nib

nib draws. It makes strokes, the kind that stay joined along their length and close round a shape, on a paper colour you pick. Think pen and ink, charcoal, a technical drawing or a cyanotype. If you want a neon outline, plenty of other effects will happily give you one.

The difference is the order it asks the questions in. A conventional edge effect asks one question per pixel: how fast is the tone changing here? The answer's a magnitude, and a magnitude has no idea of direction, so it can't tell a stroke from a speck, and what it decides at one pixel has nothing to do with what it decides at the next one along the same line. nib asks about direction first, builds a flow field over the whole frame, and then steers two filters by it, one across it and one along it: a band-pass across the flow, which is the only direction a band-pass measures an edge in at all, and a line integral convolution along it, which is what joins the per-pixel answers up into a stroke.

You can check that two ways. Turn Flow to zero and the second filter is skipped, leaving plain isotropic XDoG, the effect without the idea in it. That comparison ships as one of the presets, so it's one click rather than an argument. And the flow field itself is measured: the offline harness renders concentric rings whose tangent direction is known in closed form, adds noise, and reports how many degrees the plugin's own answer is off, using the same shader code that ships.

Detection runs on whichever channel actually carries the drawing, and that isn't always luminance. A red line on a blue field has loads of chroma contrast and almost none in luma, so detecting on saturation is an ordinary setting rather than an exotic one. Over time it's lopsided on purpose: ink appears at once and fades slowly, because a line that arrives late has been drawn on the wrong frame.