在matlab中怎样对一个函数式进行取对数  如用数学函数式表达为:f ( x, y) = fi ( x, y) fr ( x, y)  
                                                           对两边取对数, 则有:
                                                                               lnf ( x, y) = lnfi ( x, y) + lnfr ( x, y)
然后再将两边进行傅里叶变换呢？用matlab实现他们······求助！！！