Hi mifa,
For your reference here are two sources that can help you with the basics and understanding of lightmapping your assets for Unreal Engine 4.
- UE4 Documentation for Lightmapping
- World of Level Design. While these tutorials and information is with UDK/UE3 it’s still using the same process that would be needed for creating Lightmaps for Unreal Engine 4.
With the curtain from the Realistic Rendering, The entire mesh has been stretched out to use the 0,1 UV space to get the best results. If you look at UV Channel 1 you will see that the mesh has been mapped so that there is no stretching for the texture that is applied. When it comes to the lightmap, you can adjust and stretch faces to fit the entirety for better results and to maximize the 0,1 space for the best results. The idea is also to use the lowest lightmap resolution you can to get the best results, since this will use texture memory which can be costly with higher lightmap resolutions and lots of objects. Typically those in ArchViz are not as concerned about FPS and tend to use higher lightmap resolutions though, but it still will require a good UV layout for lightmaps.
For something like your next question with something using a lot of geometry that needs to be broken up or separated it can be very difficult to get a good quality lightmap, but these are also very small pieces that when using the flat mapping method in 3Ds Max or any modeling program will not produce a good UV layout for lightmaps. With something like this, you could probably get better results by breaking the mesh into smaller pieces that are their own sections that can be reassembled. This way you can maximize the space in the Lightmap UV space.
Personally, I would look at making this a movable (dynamic) object and then in the World Settings I would increase the Volume Light Sample Placement density by using a lower value. Default value is 1.0. Try something like 0.5 to 0.1. Then in your scene use a couple of stationary lights in this area. This will then be able to cast dynamic shadows but it will also use the static lighting samples to get the indirect lighting of static without the need for a lightmap. It wouldn’t come without it’s own testing and setup to get better results, but it’s better than fighting with a UV for the lightmap on a complex piece of geometry.
You can read more about Indirect Lighting Cache here: Indirect Lighting Cache in Unreal Engine | Unreal Engine 5.1 Documentation
I hope this helps.
Tim