True Dimensionality maybe need > 2D to show real structure find knee in error-dimensions curve show as multiple scatterplots linear (PCA) nonlinear (charting) [A Data-Driven Reflectance Model, SIGGRAPH 2003, W Matusik, H. Pfister M. Brand and L. McMillan, graphics.lcs.mit.edu/~wojciech/pubs/sig2003.pdf] 113