Butterworth Bandpass Filter for Image Processing

This function performs butterworth bandpass filtering in the frequency domain

Butterworth Bandpass Filter
This simple function was written for my Digital Image Processing course
at Eastern Mediterranean University taught by
Assoc. Prof. Dr. Hasan Demirel
for the 2010-2011 Spring Semester
for the complete report:
http://www.scribd.com/doc/51981950/HW4-Frequency-Domain-Bandpass-Filtering

Written By:
Leonardo O. Iheme ([e-mail])
23rd of March 2011

I = The input grey scale image
d0 = Lower cut off frequency
d1 = Higher cut off frequency
n = order of the filter

The function makes use of the simple principle that a bandpass filter
can be obtained by multiplying a lowpass filter with a highpass filter
where the lowpass filter has a higher cut off frquency than the high pass filter.

Usage BUTTERWORTHBPF(I,DO,D1,N)
Example
ima = imread('grass.jpg');
ima = rgb2gray(ima);
filtered_image = butterworthbpf(ima,30,120,4);