How to Vectorize a Sketch or Line Drawing

A flatbed scan at 300 dpi or more gives you even lighting, no perspective distortion and no shadow. A phone photograph gives you all three, plus a gradient across the page from whatever light you were standing under. If a photograph is all you have, at least shoot it flat under diffuse light and correct the perspective afterwards.

Then push the contrast until the marks are unambiguously darker than the paper. In practice this means a levels adjustment where the white point comes down to just above the brightest paper tone and the black point comes up to just below the darkest ink. Faint pencil that is only slightly darker than the page is the single biggest cause of speckled traces.

If the drawing is pure line art, threshold it to black and white outright. You lose the tonal subtlety, which for a line drawing you were not using anyway, and you gain a source with no ambiguity in it at all.

The engine finds regions of similar colour and draws their boundaries. For a pencil line, the region is the dark stroke itself, so the output is the outline of the stroke — a long thin closed shape that follows both sides of your line. That is what you want for printing, filling and cutting.

It is not a centreline. If you need a single-stroke path — for a pen plotter, for engraving, or for a stroke you intend to restyle with different weights — that is a different operation called centreline tracing, and the usual route is Inkscape's centreline tracing mode after you have a clean raster.

Scanning introduces a fine tremor along every edge: the sampling grid is not aligned to your line, so the traced boundary steps in and out by a fraction of a pixel. A little smoothing removes that entirely and the line reads as intended.

Too much and the drawing stops being yours. The small irregularities in a hand-drawn curve are what make it look hand-drawn, and a maximum-smoothed sketch has the slightly dead quality of clip art. On Pro and above, the smoothing control is a four-step scale; Strong is the default and keeps genuine corners sharp, and Maximum rounds everything including the corners you meant.

Every speck of dust, every torn edge, every ghost of an erased construction line is a shape in the output. Deleting fifty stray specks in a vector editor is tedious; painting them out with a white brush in the raster takes seconds and is much more reliable.

Pay particular attention to the edges of the scan. Paper rarely sits perfectly flat, so the boundary of the sheet often traces as a long thin shadow shape that surrounds everything else. Crop inside the paper edge and the problem disappears.