Harquail, NookAllen, MarissaWhiting, EmilyA. Medeiros e Sa and N. Pietroni and K. Rodriguez Echavarria2016-05-042016-05-042016978-3-03868-003-1not yet assignedhttps://doi.org/10.2312/gdf.20161076Crafting a 3D paper pop-up is a creative and playful experience, which can help develop spatial reasoning skills. However, designing the cuts and folds is often a frustrating trial-and-error process due to the tight set of geometric constraints. We introduce Foldlings: an iPad application that assists in this exploratory process. Our tool-based approach allows users of all skill levels to create complex cards with ease, by separating folding geometries into logical units. We present a novel user interface for creating popup cards from user input, leveraging the practical two-dimensional format of paper. Our approach uses a modular set of simple drawing tools, which can be combined to create complex designs. We guarantee the validity of the folded card at all stages of design, and our interface provides an intuitive set of visual aids to help users understand the relationship between 2D patterns and 3D geometry. We describe the algorithms and data structures used in interpreting 2D user input and visualizing the 3D geometry of the folded card. We discuss results of user studies, which guided early stages of UI design and demonstrate the efficacy of the final prototype.10.2312/gdf.2016107631-38