Let's Figure Out 3d Volumetrics!

Even a subtle mist can completely change the image. You’ll be amazed by how much! What if we went nuts and introduced a thick and massive layer of fog? Let’s tame all of those atmospheric effects today!

By COMMONPOINT / Nov 01, 2021 / 11min READ

Common Knowledge

Let's start with the basics. Volumetric effects are a fancy way of saying fog, mist, smoke, etc. Pretty much anything that stays in the air and cuts down the visibility of objects behind it. To make any volumetric effects possible, we need some kind of air pollution, like dust or water droplets. That causes the light to scatter more than usual, resulting in unique atmospheric effects. While some can be massive and thick (like fog), they usually evaporate quickly and cover a small space (like mist or smoke). Still, they have something in common - they look fantastic!

Volumetric effects give that dramatic, almost sinister feeling to the image. It’s not something we see every day, and it truly grabs attention. That’s the big reason people in various industries use those effects, and so should you. Filmmakers spread smoke across forests to create that creepy/eerie feeling. Photographers wait for days to capture fog in that perfect moment in nature. As artists, we can sit comfortably in our chairs and make it happen right away. So, let’s figure out how to approach 3d volumetric effects today!

What is Volumetrics Really About?

You may think it’s about reducing visibility, and that’s true to some extent. Fog, in particular, covers the objects and removes the details in textures. So while using it, you might think you are mostly hiding things. But actually, it’s the other way around. Ask yourself: What should I keep in focus? What should come forward and grab attention? Figure out what’s essential, and then mute down the rest. Bearing that in mind, can you find a sweet spot where the architecture really pops up?


The fogging impact is quite massive, isn’t it? At first, we have a flat composition that looks unnatural (0cm). Even a small addition of fog helps to bring dimension and depth to the image (80000cm). Slowly, we fade down the surroundings and bring the cabin forward (7500cm), and that’s our sweet spot. We emphasize the cabin silhouette and establish it as an anchor point. Eventually, the fog is so intense (2000cm) that we cannot see the sky color anymore. We can only see the interior lighting taking over the image. This example can show us so much more than keeping things in focus. Because volumetric effects are really about:


There are many things connected to volumetric effects, but we want to emphasize the composition very strongly. Fog can diffuse light, soften the background and make the foreground look sharp. So to make it work, objects in your foreground need to have strong silhouettes in the first place. You can introduce fog to any image you like, but it will really pop if your composition is suitable for that. Keeping that in mind, let’s figure out how to approach 3d volumetrics in Corona Renderer.

Volume Effect - The Easy Way

It’s time to jump into 3ds max and start doing things. Let’s start with a simple way of adding atmospheric fog to your exterior views. There’s a very efficient way of doing it in Corona Renderer (7.0 and above). But before stepping into it, let’s briefly make an overview of our test scene. To make this trick work, we need to use CoronaSky/Sun for the lighting system. You can find the settings and scene diagram in the images below:


If you look at the CoronaSky Map above, you can notice an option called “Volume Effect.” We recommend turning it on pretty much any time you use Corona Sky & Sun. It will instantly add some extra realism to your scene. It makes distant objects inherit sky color. It’s essentially overlaying sky colors on top of geometry, making the color fade away as we approach the horizon. It produces fantastic results but just to a certain point. Check the example below to see its behaviors across the entire spectrum.


The setup is painfully easy, and it works really fast. At lower values, we get that crisp and clear feeling. Unfortunately, it looks superficial when we go too high. We can see unwanted bleeding across objects in the foreground. Also, there’s a huge downside to using this technique. It doesn’t interact with any light in your scene. So, you wouldn’t achieve proper “god rays” using it. Still, it’s a perfect solution for bringing aerial perspective into a distant landscape. Now, let’s get to the real business.

“CoronaVolumeMtl” is the primary medium for volumetric effects in Corona Renderer. It defines the ability of space to interact with light. By default, light rays travel freely through space until they hit 3d objects. But with “CoronaVolumeMtl” applied, they might behave differently. Change the direction while traveling or even die before reaching things in your scene (which results in dark colors.) That might seem abstract, but we’ll go through the examples and technical side of it too. Today, you’ll show you the setup for both scenes in the examples down below:

Those are unique moments that truly grab attention. And honestly, you can achieve so much more, show your true creativity and stand out from the competition. But before jumping into it, you need to be aware of a couple of things. At first, it might feel a little puzzling. The learning curve is quite steep, but you’ll get there through experiments. Also, the rendering times can sky-rocket sometimes. So don’t get discouraged quickly. We’ll do our best to explain the settings and share some optimization tricks to make it faster. Let’s start with the most critical settings down below:

Let's tackle them one by one:

Part 1/3 - Absorption

The “Distance” dictates how dense the fog is. The higher value, the thinner the fog, nothing more to it. “Color” serves as an additional mask where white – lack of fog, black – extremely dense fog. For most cases, we recommend leaving it at middle grey (RGB 128, 128, 128.) You can do some interesting tricks here, but we will cover them in Part II of this article. The effect of absorption is pretty straightforward, so let’s see it in action:


Part 2/3 - Scattering

The “Color” defines the hue of fog. There’s a cool use of it in Example #1 later on, so definitely check it out. „Directionality” value lets you control how light behaves in space. Even though it’s one value only, there are many situations to consider here. We broke them down one by one later, so don’t worry about it now. It can generally force striking light gradients in space or boost up the god rays. Leaving it at 0 is totally fine if you’re just starting to play with volumetrics. Lastly, we do not recommend turning on “Single bounce only” as it produces artificial results. Let’s see what kind of different behavior you can squeeze out of it.


Part 3/3 - Emission

The “Color” defines the hue of the glowing effect. It's not practical to set it up across the whole fog, but it might become helpful to do some advanced effects (like glowing aura). “Distance” value affects the rays traveling through volume. The lower the value, the higher the glowing effect. Corona Tooltip refers to it as a glowing plasma effect, and we couldn’t agree more!


That’s pretty much it! In the next chapter, you’ll learn how to apply “VolumetricMtl,” and we would strongly encourage you to play around with all of the parameters. It takes experience to master them all. Don’t get discouraged if it doesn’t work right away. Eventually, you’ll get there, one step at a time.

VolumetricMtl - Setup

There are two ways of applying “VolumetricMtl.” You can drag and drop it to “Global Volume Material” under “Scene Environment,” and you’re ready to go. It’s pretty straightforward and allows you to set it uniformly across the whole scene. It’s a perfect solution for a slight atmospheric fog we just covered in the previous chapter. It works with HDRI lighting so go for it whenever you want that extra realism and depth to your image. The option we would recommend, though, is applying it on top of 3d geometry. So it works locally, in a specific region of your scene. Just as it’s shown in the examples below:

There are three big reasons for doing so. It’s faster, gives you more control and creative freedom. Volumetric Effects are very time-consuming, so we need to be smart about them. Rendering times will go much lower when you focus on the space that actually needs fog, which usually is not that big. You’ll be able to keep some of the objects not affected by it too. There’s no need to have a fog between the camera and your main subject in most cases. Lastly, it allows you to create unique effects that wouldn’t be possible otherwise. That involves using fog as a “painting canvas,” but we will expand on that in Example #2.

We have all the tools we need. Let’s jump to practice!


Example #1


Buckle up and prepare for a ride; THIS IS GOING TO BE FUN! :) The following example is a play between color and lighting. It wouldn’t be possible without volumetric effects. The selection of colors creates an excellent harmony, but let’s focus on the lighting. You can notice that the background works as a canvas, even though it is only air. The negative space around our main subject is basically “painted” with light. We can experience how the fog is carrying illumination. How cool is that? Let’s check the setup and additional comments below:


We applied “VolumetricMtl” to a box around the subject. We can notice that the box itself is not so big, which is perfectly fine. We purposefully left an empty space in front of the camera, so the foreground is not affected. We changed the “Scattering Color” and set a relatively low distance, producing thick, deep red fog. Two lights (1 and 3) play the biggest role in this scenario. It is essential to keep a good balance between their intensity. They produce the lighting gradients, which is the heart of this setup. Lastly, you can make the image pop even more with two rim lights (2) that affect the central structure. It’s worth mentioning they have a cyan color that blends the composition even more.

Example #2


Fog can become a creative tool, and the following example shows that perfectly. It is a fresh way of looking at “god rays,” too. The image has a dynamic feeling, and volumetric effects help to bring a lot of nuance to the whole composition. It consists of a very thick layer of milky fog close to the ground and around the building. This scene exemplifies the use of custom-shaped fog too. Check the diagram below and its peculiar shape:


The setup is very simple in terms of the lighting. There’s Corona Sun shining from the side and nothing more. The strategically placed tree helps to bring the shadows in the foreground. It evokes the impressions of dynamism right away. What’s the heart of this setup is the custom-shaped object. It serves as a medium for “VolumetricMtl.” Higher scattering directionality allows for more distinct light rays. Also, it creates a bright glow on the right side of the central structure (this is due to the selective amplification of bounced rays, explained in the last chapter).

Artur: “My jaw went straight to the floor when I saw Bartosz doing those weird-shaped boxes. Soon I realized the ingenuity of the idea. We can go way beyond a simple box to apply Volumetrics Effects. And that opens a huge window of opportunities.”

Common Mistakes

In the list below, we tried to pinpoint when you’ve gone overboard one way or another. It will help you stay focused and keep fine-tuning even smarter:

  • 01.

    Bad composition

    volumetrics effects look unappealing if your scene is cluttered or doesn’t have strong silhouettes

  • 02.

    Uniform lighting setup

    working with lights is all about gradients, and it’s even more critical with volumetric effects

  • 03.

    Lack of volumetric effects

    even a small addition can give depth to your scene in any lighting conditions

  • 04.

    Visible light cones

    unless you create a street light, be careful with the directionality, so it doesn’t look artificial

  • 05.

    Camera inside the box

    volumetric effects are not visible in such a situation

  • 06.

    Hard edges within the frame

    don’t forget to smooth out all the edges for the volume that contains fog

  • 07.

    Impractical approach

    try to isolate the most essential objects/lights and use that for the initial volumetric tests only

  • 08.

    Endless rendering times

    it’s fine to stop rendering an image with thick fog at higher noise than usual

  • 09.

    Incorrect light balance

    remember that you can always duplicate the light and exclude/include it to only affect the fog

That's pretty much it, or is it? Let's jump to "Directionality" once again!

Bonus Part - Directionality

A deep understanding of the „Directionality” parameter might be a bit challenging. We strongly encourage you to play around with it, feel it out before you really dive into it. Anyway, let’s roll.

“Directionality” affects how bright or dark the space appears to be. That might seem pretty vague, but we’ll slowly expand on that. It doesn’t change the light intensity but boosts or kills some rays before they reach the camera. We can get stronger lighting gradients because some of the rays might get promoted over others. Also, “Directionality” depends on the relationship between light and camera position in your scene. The same “Directionality” values will make dynamic or flat lighting gradients, whether the light is in front or the back of the camera.

Keeping that in mind, let’s discuss 3 different scenarios:



1. Back Lighting

Negative values reduce scattering; positive values increase it. In the example below, it’s the brightest around the value of 0,8. Getting even higher deepens the scattering gradient. At the extreme value of 0,99 we can see only faint scattering close to the light position in the scene.

2. Side Lighting

The light behaves similarly with both negative and positive values of directionality. More extreme values will cut its scattering off entirely.

3. Front Lighting

Negative values increase scattering while positive decrease it. In the example scene, it’s the brightest around the value of -0,4. Getting lower from this point will dim the scattering, with a vague upward gradient becoming visible. At the extreme value of -0,99, we can see no scattering at all.

Extreme values of -0,99 and 0,99 give you that uncanny, painterly impression, which is kind of interesting. Objects will be lit themselves, but you won’t notice that much light scattering in the air. It’s also worth mentioning that directionality has a powerful effect on the lighting. It can even affect the rays that are bouncing from the objects in the scene. This might sometimes give the effect of glow or aura around the objects, which we mentioned in example #2.

Even though “Directionality” is a single parameter, we strongly encourage you to test it out and definitely shoot us a message when you do! (hello@thecommonpoint.com)

Bartek: "When I started working in the industry, rendering volumetric effects would often take too long so we just dropped zdepth channel gradients in post-production each and every time. It usually did the trick but it took me a few hardware upgrades to fully discover the potential behind the volumetrics. I found out that it is literally the medium to sculpt the light. You can create the simplest scene, run the interactive rendering session and discover on your own how easily you can reshape its apperance and even venture far into painterly territory."

Conclusion?

Not yet, though. There’s more to explore!

In the next article, you’ll find more ways to work with volumetric effects using VDB objects. You’ll find advanced examples and free 3d assets. A little sneak peek down below. See you there!




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