To test the affect of low/high light maps on the lighting quality I created a simple scene that I could manipulate to easily visualise the changes.
This first set of images shows a mix of low/high lightmaps in the first two images, and high density maps in the final two images. Blue denotes low density, red indicates high density.
There is an obvious difference to the shadow quality and brightness to the shadowed wall, and to meeting edges on the low density maps, which is much more resolved on the higher density maps.
This next set of images focuses mainly on the floor shadows.
The shadow edges are surprisingly mostly unaffected, with the most obvious visual changes being the brightness of the shadows and how it affects the material beneath.
And the last comparison is focusing primarily on the wall which has some overlapping geometry and edges for the lighting to resolve.
This shows the most obvious difference. The low density map clearly lacks the resolution to produce much of a gradient and edges are lacking in shadow.
The gif below shows the change in quality.
This demonstration scene is unrepresentative of an actual Unreal scene with the map densities pushed to the extremes, but it gives a good visual idea of how lightmaps affect the final image, and where you might reduce the densities to save on build time performance.
Adjusting lightmaps in an Unreal Project
It's clear that there are light map densities in the scene that are either too low for models that will be in view close to the camera, and other maps are far too dense for areas not clearly in view.
A few minutes adjusting the buildings and models that are out of view has drastically cut the amount of area the Unreal Engine needs to compute for each lighting build.
There are still optimisations that can be made. But I thought I'd also use this opportunity to test the lighting build quality differences.
With the adjusted light maps, the build time takes a couple of minutes each time for the "preview" quality. The "Production" quality mode took about 40 minutes to complete for this scene.
The image quality improvement are minimal. This might be due to ray tracing being enabled which is doing most of the lighting for the scene.
There are improvements to areas in the distance, or behind translucent materials such as windows. Leaving the lighting build information on "preview" quality until you the scene is finished is highly recommended. The preview gives a very accurate representation of the final image which will have only minor improvements - at least with Ray Tracing enbled.
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