Custom Materials were re-introduced along with some enhancements in Lumion 2023.1.0. See section 2.1.3: Custom Materials:
- Knowledge Base: Lumion 2023: Update to 23.1.0 - Release Notes
The thumbnail of the material may not suit when you add it to your Custom Materials Library.
Here's a way to improve that.
1. New Custom Material and Thumbnail:
1.1: The camera used for the thumbnail is the current camera in the Scene.
1.2: To create a flat rectangle thumbnail, simply move your camera to face the surface where the material is in use, before adding it as a Custom Material. If you move the camera close to the surface you will get a better look for the thumbnail:
Then click on the Save OK button.
2. Existing Custom Material and Thumbnail:
If you have already created the Custom Material and want to update the thumbnail then use the following steps.
2.1: Select a surface and click on the Custom Material thumbnail to apply it to the surface.
2.2: Delete the current Custom Material:
2.3: Then from the Material Settings tab click on the Custom Material icon to save a new Custom Material:
2.4: Save the Custom Material.
The new Custom Material thumbnail: