Compositing Realistic Clouds in VFX

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What is Sky Replacement?

Sky-replacement is a technique to change the sky in an image of video to another sky. This is one of the most powerful aspects of matte-painting. It allows you to bring out the true detail of a natural sky in your artwork. In film and animation its often the case that you have to expose either to the sky or the subject. If you want to bring back the details of the sky when it's over-exposed you can use sky-replacement.

I’ve recently explored sky-replacement with a series of personal renders about wind-farms.

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Sky-Replacement Compositing Breakdown

Below you can see the different layers that have been combined to achieve the final results. Each foreground plate has been rendered inside of Cinema 4D with Chaos Corona. And it uses a combination of a Corona Sky and a Light Material with the cloud map in the background. This helps with blending the values during compositing.

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For each render the lighting was based on a photograph from our Freely available Complete Clouds Pack. This allowed for easier compositing and matching of the color and light values.

The renders were made in Cinema 4D with Corona Renderer 8. They were composited in Adobe After Effects.

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Free Cloud Assets for Sky-Replacement

With years of compositing experience I’ve come to realise I needed a large library of Cloud assets with a high dynamic range. For that reason I’ve been continuously developing this asset collection to help beginner and experienced artists in need of clouds.

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Essential Tutorials on Sky Replacement

Sky-replacement can go from simple to advanced very quickly depending on the type of shot you are working with. If you are working with moving shots you will need to first accurately motion-track your clip before applying any clouds to the background.

When working with shots taken at different camera settings from the cloud assets you're using (which will be most of the time) you'll also have to understand color matching. This makes sure that you get a seamless integration between your sky-replacement and your background.

Below are two tutorials that I'd recommend you give a watch before you start working with sky-replacement. Good luck!

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Conclusion

Sky-replacement is one of the most common requirements in visual effects. Whether when you're creating a matte-painting or simply want to improve the look of your background skies, sky-replacement is the go-to technique to make this possible. However, with sky-replacement comes a variety of techniques you need to understand to make sure you achieve good looking results.

Having high quality sky assets is also important. For that reason we have the Complete Clouds Pack available for Free on our Resources page.

If you are working with any of our assets I'd love to see what you make with it :) Feel free to send me a message on leo@blauwfilms.com or through Instagram.

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