Images, Pixels and Blobs SOLVED

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Assignment #5
COMP 1006/1406
Submit your file to Brightspace.
This assignment has 10 marks.
Images, Pixels and Blobs
An Image is a 2-dimensional grid of Pixels. A pixel’s location in the image is specified by its row
and column in the image and either has ink (i.e., you can see it) or does not.
A blob of ink is one more pixels that have ink and are neighbours in the image. Two pixels at
positions (r1, c1) and (r2, c2), respectively, are immediate neighbours if one of the following is
• r1 = r2 and c1 = c2 ± 1
• r1 = r2 ± 1 and c1 = c2
Here ri
is the row of pixel i and ci
is the column of pixel i. Notice that you can move from one pixel
to one of its immediate neighbours by either moving exactly one position up, down, left or right in
the image grid. Diagonals are not allowed for immediate neighbours. Two pixels are neighbours
if there is a sequence of immediate neighbours connecting them.
For example, in the image below, there are THREE (3) blobs. The blob on the left is made
up of 9 pixels (coloured green), the middle blob as 3 pixels (coloured yellowish), and the last blob,
furthest to the right, consists of a single pixel (coloured blue).
In this assignment, you will implement several algorithms that explore an image to identify
all the pixels in a given blob of ink. The needed methods are defined as abstract in the provided file. You will create a concrete class called that overrides these methods
with a concrete implementation. A skeleton version of is provided with the other
needed classed (,, and
Note: in this assignment, the pixels in a given blob will be stored in a Deque (that stores Pixels)
instead of a List. Recall that the Deque ADT (double-ended queue) is a restricted list that has fast
access (adding/removing) from both the front and back of the list. For this assignment, you should
NOT need to use any other methods than the addLast(), addFirst(), removeFirst() and size()
methods of the ArrayDeque you will use. The ArrayDeque is a concrete class that implements the
Deque interface. The provided java files have already imported the needed classes/interfaces.
COMP 1006/1406 – Winter 2022 1
Assignment #5 Due Friday, April 8 at 11:59 pm
1 Recursive Approach [5 marks]
You will use recursion to find all pixels in a given blob. In particular, you will implement a method
called blobRecursiveHelper() that uses accumulative recursion.
Recall that in accumulative recursion, you have an extra input parameter that builds up the
solution to the problem. When the recursion ends, the final answer is contained in that parameter.
blobRecursiveHelper(int row, int col, Deque<Pixel> blobSoFar)
In this method, a list of pixels that you have already determined to be in the blob is passed as
input (blobSoFar). If the current pixel (specified by the input row and column) has not yet been
visited and has ink, you add this pixel to blobSoFar and then recursively visit all four of this pixel’s
neighbours. If the pixel has been visited before, then you do nothing (just return). You’ll need
to take care that your code doesn’t crash by trying to process pixels that are outside of the image.
You’ll know if there is a problem if your code crashes with an ArrayIndexOutOfBoundsException.
The provided blobRecursive(int row, int col) method will call your helper method. It
returns a Deque of all pixels in the blob that contains the pixel at the initial specified row and
column. If the initial pixel does not have any ink then the list should be empty.
As you discover a new pixel that hasn’t been visited yet, be sure to add it to the back of the
list blobSoFar. This will ensure that the ordering of the pixels will match their discovery ordering.
Marking will depend on this ordering being correct.
When making recursive calls to immediate neighbours, always visit them in the following order:
up, right, down, left . Note that up means decreasing the row value, and left means decreasing the
column value. As always, row and column values start with value 0.
2 Iterative Approach [10 marks]
A problem with the recursive approach to solving this problem is that you will run out of stack
space if the blob is too large (and your program will crash).
Instead of using recursion, we can also discover all pixels in a blob using iteration. Here is
pseudocode for the basic algorithm:
1 blobIterative(row,col) -> List of Pixels
2 blobList <- empty List
3 workingList <- empty List
4 add pixel at (row,col) to workingList
5 while workingList is not empty do
6 p <- some pixel removed from workingList
7 if p has ink AND has not been visited then
8 mark p as visited
9 add p to back of blobList
10 for each immediate neighbour q of p do
11 add q to workingList
12 output blobList
The order of the pixels in the output list will depend on the data structure used for the workingList
list and implementation details.
If you use a Stack (recall the Stack ADT) and the order you select immediate neighbours (in line
10 of the pseudocode) is OPPOSITE to the order in which you make the recursive calls to immediate
COMP 1006/1406 – Winter 2022 2
Assignment #5 Due Friday, April 8 at 11:59 pm
neighbours, then the output list will be in the same order as your recursive implementation. This
is a depth-first approach to discovering the pixels.
If you use a Queue (recall the Queue ADT), then the order of the output list will start with the
initial pixel and then move outwards like the ripple of a wave. This is a breadth-first approach to
discovering the pixels.
You will override the provided (abstract) blobIterative() method. For your implementation,
you can use either the Stack or Queue approach (you will still use the ArrayDeque to simulate
either). Regardless of your choice, you must add immediate neighbours (lines 10-11) in the following order: left, down, right, up . Note that up means decreasing the row value, and left means
decreasing the column value. As always, row and column values start with value 0.
Submission Recap
Submit your file. Do not submit any other file.
COMP 1006/1406 – Winter 2022 3


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