The mechanism in two steps

Step one: colour reduction. The tracer runs a median-cut quantiser that picks n representative colours (2–24, from the slider) and assigns every pixel to the nearest one, with no dithering. On a logo with four fills this is lossless in practice. On a photograph of a face — thousands of skin tones shading into each other — it forces every pixel into, say, eight buckets. Where the original faded smoothly from one tone to the next, there is now a hard line between bucket 3 and bucket 4.

Step two: outlining. Potrace takes each colour's pixels as a black-and-white mask and fits an outline around it. It is designed for binary images: text, line art, silhouettes. It has no concept of "this edge should be soft". Each bucket boundary from step one becomes a crisp vector edge.

Put together: a photo becomes a stack of flat, crisply outlined bands. That is posterisation, and it is inherent to region tracing rather than a defect of a particular tool. Any tracer that outputs filled paths does the same thing; the ones that appear not to are embedding the bitmap (see the guide on vector versus raster).

What the colour slider does and does not change

  • More colours (up to 24): more bands, each thinner, more paths, larger file. Skin looks like a contour map with finer lines. It never converges on a photograph, because the maximum is 24 flat fills.
  • Fewer colours (2–4): bold poster or stencil look. This is where tracing a photo is genuinely useful.
  • Smooth curves: softens the band outlines slightly; does not soften the colour steps.
  • Presets: Detailed artwork starts at 16 colours and skips the palette clean-up; Line art / Stencil converts to black and white first, which on a portrait produces a two-tone stencil.

When tracing a photo is the right call

  • Stencils and silhouettes: a high-contrast portrait at 2 colours gives a cut-ready silhouette; adjust brightness in an image editor first so the threshold lands where you want.
  • Poster and screen-print looks: 3–6 colours, then recolour the bands in the editor to your ink colours.
  • Simple product shots with flat backgrounds: remove the background, trace the object at a low count, and you have a clean flat illustration of it.

When to keep the raster

If the goal is "the same photo, but scalable", a trace will disappoint: a JPEG at a sufficient resolution is the right format for photographs, and SVG can carry one only by embedding it. Gradients in a logo are the in-between case: trace the flat parts at a low colour count, then recreate the gradient as a real SVG gradient in a desktop vector editor, which the tracer does not do.

Reading the preview honestly

The preview shows exactly the bands you will get. If it looks like a paint-by-numbers version of your photo, that is the result, and no unlock changes it. Previews are free — use them to find the colour count that gives the poster effect you want before spending a credit.