1. In a graphics processing circuit, a method for calculating an output value of a function based on an input value, wherein the input value comprises an input mantissa and an input exponent, the input mantissa further comprising a first portion and a second portion, and wherein the output value comprises an output mantissa and an output exponent, the method comprising: providing a point value based on the first portion of the input mantissa; providing at least one slope value based on the first portion of the input mantissa, wherein each of the at least one slope value is based on a LaGrange polynomial approximation of the function; and combining the point value, the at least one slope value and the second portion of the input mantissa to provide the output mantissa.