如何在保留背景图像的同时从结构js画布中删除对象

I am designing a webpage that allows the user to upload an image and take measurements (counts, lengths, and angles). Uses both jQuery and fabric.js - The user can push one of three buttons for the appropriate analysis (i.e. counts, lengths, or angles). However, currently I can't figure out how to remove objects from one canvas while preserving the background image that the user uploads. Any help greatly appreciated. I am designing a webpage that allows the user t