When gathering lighting, Lightmass can backtrack to Point, Spot and Directional light sources using photons from the Photon Mapping technique. This means that it can find small windows where light is entering from these light types and resolve the incoming lighting with high-quality. However, SkyLights and Emissive meshes don't support photon emission efficiently, so Lightmass can only find the small important lighting features with brute force. This manifests as splotchy artifacts around indoor corners. To help Lightmass better understand where the light should be coming from, you can place Lightmass Portals Actors around the areas that are critical to lighting. In the following document, we will take a look at setting up and using Lightmass Portals in your Unreal Engine 4 (UE4) projects.