A new deformation technique for F-rep objects uses algebraic sums (differences) between an object's defining function F and a function interpolating F values in given control points. The use of a volume spline for the interpolation made possible smooth deflation (left image) and inflation (right image) of a rubber cat (center).
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More details can be found in the paper:
Pasko A.A., Savchenko V.V. “Algebraic sums for deformation of constructive solids”, Third ACM Symposium on Solid Modeling and Applications, C.Hoffmann and J.Rossignac (Eds.), Salt Lake City, Utah, USA (May 17-19, 1995), ACM Press, 1995, pp.403-408.
Electronic version: http://hyperfun.org/PS_Asum_SM95.pdfPDF (407K)
Note: the model of the famous Japanese cat Doraemon (upper image, center) has been made by the second year students of the University of Aizu (Yukio Hashimoto, Kou Setoguchi, Masayuki Tetsuka, and Tetsurou Tobe) during their extra-curriculum project.