In Unity, precomputed lighting is generated in the background either as an automatic process or when manually initiated. It is possible to continue working in the editor while these processes run.
When the precompute process is running, a blue progress bar will appear in the bottom right of the Editor. There are different stages which need to be completed depending on whether Baked GI or Realtime GI (deprecated) is enabled. Information on the current process is shown on top of the progress bar.
Progress bar showing the current state of Unity’s precompute.
The stages of lighting precomputation are listed below:
Realtime GI (deprecated)
Only static geometry is considered by Unity’s precomputed lighting solutions. To begin the lighting precompute process, you need at least one GameObject marked as ‘static’ in your Scene.
When you manually initiate a precompute, all aspects of your Scene lighting are evaluated and computed. To recalculate and bake just the Reflection Probes by themselves, click the drop-down menu attached to the Generate Lighting button in the Lighting window.
NOTE: When using Auto Generate mode, Unity stores your lighting data in a temporary cache with a limited size. This means that when you exceed the cache’s size, Unity deletes old lighting data. A problem might occur when building your project if some of your Scenes rely on auto-generated lighting data that has been deleted. In this case, your Scenes might not have the correct lighting in the built project. Therefore, before building your game, you should uncheck Auto Generate, and generate the lighting data manually for all your Scenes. Unity then saves your lighting data as Asset files in your project folder, which means you have the data saved as part of your project and included in your build.