Assignment – PNG Images
Version 1.00
1. Submission Guidelines
Deadline: 9:00 AM on Friday 13 December
Submission procedure: Submit only one file labelled png.py through blackboard (via TurnItIn)
Version requirement: Your code must run using Python 3.11.4 on a PC
Allowable import modules: math and zlib only. No other modules may be imported.
2. Overview
The Portable Network Graphics (PNG) format is a format for storing images. For this assignment, you are tasked with writing code that can be used to open a PNG file, store its contents in the program, and save modified PNG image files.
This is a real-world assignment, meaning that you are working on a practical problem that requires that you find additional information to complete the task. The official PNG documentation is available online.
https://www.w3.org/TR/png/
We expect that all students will use the internet and textbook resources to learn concepts needed for this assignment. There will be many concepts, in-built functions, and other aspects of Python coding, which have not been directly taught in labs or lectures. It is up to you to fill in these gaps in knowledge by engaging in both the course and self-learning.
Keep track of the version number of this assignment and ensure that your code meets the latest specifications by the deadline. This assignment challenge is very similar to a real-world situation where specifications evolve with the person writing code for the specifications. We will be updating the assignment descriptor throughout the week to add clarity, remove ambiguities, and fix descriptor errors. Post on the Ed Discussion board any clarification requests, ambiguities, or descriptor errors. The Teaching Assistants will then raise this with the instructor who will then update the version of the assignment and announce it via email.
3. PNG Format
You must adhere to the PNG version specified in the hyperlink above. However, you will only need to implement only a small subset of the total PNG specifications.
Implement only the following chunks:
• IHDR
• IDAT
• IEND
• Other chunks. There may be other chunks present in the PNG image file. For this assignment, you do not need to implement those other chunks. However, your programme should run smoothly even if they are present. In other words, ignore the other chunks if they are present in the image.
You will only need to read and write images that have the following image header (IHDR) specifications:
• A bit depth of 8
• A colour type of 2
• A compression method of 0
• A filter method of 0
• An interlace method of 0
When you read the example image (brainbow.png), you should be able to find these values within the file.
We have uploaded brainbow.png as a sample image for you to open and use. Your code should be able to
handle other PNG files, including images with different widths and heights. We may also test very large images. We will certainly test other .png files to ensure that your code is not hard coded to produce the sample outputs.
4. Class PNG
Create a class named PNG that will allow users to interact with PNG files.
4.1. Attributes
Objects of type PNG should have the following variable attributes:
data A value of type bytes.
This represents the data bytes from the png file. The default values should be b''
info A value of type str.
This represents information about the png file. The default value should be '' and should become either the name of the file or 'file not found'.
width A value of type int.
This represents the width of the image in pixels. The default value should be 0.
height A value of type int.
This represents the height of the image in pixels. The default value should be 0.
bit_depth A value of type int.
This represents the bit depth of the image data stored in the png file. The default value should be 0.
color_type A value of type int.
This represents the color type of the image data stored in the png file. The default value should be 0.
compress A value of type int.
This represents the compression method of the png file. The default value should be 0.
filter A value of type int.
This represents the filter method of the png file. The default value should be 0.
interlace A value of type int.
This represents the interlacing method of the png file. The default value should be 0.
img A value of type list.
This represents a 3-dimensional array where the 1st dimension is the image rows, the 2nd dimension is the image columns, and the 3rd dimension is the red, green, or blue values.
The red, green, and blue values should be of type int and should range between 0 and 255 only.
The default value should be an empty list.
4.2. Methods
For this assignment, you may assume that all arguments put into each method are of the correct type.
init (self)
Return values: None
Arguments: A reference to itself, self.
Implementation: This method should initialise all of the above member attributes to their default values.
10 marks for implementation
load_file (self, file_name) Return values: None
Arguments: A reference to itself (self) and a file name (file_name) of type str.
Implementation: The load_file() method should open the file specified by file_name and store all
of the file’s data into the member attribute data.
If the file is found, then assign file_name into the info member attribute.
If the file is not found and a FileNotFoundError exception is raised when trying to open the file, then assign 'file not found' into the info member attribute.
10 marks for implementation
valid_png(self)
Return values: True or False
Arguments: A reference to itself, self.
Implementation: Reads the PNG signature and returns whether the correct values were found or not.
Returns True if the PNG signature was correctly specified in the file. Returns False if the PNG signature was not correctly specified in the file.
10 marks for implementation
read_header (self) Return values: None
Arguments: A reference to itself, self.
Implementation: Reads the image header chunk (IHDR) and updates the relevant member attributes.
Update the width, height, bit_depth, color_type, compress, filter, and interlace attributes according to the information stored in the image header chunk.
10 marks for implementation
read_chunks (self) Return values: None
Arguments: A reference to itself, self.
Implementation: Reads through all the chunks and updates the img attribute.
Read through the remaining chunks following the PNG signature and IHDR chunk. This includes IDAT and IEND chunk types that include the image data. There may be other chunk types present in the PNG file, but gracefully ignore those chunk types. Your program should still run smoothly even if those chunk types are present in the PNG file.
One of the challenges of this method is being able to extract the image data from the IDAT chunk(s). After extracting the image, store it in the member attribute img. img represents a 3-dimensional array where the 1st dimension is the image rows, the 2nd dimension is the image columns, and the 3rd dimension is the red, green, or blue values. The red, green, and blue values should be of type int and should range between 0 and 255 only.
20 marks for implementation
10 marks for efficiency
save_rgb (self, file_name, rgb_option) Return values: None
Arguments: A reference to itself, self, a file name (file_name) of type str, and a saving option
(rgb_option) of type int.
Implementation: Saves the red, green, or blue channels of the img attribute into a PNG file named
file_name as determined by the setting of rgb_option.
When rgb_option is set to 1, store the red channel of the image into the PNG file.
When rgb_option is set to 2, store the green channel of the image into the PNG file. When rgb_option is set to 3, store the blue channel of the image into the PNG file. We should be able to open your PNG file using a regular image viewing application.
10 marks for implementation
10 marks for efficiency
5. Coding style and commenting
Ensure that your code is readable and well commented. We will be marking according to spacing, naming, comments, length, and general readability.
10 marks for coding style and commenting
6. Coding rules
Only the PNG class definition and imports to the math and zlib modules should be in the global space of png.py. There should be no other global variables or definitions. You are free to define methods within the PNG class, which we will not call. A main() function could be used if you so desire, but it is not required. Only the math and zlib modules are allowed for this assignment. No other module is allowed.
7. Marking criteria
We will mark your submitted png.py code according to the following categories:
• 70 marks: Implementation and evidence of coding knowledge
• 20 marks: Coding efficiency
• 10 marks: Coding style and commenting
We will import your png module near the top of our script. to test your code. We will run several test conditions against each of your classes, including its attributes and methods. After running each method, we will investigate what was assigned in your attribute variables. So, the type, length, etc. of your attribute variables must perfectly match how we defined them in this specification. We will test far more test cases than the single example we have included in this assignment sheet.