1. A method for populating values at nodes of a look-up table (LUT) which is arranged as a rectangular grid of cells, wherein the populated values model an underlying process that transforms from a first color space to a second color space, and wherein some of the cells of the LUT are completely inside a constraint boundary of the underlying process and some of the cells are not completely inside the constraint boundary, the method comprising: an indexing step of indexing the cells in an order which starts with a cell which is inside the constraint boundary and which is also farthest away from the constraint boundary, increases as the cells draw closer to the constraint boundary, and further increases as the cells cross and then fall outside the constraint boundary; an accessing step of accessing samples of the underlying process which lie on nodes of the grid within the constraint boundary, and accessing auxiliary samples of the underlying process which lie between nodes of the grid; a first populating step of populating values of nodes for cells whose nodes are all within the constraint boundary by using the samples of the underlying process; and a second populating step of populating values of nodes for cells whose nodes are not all within the constraint boundary by visiting such cells in the indexed order and populating values for the unpopulated nodes of such cells using populated ones of the nodes of such cells and the auxiliary samples of the underlying process.