PNG-8 vs PNG-24

PNG-8 stores an index per pixel pointing into a palette of at most 256 colours. One byte per pixel, plus a small palette.

PNG-24 stores three bytes per pixel — one each for red, green and blue at 8 bits. PNG-32 adds a fourth byte for alpha, and is what most "transparent PNGs" actually are, though almost nobody uses the term.

So before compression, PNG-8 holds a third to a quarter of the data. After compression the gap narrows but usually remains substantial on content that suits it.

This is the part that gets glossed over. Converting to PNG-8 means choosing 256 representative colours and mapping every pixel to the nearest one. Information is discarded permanently.

The PNG file that results is still losslessly compressed — the format has not changed — but the image inside it is no longer what you started with. That is why a "lossless PNG optimiser" claiming 70% savings on a photograph is being misleading: the lossless part is true and irrelevant, because the destructive step happened before it.

On a logo with six flat colours nothing is lost, because there were fewer than 256 to begin with. On a photograph, plenty is.

To disguise banding after quantisation, encoders can dither — scattering pixels between two palette colours so the eye blends them into an intermediate shade.

It works visually and it costs you the compression. Dithering replaces smooth predictable areas with high-frequency noise, which is precisely what PNG's filtering and DEFLATE are worst at. A heavily dithered PNG-8 can end up larger than the PNG-24 it was meant to replace.

If you find yourself needing heavy dithering, the content is not suited to an indexed palette.

PNG-8 transparency is usually binary in practice — a pixel is drawn or it is not. The format technically permits variable alpha per palette entry, but tool support is inconsistent and most exporters produce the binary kind.

That means anti-aliased edges come out jagged, exactly as they do in GIF. A logo with a soft outline placed on a coloured background will show a stair-stepped fringe.

PNG-24 with alpha — PNG-32 — has 256 levels of opacity and handles soft edges properly. If the graphic has anti-aliasing and will sit on a varied background, this is not optional.

The PNG compressor here does not quantise. It re-encodes losslessly at full colour depth, so every pixel value in the output matches the input exactly.

That is why its savings are more modest than tools which convert to PNG-8 and still describe the result as lossless. Both approaches are legitimate; only one of them is what the word means. If you want indexed colour, use an editor's export dialogue where the choice is explicit and you can inspect the result.