What auto-trace is doing, and when to redraw instead

The algorithm, so you can predict it

Every tracer (Inkscape’s Trace Bitmap, Illustrator’s Image Trace, CorelDRAW’s PowerTRACE, the potrace command line, and the paid web services) runs roughly the same four steps.

1. Reduce to regions. For a black-and-white trace this is a single brightness threshold. For a colour trace it is a quantisation to N colours.
2. Find contours. Walk the boundary between regions. The result is a polygon whose vertices sit on pixel corners, so a diagonal edge comes out as a staircase.
3. Detect corners. Decide which staircase steps are a genuine corner and which are sampling noise. This is a tolerance you can set.
4. Fit curves. Replace each run of steps between corners with Bézier segments, within an error tolerance.

Once you know this, the failures stop being mysterious.

• Wobbly straight edges come from step 4 smoothing a staircase that was actually a straight line. The fix is a higher resolution scan, not a different setting.
• Rounded corners on a square logo come from step 3 deciding your corner was noise. Lower the corner threshold.
• Hundreds of nodes come from a tight error tolerance in step 4 plus a noisy scan.
• Blobs everywhere on a photograph come from step 1: a photograph has no flat regions to find.

The settings that actually matter

In Inkscape (Path ▸ Trace Bitmap):

• Brightness cutoff for line art. Try 0.45, then move it and watch the preview.
• Speckles: discards features below N pixels. Set it to about 4 for a clean scan, 20 for a photographed business card.
• Smooth corners: lower keeps corners sharp.
• Optimise: merges adjacent curve segments. Higher means fewer nodes and more drift from the original.
• Multiple scans ▸ Colours for a flat colour logo. Tick Stack scans and Remove background.

Two things about multi-colour trace nobody mentions. The output is a stack of opaque shapes, largest at the back, each partly hidden by the one in front, not a jigsaw of adjacent regions. So recolouring the top shape reveals the one beneath rather than changing the visible colour, and deleting a “background” shape puts holes in everything. And the shapes overlap by a pixel or two, which is invisible on screen and shows as a doubled edge on a cutting plotter.

Centreline tracing is a different job

A hand-drawn signature, a technical line drawing, a map: these are lines, not filled regions. A normal trace gives you the outline of each stroke: two paths where you wanted one. Centreline tracing gives you a single path down the middle, which is what a plotter pen, an engraver or a CNC router wants.

Inkscape 1.x has Centreline tracing (autotrace) in the same dialog. On the command line, autotrace -centerline does it. Illustrator has no true centreline mode; the workaround is to trace normally and then use Object ▸ Path ▸ Simplify on the result, which is not the same thing and usually worse.

Preparing the scan is most of the win

Ten minutes in GIMP or Photopea before tracing beats an hour of settings afterwards.

1. Scan at 600 dpi minimum, 1200 for fine line work. Photograph only if you must, and flatten the lighting.
2. Levels: drag the black and white points in until the background is white and the ink black.
3. Despeckle or a small median filter to kill paper texture.
4. Threshold to pure black and white yourself, so you control step 1 rather than the tracer.

When to redraw instead

Trace, and accept it, when: the artwork is a one-off, it will be used at roughly the size it was scanned, and nobody will cut, stitch or enlarge it. A signature on a certificate. A rubber-stamp texture. A decorative border.

Redraw, always, when: the mark is a logo, anything goes to a plotter, laser or embroidery machine, or the artwork will be enlarged. Redrawing a typical mark from a clean trace takes 30 to 60 minutes with primitives and the pen tool, and the difference is measurable: eight to fifty nodes instead of two hundred to four hundred, straight edges that are straight, and circles that are circles.

Use the trace as a locked template layer at 30% opacity and draw over it. That is the professional workflow, and it is the same one whether you paid for your software or not.

The paid vectorisers

Vector Magic, Vectorizer.AI, Adobe’s and Recraft’s tools produce noticeably better curve fitting than potrace on difficult sources, and they are faster. They do not change the underlying truth: the output has more nodes than a drawn mark, the corners are inferred rather than intended, and a plotter will still cut every wobble. Treat them as better tracers, not as a substitute for drawing.