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+Chen J, Bao H, Wang T, et al. Numerical coarsening using discontinuous shape functions[J]. ACM Transactions on Graphics (TOG), 2018, 37(4): 1-12.
+
+1. A 2D version.
+2. The quadratic programming optimization uses the null-space of matrix.
+3. Note that I didn't realize the rotation invariant, i.e. Eq. (7).
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