HDRI Lighting
Easy to learn, easy to master!

The easiest way to bring life into your scene is by using HDRI lighting. That’s it, seriously. Basic principles are simple, and they translate well to rendering engines. Let’s discuss the illumination process and how to have control using HDRI lighting!

By COMMONPOINT / Feb 01, 2021 / 12min READ

Common Knowledge

HDRI is, in fact, a High Dynamic Range Image 1. It is the opposite of a JPEG, an LDRI (Low Dynamic Range Image). That means that HDRI has a greater range of brightness (more than 0 - 255) 2. This way, you can record many real-world scenes containing very bright, direct sunlight as well as extreme shade. The parts of an image that appear pure white (or black) consist of many useful light features. What does it mean to a CG artist, though?

HDRI lets you set the lighting conditions of a given time and space. You can use it inside 3d software as an illumination system.

HDRIs used for illumination are mostly spherical maps, which we can set as our environment. When we speak about illumination, we mean all of the light traveling through the scene (global illumination). Every pixel in HDRI shoots a ray of light into the scene 3. All of those rays have different intensities and colors. Sun will shoot considerably brighter rays than the sky. Meanwhile, there will be a lot more rays shooting from the sky simply because its surface is greater. Now, what to keep in mind when you're looking for a perfect HDRI:

What makes up a good HDRI?

  • 01.

    EV (Exposure values) 4

    the measurement of the dynamic range

  • 02.

    Resolution
    the quality of illumination (amount of rays) and backplate

  • 03.

    Geolocalization
    lighting and weather conditions

This is what you might consider when choosing your perfect HDRI. We also covered the list of our most favorite HDRIs down below, so definitely check that out. Now, let’s get to the fun part and put all of that into practice!

How to set up an effective HDRI workflow?

It’s pretty much straightforward, and we will use our lovely Corona Renderer for that purpose. Load your HDRI as CoronaBitmap and plug it into CoronaColorCorrect map. Connect it to “Single map” under Scene Environment, and you’re ready to go!

We can now use “Gamma” to control sun intensity and “Exposure” for the overall brightness. Usually, we start with the “Gamma” adjustment. Go below 1.0 if you want more intense sun, and go above 1.0 if you want less. Next, you will need to raise or lower the brightness with the “Exposure” adjustment. Let’s see that in action:

So, setting up the HDRI lighting is a matter of finding the sweet spot between “Gamma” and “Exposure.” You go back and forth until you find the perfect illumination. Usually, “Gamma” values vary between 0.8 - 1.2, and “Exposure” values - between -2 and +2 (even though they can get crazy like -10 to +10, so don’t worry about that). But obviously, there are more aspects of HDRI you can play with.

We need more control!

“Gamma” and “Exposure” are the main controls when you deal with HDRI lighting. But it doesn’t mean they are the only ones. First of all, don’t forget to set the values in the Corona Frame Buffer. We usually leave the standard “Exposure” at 0, “Highlight Compression” above 3, and crank up the “Contrast” above 3 or 4. Then, we are ready to set the lighting and play around with all the parameters. Just take a look at the list down below:

  • 01.

    U offset -

    sun latitude

  • 02.

    V offset (Environment) -
    sun longitude

  • 03.

    Gamma -
    sun intensity

  • 04.

    Exposure -
    overall brightness

  • 05.

    Temperature -
    color balance

  • 06.

    Saturation -
    intensity of colors

You can control the sun's latitude, longitude, and it’s strength. You’ll easily adjust the temperature of the sky or totally desaturate it. And the beauty of this setup is that it works both in exteriors and interiors. Just start with the “Gamma” adjustment and balance the brightness with “Exposure.”

Few more things about HDRIs

It is common for HDRIs to be named with four digits (PG 1658). They represent the hour in which the photos were taken. This gives you a rough impression of the sun’s position so that you can use it to your advantage, especially in overcast situations, with blurry highlights. More often, you can find two more digits, which represent the sun's angle. Additional information is rarely provided, but you can always check the files’ metadata for the year’s exact time.

At noon, the sun’s latitude will be totally different in December in Norway than in June in Spain. Even the color of the sky and visible cloud formations will differ accordingly. Instead of pinpointing the specific location, you should just use your creative freedom to alter and adjust your HDRI, respectively. Every single HDRI will have different values to make it work. Test your “Gamma” and “Exposure” values every time, or even jump into Photoshop and take it from there.

Artur: I’d love to encourage you and test whatever HDRI gets into your hands. For the past five years or so, I have used mostly PG 1658, which I wholeheartedly recommend to this very day. But what will make you grow as an artist is to break the routine, try something new and plainly have some fun along the way

Bartosz: It is easy to appreciate an HDRI with a dramatic sky of golden or blue hour. I have contrarily chosen something much more mundane here (it's not that I don't like a good sugary sunset) and I hope it will be a more useful suggestion. PG1714 is my first go-to HDRI when I aim at the punchy commercial daylight scenario. It is bold, creates stark contrast and the deep colour of the sky tints shadows with pleasant blueish hues.

How to fix Backplate the easy way?

So, we plug our hdri into the environment slot (F10). And as a default, the same map is used for the backplate and reflection/refraction map. If you wish to have a different backplate, You can always use the same hdri, but with a separate CoronaColorCorrect adjustment. Change the "Gamma", "Saturation", anything you need. Plug it into the direct visibility slot, and that’s it, like so:

Most often, when you use your HDRI as a lighting system, it will fail as a backplate. It will most likely fail as a reflection map too. In the example below, you can see how the initial HDRI looks versus the adjusted one. What we did there is really simple. Use the same HDRI, and plug it into a separate CoronaColorCorrect, play around with values, use real life reference for the sky colors, and that’s it. Check the settings and the difference below:

One last thing, though. Even if you use the highest resolution HDRIs (16k or so), the backplate can still appear pretty blurry. It depends entirely on the focal length you use. The bigger it is, the larger a single pixel of HDRI will need to be to cover your image. Still, You can work your way around it. Go to CoronaBitmap and crank the “U tiling” and "V tiling" values above 1.0. Since we use this map for Direct Visibility anyway, we can scale down the HDRI. This way, we compress the visible portion of the sky, so more of it covers the image itself.

How does HDRI lighting work in interiors?

So far, we know that the spherical map (HDRI) shoots rays of light into 3d space. Light is coming through the windows, and it’s going inside the interior. It keeps bouncing off of surfaces until it eventually dies out. We also know that we can rotate our environment to set the sun’s direction. So, let’s see what happens when we let inside either the sunlight or just the skylight. Let’s make a full 360 rotation of HDRI:

Environment 0 - 360 Degrees

PG 1658, Gamma 0.9, Exp 2.0

It’s worth mentioning that light coming from the sun is intense and warm. Everything gets brighter. But when the sun is on the other side, we see the light coming from the sky only. Naturally, it’s getting colder and a bit darker. We can see that rays of light play a great deal in illumination. Still, we need to understand more about the illumination process.

Keep in mind the albedo level!

Light bounces off of surfaces onto other surfaces. Some portion of the light gets absorbed, and the rest keeps traveling. Eventually, all of the light fades out, finishing the global illumination. Why is that so important? If you have more light traveling inside an interior, it gets brighter in general. If you have less light traveling, it gets darker. Simple. And albedo is what controls the absorption and reflection of light.

Albedo is measured on a scale from 0 to 1, corresponding to light fully absorbed or reflected. 5

It might sound complicated, but it’s really straightforward. When we talk about albedo, we simply mean your materials’ colors. RGB (255,255,255) is interpreted as albedo 1.0, and RGB (0,0,0) as 0.0. So, if you have bright materials in the interior (Brightness +200), you’re going to get more light reflected from all of the surfaces. There will be more illumination, and it will be more difficult to achieve dark shadows. Let’s look at this example:

Why is there more contrast in the first image? The reason for that is, we have lower albedo and more light coming in. Second image has higher albedo and less light coming in. And so the outcome has less contrast. That means you need to be careful with materials in your scene. Don’t go overboard with the brightness of your materials (Brightness +220). This way, you don’t give yourself a chance for natural illumination. Be sure not to make the materials pure black for the same reason. This applies to both exterior and interiors.

What happens behind “Gamma” and “Exposure” adjustment?

This is the technical bit, and it’s not essential for understanding this technique. “Gamma” and “Exposure” values correspond to the Photoshop Exposure values. They do, really! Just put the Adjustment layer on top of your HDRI and input the same values as you would in CoronaColorCorrect. You could use this new HDRI straight in CoronaRenderer, and it would give the same illumination. We can also analyze the colors in our HDRI when we change “Gamma” and “Exposure.” Let’s use a simple histogram for that purpose:

You can see how the histogram responds to “Exposure” and “Gamma” Adjustment in Photoshop. Colors get shifted and stretched. We can see a spike on the right, which represents the sun. The rest of the histogram is simply clouds and sky. We said that we could change the sun’s intensity by “Gamma” Adjustment. Technically, we’re just moving the spike of the sun closer or further away from the sky. The appropriate way would be to say that we change the sun’s power in relation to the sky. Also, balancing the brightness using “Exposure” simply means “shifting” the colors left or right. This way, we can set the overall brightness higher or lower. All in all, setting up an illumination system with CoronaBitmap and CoronaColorCorrect is a simple color correction. We put more contrast to the HDRI and raise the level of brightness. And that’s pretty much it!

Conclusion

HDRI lighting is a straightforward process. You can control the sun’s intensity and overall brightness using the “Gamma” and “Exposure” adjustment. Keep in mind that lighting is a process of illumination. It depends on light traveling through the scene and the brightness of objects as well. Be careful with your materials, and don’t create materials that are too bright or too dark. If the albedo of your materials is correct, you’re on the right path for realistic illumination.

Thank you for staying out this long with us, and I hope to see you next time! Subscribe to our newsletter and enjoy the upcoming free content.

1) Dynamic Range is a complicated subject that we breezed through. If you wish to learn more about it, check this awesome resources down below:
Dynamic Range Explained
What Makes Good HDRI?

2) RGB color values in the RGB color model also convey what we perceive as brightness. In standard 8bit RGB file, a black pixel has 0,0,0 RGB color values and a white pixel 255,255,255. We can't assign more than 255 color value to something as bright as the sun.

3) The situation is slightly different when we use the path tracing algorithm. Though, for the sake of simplicity, we tried to describe only the portion of it. More information here:
Path tracing

4) Don’t miss the great resources from VWArtClub. Exposure Values explained:
Exposure in 3d Photography
Light in 3d Photography

5) There are no objects in nature that are fully absorbing the light or fully reflecting. More information here:
Albedo

List of our most favorite HDRI:

https://www.pg-skies.net/products/1957
https://www.pg-skies.net/products/1658
https://3dcollective.es/en/producto/real-light-hdri-067-1326/
https://3dcollective.es/en/producto/real-light-hdri-008-1846/
https://cg-source.com/Product/3086-03/Cloudy+Sky
http://noemotionhdrs.net/hdrevening.html

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