Run-Length Encoding (RLE) is one of the few compression techniques in IB Computer Science that students are expected to understand procedurally, not just conceptually. It is a form of lossless compression, and IB exam questions often require students to apply it step by step rather than simply define it.
Many students lose marks by misunderstanding how RLE works or by applying it in situations where it is not suitable. This article explains run-length encoding clearly, from basic principles to exam expectations.
What Is Run-Length Encoding?
Run-length encoding is a lossless compression method that reduces file size by:
- Identifying repeated values
- Replacing them with a single value and a count
It works best when data contains long sequences of repeated values, known as runs.
Because no data is removed, the original file can be reconstructed perfectly.
Why Run-Length Encoding Is Lossless
RLE does not remove any information. Instead, it:
- Stores how many times a value appears consecutively
- Allows the original sequence to be rebuilt exactly
This makes RLE suitable for situations where accuracy is essential.
Step-by-Step Example of Run-Length Encoding
Consider the following data sequence:
AAAAABBCCCCDD
Step 1: Identify runs of repeated characters
- A appears 5 times
- B appears 2 times
- C appears 4 times
- D appears 2 times
Step 2: Replace each run with a value and a count
The encoded result might be written as:
- 5A2B4C2D
This representation uses fewer characters than the original sequence, achieving compression.
Applying Run-Length Encoding to Images
RLE is often used with bitmap images, where:
- Large areas contain the same colour
- Pixels are stored row by row
For example, a row of pixels like:
- white, white, white, white, black
Can be encoded as:
- 4 white, 1 black
This is why RLE works well for:
- Simple images
- Icons
- Diagrams
- Images with large uniform areas
It performs poorly on complex photographs with lots of variation.
When Run-Length Encoding Is Effective
RLE works well when:
- Data contains many repeated values
- There are long runs of identical data
- The data is relatively simple
It is commonly used for:
- Simple graphics
- Black-and-white images
- Fax transmissions
When Run-Length Encoding Is Inefficient
RLE is ineffective when:
- Data changes frequently
- There are few repeated values
- Runs are very short
In these cases, RLE can even increase file size.
How Run-Length Encoding Appears in IB Exams
IB exam questions may ask students to:
- Encode a given sequence using RLE
- Decode an RLE sequence
- Explain why RLE is suitable or unsuitable
- Compare RLE to lossy compression
Clear, step-by-step working is essential for full marks.
Common Student Mistakes
Students often:
- Forget that RLE is lossless
- Encode individual values instead of runs
- Apply RLE to unsuitable data
- Skip steps in explanations
Examiners reward clarity and method.
Final Thoughts
Run-length encoding is a simple but powerful example of lossless compression. Its effectiveness depends entirely on the structure of the data being compressed. When applied correctly, it reduces file size without losing any information.
Understanding how RLE works step by step helps IB Computer Science students master compression questions confidently and accurately.
