Homework 1 — Morphological Image Processing

CSCI-1200 Data Structures
Homework 1 — Morphological Image Processing
Before starting this homework, make sure you have read the Collaboration Policy & Academic Integrity.
At the core of many advanced computer vision algorithms for image segmentation, feature detection, and
classification are several basic morphological image processing operations that modify the borders between
neighboring clusters of similarly colored pixels. The dilation operation adds pixels along the boundary to
expand or grow a cluster. The erosion operation shrinks the boundary of the cluster by reassigning pixels
along the boundary to a background color. We’ll also implement a simple replace operation to change all
pixels in the image of color A to color B.
The diagram on the right illustrates the details of the dilation and erosion operations. In both cases we will use a plus-shaped structuring
element that contains the 4 adjacent pixels to a candidate pixel. When
deciding which pixels to add to expand the boundary of the grey cluster during a dilation operation, we place the structuring element over
each candidate pixel and ask if the shape overlaps or “hits” any of the
existing grey pixels. If it does, we change its color to the foreground
color (grey). In contrast, when shrinking the boundary during erosion,
we consider removing each pixel from the cluster. If all of the pixels
of the structuring element do not fully “fit” within the grey cluster of
pixels we will set that pixel to the background color (white).
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4 5
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EROSION
add
change
no
pixel
remove
change
no
pixel
DILATION
You will work with command line arguments, file input and output, and the STL C++ string and vector
classes to manipulate manipulate a grid of ASCII characters (instead of a traditional color image file). Please
read the entire assignment carefully before starting to program.
Command Line Arguments
Your program will expect 4-5 command line arguments, depending on the requested operation. The 1st
argument is the name of the input ASCII art image file. The 2nd argument is the name of the output file
where the ASCII art image resulting from the operation should be written. The 3rd argument is the name
of one of 3 operations that should be performed: replace, dilation, or erosion. Here are some example
command lines:
./image_processing.out input4.txt output4_replace.txt replace X O
./image_processing.out input4.txt output4_dilation.txt dilation X
./image_processing.out input4.txt output4_erosion.txt erosion X \.
The number and purpose of the remaining arguments on the command line depend on the operation. For
a replace operation, the 4th argument is the character that should be removed from the image and the 5th
argument is the character that should be substituted in those locations. For the dilation operation, the
4th argument is the foreground character (clusters of pixels of this “color” should increase in size). For the
erosion operation, the 4th argument is the foreground character (clusters of pixels of this “color” should
decrease in size). We also need to know the background color that will replace pixels that are removed from
the foreground object and the 5th argument is that background character.
Note that certain ASCII characters have special meaning on the command line, and it will be necessary to
escape that meaning by preceding those characters by a backslash (\) to make sure your program receives
the desired character.
Examples and More Information
In many computer vision applications, these operations are applied sequentially, one after the other. Note
that these operations are generally not commutative. The opening operation is the result of first applying
erosion and then dilation. The effect of opening is to round sharp outer corners and remove tiny islands.
The closing operation is achieved by first performing dilation and then erosion. The effect of closing is to fill
in small gaps within the foreground object and smooth inner corners.
./image_processing.out output4_erosion.txt output4_opening.txt dilation X
./image_processing.out output4_dilation.txt output4_closing.txt erosion X \.
input4.txt output4 replace.txt output4 dilation.txt output4 erosion.txt output4 opening.txt output4 closing.txt
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You must follow the specifications for the command line arguments and functionality exactly as described
in this handout to receive full credit. Sample input and output files are posted on the course website, and
the validation and automatic grading on the submission server will also help you check your work. More
information on command line arguments and file I/O in C++ is available on the course webpage under
“Misc. C++ Programming Info”.
Image Bounds & Other Error Checking
When the structuring element is placed at the edge of the image, one or more pixels may fall off the edge
and not overlap legal image coordinates. You do not want to access data that does not exist (a memory error
that could give weird output or crash the program!). Be sure to add appropriate checks in your program to
skip these out-of-bounds coordinates.
You may assume that the input image file is properly formatted and contains a rectangular grid of non-white
space characters. However, to help with your own debugging, you should implement simple error checking
to ensure that the command line arguments are appropriate and that the input and output file streams are
successfully opened. Your program should exit gracefully with a useful error message sent to std::cerr if
there is a problem with the arguments.
Submission Details
Do your work in a folder named hw1 within your Data Structures homeworks directory. Organize your
program into logical, reasonable-length functions – don’t put all of the code in main! Be sure to read the
“Homework Grading Criteria” as you put the finishing touches on your solution. Create new test cases to
fully debug your program and don’t forget to comment your code! Use the provided template README.txt
file for notes you want the grader to read. You must do this assignment on your own, as described in the
“Collaboration Policy & Academic Integrity” handout. If you did discuss this assignment, problem solving
techniques, or error messages, etc. with anyone, please list their names in your README.txt file.
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