ECE 435/BME 403 Medical Image Processing Assignment 1 answered

1. Write a Matlab function (m-file) that performs downsampling of the original

image using a variable rate N. You will use two input image files (provided):

– ‘Xray.tif’

– ‘arteriogram.tif’

Your code and write-up will show results for N=2 and N=5 on both input images.

Downsampling is performed by uniform sampling at a spatial period of N (for both

rows and columns), as shown in the diagram on the left.

The inputs of your function will be the original image and

the down-sampling rate, while the output will be the

downsampled image. You also need to display the input and

output side by side.

This exercise is an opportunity to become familiar with

matrix indexing in Matlab, as opposed to using for loops,

which are computationally intensive and thus inefficient. For

instance, consider the matrix A

The expression 1:k creates a vector of integer values from 1

to k, so we can select a submatrix T2 of A by using indexing

with a single colon.

Using the colon operator twice allows you to access

elements of A spaced at a uniform distance. The

general syntax of this indexing is

start_location: space : end_location

2. Write a Matlab function ((m-file) that lowers the bit resolution of the original

input image by powers of 2 (B=3, 4, 5). The modification of the bit resolution is

shown in the diagram below. You will use the ‘Xray.tif’ image provided. Your code

will display the input and output image side by side, or in different figures.

As in Part 1, we need to avoid the use of for loops.

The division by 2^B will result in a non-integer

number, so a rounding operation needs to be

performed afterwards. This is implemented in

Matlab by round( ).

To display images with low dynamic range you

may use the imshow command with a specific

syntax, where the display range is specified.

To submit: one zip file, containing two m-files (one for each part) and a pdf file

containing a brief discussion of your results. Your discussion should focus on how

the quality of the image is affected by varying the spatial resolution (part 1), and the

intensity resolution (part 2).