{"course":{"slug":"vector-and-print","title":"Vectors, Logos and Things That Get Printed","url":"https://addaly.com/learn/vector-and-print","level":"beginner","track":"create"},"lesson":{"slug":"tracing-and-when-to-redraw","title":"What auto-trace is doing, and when to redraw instead","url":"https://addaly.com/learn/vector-and-print/tracing-and-when-to-redraw","position":8,"of":73,"minutes":9,"answer":"Auto-trace thresholds, finds contours on a pixel staircase, guesses which steps are corners and fits curves within a tolerance, so wobbly straights, rounded corners and node explosions are all predictable consequences.","keyIdea":"Auto-trace thresholds, finds contours on a pixel staircase, guesses which steps are corners and fits curves within a tolerance, so wobbly straights, rounded corners and node explosions are all predictable consequences, and anything being enlarged or cut should be redrawn over the trace rather than shipped as one."},"body":{"format":"markdown","text":"## The algorithm, so you can predict it\n\nEvery 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.\n\n1. **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.\n2. **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.\n3. **Detect corners.** Decide which staircase steps are a genuine corner and which are sampling noise. This is a tolerance you can set.\n4. **Fit curves.** Replace each run of steps between corners with Bézier segments, within an error tolerance.\n\nOnce you know this, the failures stop being mysterious.\n\n- **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.\n- **Rounded corners on a square logo** come from step 3 deciding your corner was noise. Lower the corner threshold.\n- **Hundreds of nodes** come from a tight error tolerance in step 4 plus a noisy scan.\n- **Blobs everywhere on a photograph** come from step 1: a photograph has no flat regions to find.\n\n## The settings that actually matter\n\nIn Inkscape (Path ▸ Trace Bitmap):\n\n- **Brightness cutoff** for line art. Try 0.45, then move it and watch the preview.\n- **Speckles**: discards features below N pixels. Set it to about 4 for a clean scan, 20 for a photographed business card.\n- **Smooth corners**: lower keeps corners sharp.\n- **Optimise**: merges adjacent curve segments. Higher means fewer nodes and more drift from the original.\n- **Multiple scans ▸ Colours** for a flat colour logo. Tick **Stack scans** and **Remove background**.\n\nTwo 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.\n\n## Centreline tracing is a different job\n\nA 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.\n\nInkscape 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.\n\n## Preparing the scan is most of the win\n\nTen minutes in GIMP or Photopea before tracing beats an hour of settings afterwards.\n\n1. Scan at **600 dpi minimum**, 1200 for fine line work. Photograph only if you must, and flatten the lighting.\n2. **Levels**: drag the black and white points in until the background is white and the ink black.\n3. **Despeckle** or a small median filter to kill paper texture.\n4. **Threshold** to pure black and white yourself, so you control step 1 rather than the tracer.\n\n## When to redraw instead\n\nTrace, 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.\n\nRedraw, 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.\n\nUse 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.\n\n## The paid vectorisers\n\nVector 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."},"check":{"question":"A shop logo is traced from a 300 dpi scan in Inkscape. The result looks acceptable at 100% on screen, but when cut in vinyl at 600 mm wide the long straight edges of the letterforms have a visible gentle waviness. Which step of the tracing pipeline produced that, and what actually fixes it?","options":[{"text":"Curve fitting smoothed a straight edge's pixel staircase into shallow Béziers; a better scan and a redraw fix it.","correct":true},{"text":"The speckle setting was too low, so paper noise was traced as tiny features along each edge.","correct":false,"tempting":"Speckles appear as separate small shapes, not as a smooth wave along an otherwise continuous edge."},{"text":"The vinyl plotter's blade offset was set wrong, which adds a small periodic deviation along every long straight cut.","correct":false,"tempting":"Blade offset shows as rounded or overshooting corners, not as waviness along a straight run, and the wobble is present in the file before it reaches the machine."},{"text":"The trace was done in RGB rather than greyscale, so the threshold varied slightly along each edge.","correct":false,"tempting":"Tracers convert to greyscale internally before thresholding; the colour mode of the source does not introduce a spatial variation along an edge."}]},"prev":{"slug":"where-vector-becomes-pixels","title":"Where vector quietly becomes pixels","url":"https://addaly.com/learn/vector-and-print/where-vector-becomes-pixels"},"next":{"slug":"fewer-points-better-curves","title":"Your curves are lumpy because you used too many points","url":"https://addaly.com/learn/vector-and-print/fewer-points-better-curves"},"authorship":"Founder and series editor: Dr Mustafa Mun. Written by AI models to a written standard. Exam keys checked blind by a second model: 3,665 of 3,666 agreed. Academic review invited.","review":{"status":"Draft, open for academic review"},"cite":"Addaly (2026) 'What auto-trace is doing, and when to redraw instead', in Vectors, Logos and Things That Get Printed. Edited by M. Mun. Addaly. Available at: https://addaly.com/learn/vector-and-print/tracing-and-when-to-redraw (Accessed: 13 October 2026).","licence":{"name":"CC BY-NC-SA 4.0","url":"https://creativecommons.org/licenses/by-nc-sa/4.0/"},"reader":{"schema":1,"title":"What auto-trace is doing, and when to redraw instead","courseTitle":"Vectors, Logos and Things That Get Printed","module":{"index":1,"slug":"what-a-vector-is","title":"The thing that is actually in the file"},"answer":null,"keyIdea":"Auto-trace thresholds, finds contours on a pixel staircase, guesses which steps are corners and fits curves within a tolerance, so wobbly straights, rounded corners and node explosions are all predictable consequences, and anything being enlarged or cut should be redrawn over the trace rather than shipped as one.","hero":{"kind":"plate","image":null,"alt":null,"caption":null},"blocks":[{"t":"h2","id":"the-algorithm-so-you-can-predict-it","runs":[{"text":"The algorithm, so you can predict it"}]},{"t":"p","runs":[{"text":"Every tracer (Inkscape’s Trace Bitmap, Illustrator’s Image Trace, CorelDRAW’s PowerTRACE, the "},{"text":"potrace","code":true},{"text":" command line, and the paid web services) runs roughly the same four steps."}]},{"t":"ol","start":1,"items":[[{"text":"Reduce to regions.","b":true},{"text":" For a black-and-white trace this is a single brightness threshold. For a colour trace it is a quantisation to N colours."}],[{"text":"Find contours.","b":true},{"text":" 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."}],[{"text":"Detect corners.","b":true},{"text":" Decide which staircase steps are a genuine corner and which are sampling noise. This is a tolerance you can set."}],[{"text":"Fit curves.","b":true},{"text":" Replace each run of steps between corners with Bézier segments, within an error tolerance."}]]},{"t":"p","runs":[{"text":"Once you know this, the failures stop being mysterious."}]},{"t":"ul","items":[[{"text":"Wobbly straight edges","b":true},{"text":" come from step 4 smoothing a staircase that was actually a straight line. The fix is a higher resolution scan, not a different setting."}],[{"text":"Rounded corners on a square logo","b":true},{"text":" come from step 3 deciding your corner was noise. Lower the corner threshold."}],[{"text":"Hundreds of nodes","b":true},{"text":" come from a tight error tolerance in step 4 plus a noisy scan."}],[{"text":"Blobs everywhere on a photograph","b":true},{"text":" come from step 1: a photograph has no flat regions to find."}]]},{"t":"h2","id":"the-settings-that-actually-matter","runs":[{"text":"The settings that actually matter"}]},{"t":"p","runs":[{"text":"In Inkscape (Path ▸ Trace Bitmap):"}]},{"t":"ul","items":[[{"text":"Brightness cutoff","b":true},{"text":" for line art. Try 0.45, then move it and watch the preview."}],[{"text":"Speckles","b":true},{"text":": discards features below N pixels. Set it to about 4 for a clean scan, 20 for a photographed business card."}],[{"text":"Smooth corners","b":true},{"text":": lower keeps corners sharp."}],[{"text":"Optimise","b":true},{"text":": merges adjacent curve segments. Higher means fewer nodes and more drift from the original."}],[{"text":"Multiple scans ▸ Colours","b":true},{"text":" for a flat colour logo. Tick "},{"text":"Stack scans","b":true},{"text":" and "},{"text":"Remove background","b":true},{"text":"."}]]},{"t":"p","runs":[{"text":"Two things about multi-colour trace nobody mentions. The output is a "},{"text":"stack of opaque shapes","b":true},{"text":", 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."}]},{"t":"h2","id":"centreline-tracing-is-a-different-job","runs":[{"text":"Centreline tracing is a different job"}]},{"t":"p","runs":[{"text":"A hand-drawn signature, a technical line drawing, a map: these are "},{"text":"lines","i":true},{"text":", 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."}]},{"t":"p","runs":[{"text":"Inkscape 1.x has "},{"text":"Centreline tracing (autotrace)","i":true},{"text":" in the same dialog. On the command line, "},{"text":"autotrace -centerline","code":true},{"text":" does it. Illustrator has no true centreline mode; the workaround is to trace normally and then use Object ▸ Path ▸ "},{"text":"Simplify","i":true},{"text":" on the result, which is not the same thing and usually worse."}]},{"t":"h2","id":"preparing-the-scan-is-most-of-the-win","runs":[{"text":"Preparing the scan is most of the win"}]},{"t":"p","runs":[{"text":"Ten minutes in GIMP or Photopea before tracing beats an hour of settings afterwards."}]},{"t":"ol","start":1,"items":[[{"text":"Scan at "},{"text":"600 dpi minimum","b":true},{"text":", 1200 for fine line work. Photograph only if you must, and flatten the lighting."}],[{"text":"Levels","b":true},{"text":": drag the black and white points in until the background is white and the ink black."}],[{"text":"Despeckle","b":true},{"text":" or a small median filter to kill paper texture."}],[{"text":"Threshold","b":true},{"text":" to pure black and white yourself, so you control step 1 rather than the tracer."}]]},{"t":"h2","id":"when-to-redraw-instead","runs":[{"text":"When to redraw instead"}]},{"t":"p","runs":[{"text":"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."}]},{"t":"p","runs":[{"text":"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."}]},{"t":"p","runs":[{"text":"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."}]},{"t":"h2","id":"the-paid-vectorisers","runs":[{"text":"The paid vectorisers"}]},{"t":"p","runs":[{"text":"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."}]}],"check":{"question":[{"text":"A shop logo is traced from a 300 dpi scan in Inkscape. The result looks acceptable at 100% on screen, but when cut in vinyl at 600 mm wide the long straight edges of the letterforms have a visible gentle waviness. Which step of the tracing pipeline produced that, and what actually fixes it?"}],"options":[{"text":[{"text":"Curve fitting smoothed a straight edge’s pixel staircase into shallow Béziers; a better scan and a redraw fix it."}],"correct":true,"tempting":null},{"text":[{"text":"The speckle setting was too low, so paper noise was traced as tiny features along each edge."}],"correct":false,"tempting":[{"text":"Speckles appear as separate small shapes, not as a smooth wave along an otherwise continuous edge."}]},{"text":[{"text":"The vinyl plotter’s blade offset was set wrong, which adds a small periodic deviation along every long straight cut."}],"correct":false,"tempting":[{"text":"Blade offset shows as rounded or overshooting corners, not as waviness along a straight run, and the wobble is present in the file before it reaches the machine."}]},{"text":[{"text":"The trace was done in RGB rather than greyscale, so the threshold varied slightly along each edge."}],"correct":false,"tempting":[{"text":"Tracers convert to greyscale internally before thresholding; the colour mode of the source does not introduce a spatial variation along an edge."}]}],"order":[3,2,1,0],"why":null},"prev":{"slug":"where-vector-becomes-pixels","title":"Where vector quietly becomes pixels","minutes":10},"next":{"slug":"fewer-points-better-curves","title":"Your curves are lumpy because you used too many points","minutes":10}}}