What the conversion is deciding
A raster image is a grid of measurements. A vector image is a set of instructions: start here, curve to there with this control point, fill the enclosed region with this colour. Nothing in the first description tells you where one shape ends and another begins, so every tracer has to make that judgement, and the judgement is the whole product.
Two decisions dominate. First, segmentation: which pixels belong to the same region. Second, simplification: how few curve segments can follow that region's boundary without visibly departing from it. Push simplification too hard and corners round off and small features vanish. Do not push it at all and you get a path for every pixel cluster, which technically reproduces the image and is useless as artwork.
That tension explains the whole success profile. Flat art with hard boundaries makes both decisions trivial. A photograph makes both unanswerable, because there is no correct segmentation of a gradient and no small set of curves that describes grain.
Preparing an image that traces well
Most bad conversions are fixed before the tracer runs rather than in its settings.
- Reduce to few flat colours. Posterise deliberately to the number of colours the final art should have, and check the result before tracing.
- Get hard edges. Anti-aliased boundaries create a thin intermediate band that becomes its own path. Rendering or exporting with hard edges removes an entire class of artefact.
- Remove effects. Drop shadows, glows, gradients and texture all trace into either garbage geometry or hundreds of near-identical shapes.
- Work at high resolution. More pixels along a boundary means a better curve fit, even though the output has no resolution itself.
- Trace on a plain background. A busy background doubles the segmentation problem for no benefit, since you will discard it anyway.
If you need to vectorize image assets that only exist as small screenshots or compressed JPEGs, upscale and clean them first. Compression artefacts trace as real geometry, and the tracer cannot tell the difference between a JPEG ring and an intentional edge.
Judging the output
Look at three things, in this order, and ignore how it appears at the size you happen to be viewing.
Path count first. It is the fastest proxy for whether the tracer understood the image. A logo in hundreds of paths has been traced structurally; the same logo in five thousand has been traced as noise.
Then zoom to the corners. Simplification errors always show up first where two lines meet at an angle, because a curve fit rounds them. Text and sharp geometric marks are the least forgiving cases.
Then recolour one shape. This is the real test of whether the file is usable, because it reveals whether the shapes are the shapes you think they are. If changing one fill produces a patchwork, the regions were split along invisible boundaries and the file will fight every future edit.
Tracing versus generating vectors
There are two routes to a scalable file, and they suit different situations. Tracing starts from artwork that must be preserved: an existing logo, a scanned drawing, a frame you already approved. Its output geometry is whatever approximates the pixels, which is rarely how a designer would have drawn it.
Generating vectors directly, from models that output SVG rather than pixels, gives you path structures that were intended as structures, usually with sensible grouping and a small colour set. That is much easier to edit and recolour, and it is the better choice whenever the exact original does not matter. The practical rule: if the source is sacred, trace it; if only the idea matters, generate it. And in either case, ask for flat colour and hard edges up front, because both routes reward the same input discipline.