辅导 COP3502、Python程序设计讲解
P2: RLE with Images Python
Overview
In this project students will develop routines to encode and decode data for images using run-length encoding
(RLE). Students will implement encoding and decoding of raw data, conversion between data and strings, and
display of information by creating procedures that can be called from within their programs and externally. This
project will give students practice with loops, strings, Python lists, methods, and type-casting.
Run-Length Encoding
RLE is a form of lossless compression used in many industry applications, including imaging. It is intended to
take advantage of datasets where elements (such as bytes or characters) are repeated several times in a row in
certain types of data (such as pixel art in games). Black pixels often appear in long “runs” in some animation
frames; instead of representing each black pixel individually, the color is recorded once, following by the number
of instances.
For example, consider the first row of pixels from the pixel image of a gator
(shown in Figure 1). The color black is “0”, and green is “2”:
Flat (unencoded) data: 0 0 2 2 2 0 0 0 0 0 0 2 2 0_
Run-length encoded data: 2 0 3 2 6 0 2 2 1 0_.
Figure 1 – Gator Pixel Image
The encoding for the entire image in RLE (in hexadecimal) – width, height, and pixels - is:
1E|162032602220121F10721AF21092301210326032308250
\W/ \H/ \------------------------------------------PIXELS-----------------------------------------------/
Image Formatting
The images are stored in uncompressed / unencoded format natively. In addition, there are a few other rules to
make the project more tractable:
1. Images are stored as a list of numbers, with the first two numbers holding image width and height.
2. Pixels will be represented by a number between 0 and 15 (representing 16 unique colors).
3. No run may be longer than 15 pixels; if any pixel runs longer, it should be broken into a new run.
For example, the chubby smiley image (Figure 2) would contain the data shown in Figure 3.
Figure 2 Figure 3 – Data for “Chubby Smiley”
NOTE: Students do not need to work with the image file format itself – they only need to work with lists and
encode or decode them. Information about image formatting is to provide context. Requirements
Student programs must present a menu when run in standalone mode and must also implement several methods,
defined below, during this assignment.
Standalone Mode (Menu)
When run as the program driver via the main() method, the program should:
1) Display welcome message
2) Display color test (ConsoleGfx.test_rainbow)
3) Display the menu
4) Prompt for input
Note: for colors to properly display, it is highly recommended that student
install the “CS1” theme on the project page.
There are five ways to load data into the program that should be provided and four ways the program must be
able to display data to the user.
Loading a File
Accepts a filename from the user and invokes ConsoleGfx.load_file(filename):
Select a Menu Option: 1
Enter name of file to load: testfiles/uga.gfx
Loading the Test Image
Loads ConsoleGfx.test_image:
Select a Menu Option: 2_
Test image data loaded._
Reading RLE String
Reads RLE data from the user in hexadecimal notation with delimiters (smiley example):
Select a Menu Option: 3
Enter an RLE string to be decoded: 28:10:6B:10:10B:10:2B:10:12B:10:2B:10:5B:20:11B:10:6B:10
Reading RLE Hex String
Reads RLE data from the user in hexadecimal notation without delimiters (smiley example):
Select a Menu Option: 4
Enter the hex string holding RLE data: 28106B10AB102B10CB102B105B20BB106B10
Reading Flat Data Hex String
Reads raw (flat) data from the user in hexadecimal notation (smiley example):
Select a Menu Option: 5
Enter the hex string holding flat data:
880bbbbbb0bbbbbbbbbb0bb0bbbbbbbbbbbb0bb0bbbbb00bbbbbbbbbbb0bbbbbb0
Displaying the Image
Displays the current image by invoking the ConsoleGfx.display_image(image_data) method.
Displaying the RLE String
Converts the current data into a human-readable RLE representation (with delimiters):
Select a Menu Option: 7 RLE representation: 28:10:6b:10:10b:10:2b:10:12b:10:2b:10:5b:20:11b:10:6b:10
Note that each entry is 2-3 characters; the length is always in decimal, and the value in
hexadecimal! Displaying the RLE Hex Data
Converts the current data into RLE hexadecimal representation (without delimiters):
Select a Menu Option: 8
RLE hex values: 28106b10ab102b10cb102b105b20bb106b10
Displaying the Flat Hex Data
Displays the current raw (flat) data in hexadecimal representation (without delimiters):
Select a Menu Option: 9
Flat hex values: 880bbbbbb0bbbbbbbbbb0bb0bbbbbbbbbbbb0bb0bbbbb00bbbbbbbbbbb0bbbbbb0
Class Methods
Student classes are required to provide all of the following methods with defined behaviors. We recommend
completing them in the following order:
1. to_hex_string(data)
Translates data (RLE or raw) a hexadecimal string (without delimiters). This method can also aid debugging.
Ex: to_hex_string([3, 15, 6, 4]) yields string "3f64".
2. count_runs(flat_data)
Returns number of runs of data in an image data set; double this result for length of encoded (RLE) list.
Ex: count_runs([15, 15, 15, 4, 4, 4, 4, 4, 4]) yields integer 2.
3. encode_rle(flat_data)
Returns encoding (in RLE) of the raw data passed in; used to generate RLE representation of a data.
Ex: encode_rle([15, 15, 15, 4, 4, 4, 4, 4, 4]) yields list [3, 15, 6, 4].
4. get_decoded_length(rle_data)
Returns decompressed size RLE data; used to generate flat data from RLE encoding. (Counterpart to #2)
Ex: get_decoded_length([3, 15, 6, 4]) yields integer 9.
5. decode_rle(rle_data)
Returns the decoded data set from RLE encoded data. This decompresses RLE data for use. (Inverse of #3)
Ex: decode_rle([3, 15, 6, 4]) yields list [15, 15, 15, 4, 4, 4, 4, 4, 4].
6. string_to_data(data_string)
Translates a string in hexadecimal format into byte data (can be raw or RLE). (Inverse of #1)
Ex: string_to_data ("3f64") yields list [3, 15, 6, 4].
7. to_rle_string(rle_data)
Translates RLE data into a human-readable representation. For each run, in order, it should display the run
length in decimal (1-2 digits); the run value in hexadecimal (1 digit); and a delimiter, ‘:’, between runs. (See
examples in standalone section.)
Ex: to_rle_string([15, 15, 6, 4]) yields string "15f:64".
8. string_to_rle(rle_string)
Translates a string in human-readable RLE format (with delimiters) into RLE byte data. (Inverse of #7)
Ex: string_to_rle("15f:64") yields list [15, 15, 6, 4]. Submissions
NOTE: Your output must match the example output *exactly*. If it does not, you will not receive full credit for
your submission!
File:
Method:
rle_program.py
Submit on ZyLabs
Do not submit any other files!
Part A (5 points)
For part A of this assignment, students will set up the standalone menu alongside the 4 requirements listed on
page 2 of this document. In addition to this, students should also set up menu options 1 (loading an image), 2
(loading specifically the test image), and 6 (displaying whatever image was loaded) in order to help grasp the
bigger picture of the project.
This involves correctly setting up the console_gfx.py file and utilizing its methods. You will use
ConsoleGfx.display_image(...) to display images. Notice how it takes in a decoded list. This is the
format in which you will locally (in your program) store any image data that you are working with. When
the document mentions that something is “loaded” it means that something is stored as a list of flat
(decoded) data.
Part B (60 points)
For part B of this assignment, students will complete the first 6 methods on page 3 of this document. They
must match specifications and pass test cases on chapter 12.2 in Zybooks, which will be your means of
submission for this part of the assignment. Your grade will be the score received on Zybooks. To guarantee
functionality moving forward to part C, it is expected that you will receive full marks for this section.
Part C (35 points)
For part C of this assignment, students will now complete the final 2 methods on page 3 of this document as well
as the remainder of the project involving the menu options and understanding how all the individual methods are
intertwined with each other. You will submit your whole program including the 8 methods listed above and the
main method in chapter 12.3 in Zybooks. We will only test your remaining 2 methods and the main method in
part C.