Hi weihao, I made that shelf 2 years ago and lost my files on my previous computer. but it's quite simple. if you send me your definition I'll try to do the same on yours! Just briefly explaining you need to go through these steps:

-Find each corner

-locate one, two or as many joint you need on each edge

-Place a plane on those points

-Get the intersection with surfaces

-Orient it on xy plane

-depending on how long you need your joints to be cut the intersection result

-offset on both sides

-fillet the angle

-connect offset lines and Join them

you are almost there!

If you want to change anything manually you can take it from there!

I hope it helps.

I don't know whether I want to do is the same as yours. The first picture is the reference I found, and the second picture is the shelf I am working on. I would like to use the joint in the reference. I will try whether I can figure it out according to your method. Thank you for your time ~

Ok. I made a quick example which shows how to get intersection and offset them, basically showing the logic and I think you can take it from there, I hope it helps:

 Find the GH file here:

https://www.dropbox.com/s/yij3ka1fjreu1d4/QuickexampleForShelf.gh?dl=0