Image Compression Explained: Lossy vs Lossless

Every image you encounter on the web has been compressed in some way. Understanding the two fundamental approaches to compression, lossy and lossless, helps you make better decisions about image formats and quality settings.

What is Lossless Compression?

Lossless compression reduces file size without discarding any data. When you decompress a lossless file, you get exactly the original data, bit for bit. Think of it like a ZIP file for images.

Lossless formats: PNG, TIFF, BMP, WebP (lossless mode), AVIF (lossless mode)

What is Lossy Compression?

Lossy compression achieves much smaller files by permanently removing data that human eyes are less likely to notice. Once removed, this data cannot be recovered. The amount of data removed depends on the quality setting.

Lossy formats: JPG, WebP (lossy mode), AVIF (lossy mode)

How Lossy Compression Works

JPG compression, for example, works by dividing the image into 8x8 pixel blocks, transforming them into frequency components using DCT (Discrete Cosine Transform), then quantizing (rounding) the less important frequencies. Higher quality settings preserve more frequency detail, while lower settings discard more.

When to Use Each

Use Case	Compression Type	Recommended Format
Web photos	Lossy	WebP, JPG
Logos and icons	Lossless	PNG, SVG
Screenshots	Lossless	PNG
Print images	Lossless	TIFF, PNG
Social media	Lossy	JPG, WebP
Archival	Lossless	PNG, TIFF

The "Generation Loss" Problem

Each time you save a lossy file, quality degrades further. Editing a JPG and saving it again compounds the quality loss. This is why photographers and designers work with lossless formats and only export to lossy formats as the final step.

Finding the Right Balance

For web images, lossy compression at quality 80-85% is usually the sweet spot: files are significantly smaller than lossless, but the quality difference is imperceptible to most viewers. Modern formats like WebP and AVIF achieve this balance better than older formats.