lineweight

Vector linework with weight, for the case where there is no tablet.

A stroke in SVG has one width from end to end, which is why vector drawings read as diagrams next to drawings made by hand. lineweight models the thing a tablet measures — pressure — and expands each stroke into a filled outline whose width varies along it, so plain SVG can carry line weight without any dependency on a drawing application.

It exists because a language model asked to draw has no hand. Pressure cannot be felt, so it is computed from what is observable about a line, and the numbers can be calibrated against real artwork rather than guessed.

What it does

• Four brushes (fine, ink, pencil, wash), each the same set of numbers a paint program exposes: width, opacity, dab spacing, and jitter, plus two taper curves.
• A pressure model. Pressure follows four effects that a real stroke shows: it is lighter when the stroke is moving fast, lighter through a sharp turn, tapered at both ends of an open stroke, and drifting slowly underneath. Closed contours are walked once and never tapered, because a loop has no ends to taper.
• Stroke to outline. SVG cannot vary a stroke's width, so the width profile is expanded into an outline — offset the path to both sides by half the local width and fill the result. This is the same operation a drawing application performs when a variable-width stroke is expanded, done here so the output stays plain SVG.
• Whole-document inking. inked_svg() walks an existing SVG and gives every shape large enough to be part of a silhouette a weighted contour, leaving small details alone. Extent decides, not a list of names, so a shape added later is inked without anybody remembering to.
• Calibration. --fit measures a real drawing's linework — the distribution of ink across rows and columns — which is the appearance a brush's curves have to reproduce.

What it is not

It is not an automation bridge into a paint application. Most of them expose no scripting interface at all, and those that do expose a different one each; a tool that claims otherwise is a tool that will break. What is portable is the model of pressure and the measurement of a real drawing — so the workflow is: draw a test sheet in whatever program you have, at whatever pressures you can control, and fit the brushes here to it.

It is not a renderer and not a drawing program. It produces path data. What you fill it with is your business.

Install

pip install -e .          # add [fit] to measure images instead of only drawing
python -m lineweight --out strokes.svg
python -m lineweight --fit reference.png

Use

from lineweight import stroke, inked_svg

# one stroke: a path in, a filled outline plus an opacity out
d, opacity = stroke([(40, 40), (140, 30), (240, 60)], 'ink')
svg = f'<path d="{d}" fill="#1A1620" opacity="{opacity:.2f}"/>'

# or ink a whole document that something else drew
inked = inked_svg(open('figure.svg').read(), min_extent=46, brush='ink', colour='#2A1E26')

Calibrating to your own hand

1. Draw a sheet of strokes with one brush: a long line pressed from light through heavy and back, the same line drawn fast and then slow, a right-angle turn, a hairpin, and a tapered flick.
2. python -m lineweight --fit sheet.png prints the ink distribution.
3. Adjust BRUSHES until the model's distribution matches. The four effects in pressures() are the dials.

Licence

MIT — see LICENSE.