Canny Edge Detector

Walk across the edge of a DAPI-stained nucleus and the brightness climbs over a few pixels. The gradient is large along that whole climb, so a plain thresholdLoading... on the gradient marks a band, not a line. Canny's steps turn the band into a line and determine which lines are real.

The Engines Reference lists the steps this way:

1. Find the intensity gradients of the image.
2. Apply non-maximum suppression to get rid of spurious responses.
3. Apply a double threshold to determine potential edges.
4. Track edges by hysteresis, suppressing the weak edges that are not connected to strong ones.

You'll find it in the FiltersLoading... list of the Basic Operations Module. The reference example takes an 8-bit grayscale DAPI image and returns an 8-bit grayscale image matching the input. The list starts at the gradient: it names no smoothing step, and the engine has no smoothing setting.

Two neighbors in Filters measure intensity change without the deciding steps. The Sobel filtersLoading... return a derivative along one axis or the diagonal, and the Laplace filterLoading... highlights rapid change in every direction; both return graded images that still need a threshold. The Membrane detectorLoading... is the specialist for thin lines, which are a double change of intensity. In general, a thin bright line has an edge on each side, so an edge detector reports it twice.

In image processing generally, the detector comes from John Canny's 1986 paper, which set three goals: detect real edges and few false ones, place each edge where it truly is, and respond to each edge once. Non-maximum suppression serves the second and third goals. Hysteresis serves the first: a faint stretch of a real boundary survives because it is attached to a strong stretch, while an isolated faint response does not.

The textbook algorithm begins with a Gaussian blur whose width sets the scale of the edges found, then computes the gradient, often with Sobel kernels. StrataQuest's documented steps begin at the gradient, so the scale is chosen by whatever smoothing you place ahead of the detector, such as the Gauss filterLoading... and its Standard deviation. The reference doesn't say which gradient operator the engine uses.

Two thresholds are used rather than one because a single cut forces a choice between broken edges (set high) and speckle (set low). Hysteresis settles it with connectivity instead of brightness alone. Canny's own suggestion, which the reference repeats, is an upper-to-lower ratio of two or three to one, based on predicted signal-to-noise ratios.

Both thresholds start at 0, so set them before trusting the output. One workable path: raise the Upper threshold until spurious edges in flat regions disappear, then set the Lower threshold to a half or a third of it, as Canny recommends. If real edges break into dashes, lower the Lower threshold; if grain still produces edges, smooth the input first.

Source: StrataQuest 8.1 Engines Reference, ch. 6.3 (Canny edge detector pp. 122–124; Gauss filter p. 129; Laplace filter p. 132; Membrane detector p. 138; Sobel filters pp. 144–151) and ch. 6 (p. 19).