Hi, let's assume I use a lot of images in my UI. Is there a way to use an atlas for all these images to improve performance? Or is the new UI system automatically generating an atlas in the background and use it?
With Sprites (like UI elements) Unity uses something called a "Sprite Packer". It automatically packs your sprites to be more efficient by creating Atlases. You can read more about it here: http://docs.unity3d.com/Manual/SpritePacker.html
