# EE440 – Introduction to Digital Imaging Systems Homework Assignment 3

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1. The color of 3_1.bmp does not look right. Please write your own program to make the
color look more natural. You could try different methods to make it look better. Explain the
final method you use and how it works. (30 points)
Hint: For example, you might think there is one color that seems overwhelming, or maybe
some colors are not strong enough. You can play with the colors to figure out how to make
the image look better….
an image which the color does not look right a more natural image
2. You have two low-quality images (3_2.jpg, 3_3.jpg) that you want to enhance. You will
exam the following three general histogram-based enhancement algorithms:
a.Linear stretching;
One simple way to do linear stretching is, for example, if majority of original image’s
brightness (V channel in HSV or I channel is HSI) values fall into range X to Y, we could
define a linear function that map from X—Y to 0—255. Note: Linear stretching may not
work well for some images if their original dynamic range is already in 0-255.
b.Histogram equalization;
c.Histogram specification.
For each image and each method, please submit the original and the new image after
processing. For the original and new images, please also include their histogram and cdf plots
for you to get a better understanding of what’s going on with these processing in regard with
the image histogram. Compare the effects of these methods. For each image, please comment
on the effects and pick up the best method for that image in your opinion. (70 points)

Hints:
1. You need to implement your own functions for linear stretching, histogram
equalization, and specification. You may find these commands helpful: rgb2hsv,
hsv2rgb, subplot, imshow, cumsum, imhist.
2. You may want to convert the image to HSV first before taking any operations;
specifically, you want to process the V channel (value, i.e., intensity) only. After the
V channel is altered, you want to combine it with the original H and S channels and
convert it back to RGB.
3. For histogram equalization, compute pdf and cdf first and then use the cdf as the
transform function. You can use command imhist to help you get the image histogram
and then use command cumsum to get the cdf from histogram, but please DON’T use
command histeq to directly perform histogram equalization.
4. For histogram specification, you may specify any histogram to get the best result. Note
that if you specify the target histogram as a uniform distribution, you’re doing
histogram equalization. You can also try a normal distribution as the specified
histogram to get the transform function and see how the result turns out. Use
command normpdf to generate a normally distributed sequence in MATLAB. Use
command help normpdf to see how to use this function to generate a desired normally
distributed histogram. Remember this histogram has 256 bins.