays a part in a more significant color strategy. It might be naturalness and realism that leaves all the exposition for the main subject. Or just the opposite - dramatism and dynamism. An architect client will have different expectations than a marketing agency. You might want to crank up the saturation if the image tries to market a tropical retreat or tone down the sky if the architecture needs to stand out. You can always research your clients’ preferred style and understand their needs beforehand. Also, we all come from different backgrounds, and there’s cool trivia associated with that. Take a look at the squares below. Each of them represents what is called a „sky blue” color in a specific country.

  • 01.

    Mediterranean

  • 02.

    France

  • 03.

    Argentina

  • 04.

    Spain

  • 05.

    Italy

  • 06.

    Japan

You can see how much they differ, which gives you a glimpse into cultural biases, yet you should treat this aspect rather lightly. If you live in a country with 300 sunny days a year, you will probably remember the sky's color differently from a person with only 100 of those. That would be true even if the clear sky were the same in both places. Similarly, someone living up in the North might have a different perception of a sunset (long and mellow) than a person from the Equator (quick and dramatic). That's even if the measured hues in the specific moment were the same. It's all about the context and the memories we build around the sky.

Bartek: "I come from an architectural background in which using vivid blue skies was considered more or less kitschy. I know now that it has never really been about the sky itself but rather its role in the entire composition and how it corresponds with it. You can play bold and still stay in style. "

Common Mistakes

We hope that you’ll have different perspectives on the sky from now on, but we want to give you some extra tips, anyway. Those are just some red flags that the setup is not proper:

  • 01.

    The natural sky isn’t green/magenta - watch out for color balance

  • 02.

    Too much saturation - remember to lower the saturation when you make the sky brighter

  • 03.

    Shadow tint doesn’t match sky tint (often, shadows are desaturated, which is visible on white walls)

  • 04.

    Flat/dramatic sky gradients - keep in mind your lens and atmospheric effect of keeping proper sky gradients

  • 05.

    Clamped sky - watch out for tone mapping (highlight compression in particular when using sky models)

  • 06.

    A mismatch between the color of the sky at the horizon and aerial perspective in the scene

  • 07.

    Using wrong reference (color graded, taken in different conditions, mismatched camera settings)

  • 08.

    Using wrong backplate sky (color graded, taken in different conditions, mismatched camera settings)

  • 09.

    Trying to use the same hdri setting for scene lighting and sky visibility

  • 10.

    Watch out for clean gradients as you might end up with banding effect when compressing for web. You could try to avoid it by adding some noise to it.

That’s pretty much it. We hope we didn’t charm the sky away. We actually tried to do the opposite: make it even more magical by helping you understand how it really works. Lastly, let’s jump into some visual candy!

Examples



Clear sky creates a dramatic juxtaposition with the vivid fabric cover. The bottom tint indicates dusty conditions.

Technique: Sun & sky with bottom clouds pasted on top of it



The colors of the sky are very vibrant and provide the image with a commercial punch. Deep shadows and elevated mid-ranges create a fresh seaside feeling.

Technique: Backplate integration



Natural, slightly desaturated sky puts the main subject in the spotlight.

Technique: HDRI



The composition is built around the vertical cloud formation. Pale, cyan-shifted sky indicates humid, convective conditions.

Technique: Backplate integration

Artur: “I haven’t dive into one particular color so deep until now. Yes, the short lesson from all of this is to nail the perfect hue and brightness. But behind that, there’s a fascinating thinking process you can have total control of.”

By now, we understand how to approach sky color on different levels. We can decide which kind of hue will work the best and do a proper setup both in 3d and color grading software. But what if we jump into pixel-perfect colors one last time?

Color Science

This chapter is for all the people that like to analyze pixels in and out. Even though it might be a more challenging read, we hope to walk you through it as painlessly as possible. In general, the average color of the sky varies widely in brightness, saturation, and hue. We selected a couple of daytime renderings in the following collage, extracted sky gradients, and placed them side by side. Then, we used Davinci Resolve to plot the graphs below:


Cyan / Blue Range

On the left graph, we can see how many pixels fall into the specific color spectrum. On the right, the graph shows 0% saturation in the center and reaches 75% at the marked points. Interestingly enough, there’s a consistent range of hues that peaks in the middle between cyan and blue (shown both on the left/right graph). Do you notice how the colors create an arc on the right graph? That means more saturated colors tend to go blue, and desaturated colors shift towards cyan. Let’s dig a little deeper.



Brightness Range


On the left, you can see a “Waveform,” which tells you the brightness of the pixels. The brightest colors rarely go above 90% (besides the white spike in the center.) If you check the colors on the scope, you can also notice that the higher we go, the whiter the colors are (desaturated). Pixels below 65% seem to have more color (saturation)

Let’s make one more test, and check how brightness and saturation co-relate. In the chart below, we changed the saturation and brightness only, and we noticed quite a peculiar thing:


Brightness / Saturation Relationship

When images get lighter and more saturated, they become cyan, while darker and paler ones will be moved further towards magenta. We can see it very clearly in our example chart, even though we directly change brightness and saturation only. So, anytime you make adjustments, be aware of the nature of colors and compensate for it if needed.


Conclusion

Nature is full of beauty. Something as mundane as a clear blue sky will never stop amusing us. Think about all of those nuances that make it look the way it is. As an artist, working with color might be hard to master, but understanding how we perceive it, helps you make deliberate decisions. It is the first step to becoming a better 3d artist. So from this point on, look up, break down the color of the sky, and gain your experience, whenever a sunny day comes.

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