How to realize Composition

Thanks for your great work.
I just want to ask that as shown the image, besides generation, your work can composite with user defined backgrounds But I did not find the way to realize it.

感谢作者您杰出的工作

不过introduction中的图中除了generation外,您的工作似乎还能完成composition的任务.可以询问下如何实现吗,谢谢.