Reduce the Number of Colors in a PNG

median-cut palette · Floyd–Steinberg dithering · before/after size shown

PNG compresses repetition. The fewer distinct colours an image contains, the better it compresses — which is why cutting an icon or screenshot from 40,000 colours down to 32 can halve or quarter the file with no visible change. Pick a colour count, and the output row tells you exactly what you saved.

Where to set the colour count

Flat UI screenshots, logos and line art usually survive 16 to 32 colours with no visible loss at all — this is where the big wins live. Illustrations with soft shading want 64 to 128. Photographs generally shouldn't come here: banding appears well before the file gets meaningfully smaller, and WebP or Compress will beat quantisation comfortably on photographic content.

Check Color count first. If an image already uses only 200 colours, quantising to 128 buys you very little; if it reports 60,000, there's real room.

Dithering is a trade

Without dithering, each pixel snaps to the nearest palette colour and smooth gradients break into visible bands. Floyd–Steinberg dithering spreads each pixel's rounding error into its neighbours, so gradients stay smooth at the cost of a fine speckle. That speckle is noise, and noise compresses badly — a dithered file is often 20–40% larger than the same image undithered. Turn it off for flat graphics, leave it on for anything with a gradient or a soft shadow.

The all-or-nothing transparency option snaps every pixel to fully opaque or fully transparent. It's a real saving on icons with soft anti-aliased edges, and it makes those edges hard — good for pixel art and sprite work, wrong for a logo that sits on varying backgrounds.

One honest limitation

A browser canvas always writes 32-bit truecolour PNG, so this produces a PNG whose pixels use few colours rather than a true indexed PNG-8 with a palette chunk. You still get most of the saving, because DEFLATE is what's doing the work either way. For the last slice — a genuine PLTE-based 8-bit PNG — you need pngquant or oxipng on your own machine. Pairing this with Remove metadata is worth a few more percent for free.

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