What is Compression?
Files can be very large — a high-resolution photo, a long piece of music, or a movie can take up gigabytes of storage. Compression reduces file size by encoding the data more efficiently, making files quicker to send and cheaper to store.
There are two types of compression: lossless and lossy. This article covers lossless compression.
What is Lossless Compression?
Lossless compression reduces file size while preserving every single bit of the original data. When the file is decompressed, you get back an exact, perfect copy of the original.
Think of it like a clever shorthand: instead of writing "very very very very happy", you could write "very×4 happy" — shorter, but no information is lost.
Lossless compression is used whenever losing any data would be unacceptable — for example, documents, spreadsheets, software programs, and PNG images.
How Lossless Compression Works
Lossless algorithms find patterns and redundancy in data and replace them with more compact codes. Common techniques include:
- Run-Length Encoding (RLE): Replaces repeated values with a count and a value. For example, 000000111 becomes "6×0, 3×1".
- Huffman Coding: Assigns shorter codes to more frequent characters, so common letters take up less space than rare ones.
- Dictionary-based methods (like LZ77/LZ78): Builds a dictionary of repeated sequences and replaces them with shorter references.
ZIP files use a combination of these methods.
Where is Lossless Compression Used?
| Format | Use Case |
|---|---|
| ZIP / RAR | Compressing folders and files for sharing |
| PNG | Images (especially graphics, logos, screenshots) |
| FLAC | Audio (lossless music archiving) |
| GIF | Simple web graphics and animations |
| PDF (partially) | Text and vector graphics in documents |
Compression Ratio
The compression ratio measures how effectively a file has been compressed:
Compression ratio = Original size ÷ Compressed size
If a 1,000 KB file compresses down to 250 KB, the ratio is 4:1 — meaning the file is now four times smaller.
Lossless compression works best on files with lots of repetition, like text documents, simple graphics, or database exports. It works poorly on data that is already compressed (like MP3 audio), and it cannot achieve as small a file size as lossy compression.
You will never lose quality with lossless compression, no matter how many times you compress and decompress the file. This makes it ideal for files you plan to edit and re-save.
Practice Problems
A text file of 800 KB is compressed with a lossless algorithm to 200 KB. What is the compression ratio? What does this tell you about the amount of repetition in the original file?
Show answer
Why is lossless compression preferred over lossy compression for a software executable (.exe) file, even if the compressed file size would be larger?