Curve-tracing method#

Curve tracing explains how to use the feature. This page explains what its preview represents.

GraphX reads the original local image raster after you choose a seed. It samples colour around that point, compares the sample with its local background, and identifies likely curve pixels when the selected colour is sufficiently distinct.

From that seed GraphX follows compatible pixels in both directions across the calibrated plot. At each step it considers colour similarity and continuity from the preceding path, can bridge bounded gaps, and derives a representative centreline rather than treating every matching stroke pixel as a separate point. The detailed path is simplified for a readable preview while its intrinsic image positions remain the basis for tracing accuracy.

Tolerance and resampling#

Tolerance changes the acceptable colour-distance range and reruns the detection from the same seed. A larger tolerance can bridge anti-aliasing, compression, or small gaps, but can also admit grid lines, text, or another similarly coloured curve.

The Points control does not rerun detection. GraphX retains the raw path and resamples it at evenly spaced normalised plot-X parameters. On a linear X axis this corresponds to equal intervals across calibrated X; on a logarithmic X axis it corresponds to equal intervals in the logarithmic calibration parameter, not equal arithmetic differences in X. Accepted samples are converted into ordinary editable points in the active dataset as one Undo action.

Ambiguity#

Tracing is colour-guided path following, not semantic image recognition. Intersections, nearby matching colours, grid lines, markers, thick strokes, weak contrast, JPEG artefacts, gaps, and near-vertical or rapidly changing geometry can make continuation ambiguous. The preview exists so you can inspect the detected line and its outlined output markers before accepting it. See Limitations and accuracy.