Method of digitally generating data indicative of a synthesized appearance of a simulated material having physically plausible appearance attributes, with the synthesized appearance being based on a physically tangible reference material and at least one value of a selected appearance attribute of a physically tangible source material different from the reference material, comprising the steps of: a) accessing a first set of data indicative of the values of measured appearance attributes of the reference material, with the measured appearance attributes being measured at a plurality of locations on the reference material and for a plurality of illumination directions or a plurality of viewing directions relative to each of the locations; b) accessing a second set of data indicative of a value of at least one measured appearance attribute of the source material, wherein the appearance attribute of the source material includes at least one, but not all, of the appearance attributes being measured on the reference material; and c) determining a third set of data indicative of the synthesized appearance of the simulated material based at least in part on data from the second set of data associated with the physically tangible source material and at least in part on data from the first set of data of measured attributes of the physically tangible reference material different from that of the second set of data, characterized in that: step c) comprises: i) determining a third set of data indicative of the synthesized appearance of the simulated material based at least in part on the values of the measured appearances attributes of the source material and the values of the measured appearance attributes of the reference material, ii) determining an error value consisting of iia) the difference between a parameter of a measured appearance attribute of the source material and the same parameter of the corresponding appearance attribute in the third set of data, iib) a physical plausibility value based on the difference between a parameter of a measured appearance attribute of the reference material and the same parameter of the corresponding measured appearance attribute in the third set of data, iii) if the error value is greater than a predetermined threshold, then revising the determining of the third set of data indicative of the synthesized appearance of the simulated material, and iv) repeating steps i) through iii) until the error value is less than the predetermined threshold. a) accessing a first set of data indicative of the values of measured appearance attributes of the reference material, with the measured appearance attributes being measured at a plurality of locations on the reference material and for a plurality of illumination directions or a plurality of viewing directions relative to each of the locations; b) accessing a second set of data indicative of a value of at least one measured appearance attribute of the source material, wherein the appearance attribute of the source material includes at least one, but not all, of the appearance attributes being measured on the reference material; and c) determining a third set of data indicative of the synthesized appearance of the simulated material based at least in part on data from the second set of data associated with the physically tangible source material and at least in part on data from the first set of data of measured attributes of the physically tangible reference material different from that of the second set of data, i) determining a third set of data indicative of the synthesized appearance of the simulated material based at least in part on the values of the measured appearances attributes of the source material and the values of the measured appearance attributes of the reference material, ii) determining an error value consisting of iia) the difference between a parameter of a measured appearance attribute of the source material and the same parameter of the corresponding appearance attribute in the third set of data, iib) a physical plausibility value based on the difference between a parameter of a measured appearance attribute of the reference material and the same parameter of the corresponding measured appearance attribute in the third set of data, iia) the difference between a parameter of a measured appearance attribute of the source material and the same parameter of the corresponding appearance attribute in the third set of data, iib) a physical plausibility value based on the difference between a parameter of a measured appearance attribute of the reference material and the same parameter of the corresponding measured appearance attribute in the third set of data, iii) if the error value is greater than a predetermined threshold, then revising the determining of the third set of data indicative of the synthesized appearance of the simulated material, and iv) repeating steps i) through iii) until the error value is less than the predetermined threshold.