A method includes accessing a structure model defining a cross-sectional profile of a structure on a sample. The cross-sectional profile is at least partially defined using a set of blocks. Each of the blocks includes a number of vertices. One or more of the vertices are expressed using one or more algebraic relationships between a number of parameters corresponding to the structure. Information is evaluated from the structure model to produce expected metrology data for a scatterometry-based optical metrology. The expected metrology data is suitable for use for determining one or more of the number of parameters corresponding to the structure. Apparatus are also disclosed.