How JPEG Compression Works

2026-09-15

The problem JPEG was designed to solve

An uncompressed photo is enormous. A 12 megapixel image needs roughly 36 MB before you do anything to it, which was unusable on 1990s connections and is still wasteful today.

JPEG was built on one insight: human vision is far more sensitive to brightness than to colour. So it keeps brightness detail and throws most colour detail away. That single trade-off does most of the size reduction before any quality slider is involved.

What actually happens, in order

• The image is split into 8x8 blocks of pixels.
• Each block is converted from RGB into a brightness channel plus two colour channels.
• The colour channels are subsampled, usually keeping only one sample for every four brightness samples.
• Each block is transformed into a set of frequencies, which separates coarse structure from fine detail.
• The frequencies are quantised, meaning fine detail is rounded off more aggressively than coarse structure.
• The result is entropy coded, which packs the remaining numbers tightly.

The quality slider controls one step

Contrary to how it is marketed, the quality setting does not change how the algorithm works. It changes the size of the quantisation step. Higher quality means finer frequency detail is preserved; lower quality rounds it away.

This is why quality reductions are not linear. Dropping from 100 to 90 costs almost nothing visually. Dropping from 50 to 40 is where visible damage usually appears.

Why artifacts cluster around edges

Sharp edges contain high-frequency energy. Quantisation removes high frequencies first, so the parts of the image with the most high-frequency content lose the most. That is exactly where the familiar blocky and ringing artefacts show up.

It also explains why text and line art compress badly. They are almost entirely high-frequency content, which is the first thing JPEG discards.

Choosing a setting

• Photo archive: quality 90-95, because size is not the constraint and you may re-edit
• Web hero image: quality 75-85, which is where most photos stop showing visible loss
• Web thumbnail: quality 60-70, because the small display size hides the loss
• Repeated re-save: avoid entirely, since each save compounds the loss

The rule that matters most

Never edit and re-save a JPEG repeatedly. Every save applies quantisation again to already-quantised data, and the damage accumulates. Keep a lossless master and export a JPEG only at the end.

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