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Garden Studios Tech Blog: Colour Correct Regions Ue4 – Part 1‍

Posted: 9 Aug 2024

Written by Mark Pilborough-Skinner, Virtual Production Supervisor

One of the most important things in selling a ICVFX shot is that it provides the ability to blend your physical foreground with the digital background. This can be achieved by either not showing the floor, occluding the bottom of the LED wall with physical props, or by blending your floor into the digital content. A tool in UE4 that is perfectly suited for this is, Colour Correct Regions.

This tool behaves like local post-process volumes, limited to the colour-grading toolset, that can be placed in 3D space.

Below is a step-by-step guide on how to achieve this:

First, enable Colour Correct Regions in the Plugins menu of your project.

Once your project has been restarted, Colour Correct Regions will be added to the Place Actors tab. You can now drag one into your scene.

Next, select the actor and in the details panel, you will see several useful options:

Type: This is the shape of the CCR, Sphere and Box will be the most used, but Cylinder and Cone are also available.
Priority: When multiple CCR are overlapping this will set the layer priority.
Intensity: Controls the overall value of the CCR.
Inner: The hardness of the inner mask.
Outer: The hardness of the outer mask.
Falloff: The smoothness of the lerp between the Inner and Outer masks.
Invert: Inverts our CCR.
Temperature: Controls the default temperature of the CCR.
Colour Grading Settings: The post-process grading options available to us.

Through these options the CCR can be used for a multitude of local post-process effects.

For a use-case and to describe this process, we are going to use them to blend the physical floor with our digital floor.

Once the CCR has been placed into the scene, you can now set the Falloff to0 and set the Gain in the Colour Grading Settings to 0 for bright scenes and 2for dark scenes. This will allow us to better see the area we are affecting.

Position and scale the CCR so that it is affecting a portion of the digital floor we wish to blend.

Our stage has a curved screen and aspherical shape, that’s been scaled to an oblong to suit our needs. For the best results we place the CCR, so it intersects with your display screen actor.

Check your camera feed to see how much of the black area is visible, how it follows the curve of your screen, and adjust the shape and size until you’re happy with the result.

For now, we will be blending a neutral textured floor, but if you’re attempting to blend anything more complex, such as a wooden floor, your physical stage will have to match.
Start to feather the CCR by adjusting the inner and outer values, as well as the fall off.

You should experiment with the setting that work best for your use case. I’ve found that the default inner and outer values work most of the time with a Falloff of 0.25.

The next step is to adjust the Material of your floor texture, so that the flattest, well-lit area of your physical floor has the same saturation and value.
Lastly, return all the values of the CCR’s colour grading options to default, and begin to dial in the grade you want visually by watching your camera feed.

One thing to consider is that CCR should only be used once your lighting scenario is locked: both physical and digital. Any change in intensity, colour, or shadows will change the blend that is achieved.

If you’re lucky this might be a minor detail. To prevent lost work, only attempt to blend once the other departments are locked and signed off.
This is the end of Part 1 of our Colour Correct Region breakdown. In Part 2, we will cover advanced blending techniques, tips from live on set to improve and assist blending, along with some real-world examples!

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