Brush strokes on roads

A road drawn with stroke is a crisp constant-width band — correct, and obviously machine-made. A road drawn with line is a brush dragged along the geometry, with pressure and speed varying over the stroke.

"glazing":       { "type": "brush", "src": "file:brushes/watercolor_glazing.myb" },

"glazing_brush": { "op": "brush-file", "src": "@glazing" },
"roads_trunk_f": { "op": "features", "layer": "roads",
                   "min-zoom-field": "min_zoom",
                   "filter-expr": ["in", ["get", "kind_detail"],
                                   ["literal", ["trunk", "primary", "motorway"]]] },
"roads_trunk": {
  "op": "line", "features": "@roads_trunk_f", "brush": "@glazing_brush",
  "color": "$road", "radius-px": 1.6, "opacity": 0.6,
  "pressure-base": 0.7, "pressure-jitter": 0.2, "dtime": 0.05,
  "radius-stroke-curve":  [[0.0, -1.4], [0.12, 0.0], [0.88, 0.0], [1.0, -1.8]],
  "opacity-stroke-curve": [[0.0, -0.25], [0.1, 0.0], [0.9, 0.0], [1.0, -0.35]],
  "dtime-stroke-curve":   [[0.0, 2.5],  [0.15, 1.0], [0.85, 1.0], [1.0, 3.0]]
}

From watercolor.json.

pencil-sketch.json: ten stroke layers, one sketchy-line function, wobble and all.

Brushes come from you

Nothing is bundled. Declare any MyPaint .myb brush as a source and load it with brush-file. The example styles ship CC0 brushes by David Revoy next to their style JSON; file: paths resolve against --assets-dir, so render those from a checkout. For a portable style, use an http(s):// or data: source instead — see sources.

brush-solid synthesizes a brush when you want the stroke machinery without an asset.

Stroke curves are the whole trick

A *-stroke-curve is a list of [t, delta] control points where t runs 0→1 along the stroke. The shape above says: start thin, hold, taper at the end.

That is what makes a stroke look like it was begun and ended rather than extruded. Three curves are worth setting:

• radius-stroke-curve — negative deltas at both ends give tapered entry and exit. Asymmetric values (−1.4 in, −1.8 out) read as a hand lifting.
• opacity-stroke-curve — fading the ends slightly keeps intersections from building into dark knots where many roads meet.
• dtime-stroke-curve — speed. Fast at the ends, slow through the middle, which changes how much paint the brush lays down.

Stroke curves are arrays, so a plain $param cannot reach into them — but a function argument substitutes structurally and can. That is how one stroke recipe serves many layers; see functions.

Wobble: wave before line

Pressure variation alone still follows the survey line exactly. Perturb the geometry first:

"wob":  { "op": "wave", "features": "@roads_f", "amplitude-px": 0.8 },
"draw": { "op": "line", "features": "@wob", "brush": "@pencil",
          "color": "$ink", "radius-px": 1.2 }

Sub-pixel amplitude is enough — 0.8 px reads as a hand not quite following a ruler, while 3 px reads as bad data. wave is a Features → Features transform, so it composes with any downstream paint op.

Factor it into a function

Ten road classes differing only in layer, colour, and width should be one declaration called ten times. This is exactly what pencil-sketch.json does with its sketchy-line function:

"functions": {
  "sketchy-line": {
    "inputs": { "features": { "kind": "features" }, "brush": { "kind": "brush" },
                "color": { "kind": "scalar" }, "radius": { "kind": "scalar", "default": 1.0 } },
    "output": "@draw", "output-kind": "raster",
    "nodes": {
      "wob":  { "op": "wave", "features": "@features", "amplitude-px": 0.8 },
      "draw": { "op": "line", "features": "@wob", "brush": "@brush",
                "color": "@color", "radius-px": "@radius" }
    }
  }
}

Calls with identical arguments share cache entries, so inlining costs nothing.

Cost

Brush strokes are the most expensive thing in a painterly style — every dab along every line is real work. Two levers: gate minor classes behind min-zoom on the features node so they only appear where they matter, and use stroke rather than line for hairline classes where the brush texture would not be visible anyway.