Compress PNG

Lossless compression rewrites the file so it takes up less space while every pixel value survives exactly. Decode the result and you get the original back, byte for byte. PNG, GIF and lossless WebP work this way, and so does a ZIP archive.

Lossy compression throws information away on purpose. JPEG converts the image to a frequency representation, then discards the high-frequency detail your eye is least likely to miss. The result cannot be reversed: the discarded detail is gone. That is also why lossy formats reach file sizes lossless ones cannot approach — a photo saved as PNG is routinely five to ten times larger than the same photo saved as JPEG at a sensible quality.

Neither one is better. Photographs, with their smooth gradients and noise, suit lossy encoding. Screenshots, logos, line art, charts and anything with large flat areas and hard edges suit lossless encoding, because lossy encoders produce visible haloes around sharp boundaries.

JPEG has no alpha channel. It cannot store transparency in any form. When a transparent PNG is converted to JPEG the transparent pixels have to become something, and most tools silently fill them with black — which is why a logo exported to JPEG so often arrives with an unwanted dark box around it.

If your image has transparency you have three sensible options: keep it as PNG, use WebP (which supports alpha and is typically 25–35% smaller than PNG for the same image), or flatten it onto a known background colour deliberately rather than letting an encoder choose for you.

PNG's own compression is lossless and works best on images with few distinct colours and large uniform regions. A screenshot of a text document compresses beautifully. A photograph saved as PNG does not, because virtually every pixel differs from its neighbour and there is no redundancy for the algorithm to exploit.

PNG compresses in two stages. First it filters each row of pixels, replacing each value with its difference from a neighbouring pixel — so a smooth gradient becomes a long run of near-identical small numbers. Then it runs DEFLATE over the result, which is the same algorithm a ZIP archive uses.

That design explains exactly which images PNG is good at. Flat colour, repeated patterns, sharp edges and text all produce long predictable runs after filtering, and DEFLATE collapses them dramatically. Photographic noise produces nothing predictable at all, which is why a photograph in PNG is enormous.

It also explains why re-compressing a PNG gives modest results: the algorithm is deterministic, and if the original encoder chose good filters there is little left to find.

The quality number in a JPEG or WebP encoder is not a percentage of anything. It is an index into a set of quantisation tables that control how aggressively detail is discarded. The relationship to file size is steep and non-linear.

For a typical 12-megapixel phone photograph the curve looks roughly like this:

• Quality 95–100 — near-original appearance, but the file is often only 20–35% smaller. Rarely worth it for the web.
• Quality 80–90 — the sweet spot for photography you care about. Typically 50–70% smaller with no artefact you would notice at normal viewing distance.
• Quality 70–80 — the sweet spot for ordinary web images. Usually 70–85% smaller. Artefacts appear only if you zoom in on flat sky or skin.
• Quality 50–70 — visible softening and blocking in smooth areas, though thumbnails and background images survive it well.
• Below 50 — obvious blocking, ringing and colour banding. Useful only for placeholders.

Diminishing returns set in fast at the top of the range. Going from quality 100 to quality 90 usually halves the file for no perceptible change; going from 90 to 80 saves far less and costs slightly more. If you only remember one number, use 80.

A digital image has exactly one physical property: its pixel dimensions. A 3000 × 2000 photograph contains six million pixels whether its metadata claims 72 DPI or 300 DPI.

DPI is a printing instruction stored in a metadata field. It tells a printer how many of those pixels to place per inch of paper, which determines the printed size — 3000 pixels at 300 DPI is a 10-inch-wide print; the same file at 72 DPI is a 41-inch-wide print of the same six million pixels. Changing the number changes nothing about the file except a handful of bytes in a header.

So "save this at 72 DPI for the web" does not make a file smaller. Reducing pixel dimensions does. If a print shop asks for 300 DPI at A4, what they actually need is about 2480 × 3508 pixels; supply that many pixels and the DPI tag can be corrected in seconds.