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Properly Matching Emissive Materials to Exposure

With the addition of Lumen to the Unreal 5 system Lighting has become an ever more talked about topic in Games. It is a powerful tool that does a lot of heavy lifting and so now there are more people interested in the topic due to it seeming more accessible.

Today’s lighting tip deals with matching our material emissive brightness value to the brightness value of the scene.

In lighting we deal with vastly different scales of intensities depending on location. For example an office interior has a brightness of roughly 600 Lux while a park bench under direct sunlight could have a brightness of 100000 Lux.
Depending on the intensity of lighting, we use a different exposure setting to capture as much light information as possible (Exposure Value or EV).

In games this means that as a character is transitioning from a dark corridor into a bright open battle arena the camera needs to adjust its exposure so that the player can see the world and nothing is blown out.

When the light changes this much it can sometimes be tricky matching Emissive elements in your scene so that they always seem to be glowing.

For example, in the image below I have a default emissive material setup in an interior space. The colour node is plugged directly into the emissive slot with no multiplier. The brightness of this interior space is very dark and so the EV value for the camera needs to be set to -8 to fully see this emissive.

When I take the same emissive ball outside it looks almost black. This is because the exterior world is so bright. The exterior world’s EV Value is 6 and the low brightness of the emissive can’t compete.

We can help with this by adding a simple couple of nodes in the material graph and use some simple math. Below we have a colour node being multiplied by a Power node. We have a scalar node with a value of 2 plugged into the Base slot and a parameter (similar to a scalar node) plugged into the Exp (Exponential) node.

In exposure, when we move from one EV unit to another we are essentially either doubling or halving the camera’s sensitivity to light. This is where the scalar value of 2 comes in. We then decide how many EV stops we need to move up by and slot it into Exp slot using our Parameter value.

We know that the interior EV value is -8 and the exterior EV value is 6. So we take the difference of 14 and put that into our Material Graph. And now when we look at the Emissive Material in the exterior scene it has the same visual intensity as the interior scene.

Not all artists will want to be thinking in such a complex manner when it comes to matching values. In practical terms we can adjust this material graph to be more accommodating. Here artists play 1st with the brightness multiplier to get the look they want and then when they encounter a brighter area of the game they simply input the number of EV stops difference and maintain all their previous artistic efforts

Hopefully this is helpful and you can use it in your own workflows.

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