bulgedPolyline.js

Area: Geometry

An interactive builder for a polyline where each section is defined by an endpoint and a bulge value, drawn with CurveBuilder.bulgeToXY. The dialog supports absolute or relative endpoints, computes the resulting chord, versine (sagitta), sweep angle, and radius live, and adds sections one at a time. A great reference for the bulge/versine curve model and for PolyCurveNodeDefinition.create(...).

Source: bulgedPolyline.js

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Example scripts

bulgedPolyline.js

Download bulgedPolyline.js

bulgedPolyline.js

/*
BSD 3-Clause License

Copyright (c) 2026, Canva Pty Ltd.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

Redistributions of source code must retain the above copyright notice, this
list of conditions and the following disclaimer.

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this list of conditions and the following disclaimer in the documentation
and/or other materials provided with the distribution.

Neither the name of the copyright holder nor the names of its
contributors may be used to endorse or promote products derived from
this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/

'use strict';

// Run from the Script Editor with:
// require('/examples/bulgedPolyline.js').main();

const { app } = require('/application.js');
const { BlendMode } = require('affinity:common');
const { SVG11 } = require('/colours.js');
const { Dialog, DialogResult } = require('/dialog.js');
const { Document } = require('/document.js');
const { FillDescriptor } = require('/fills.js');
const { CurveBuilder, PolyCurve } = require('/geometry.js');
const { LineStyleDescriptor } = require('/linestyle.js');
const { PolyCurveNodeDefinition } = require('/nodes.js');
const { UnitType } = require('/units.js');

const EndpointMode = Object.freeze({
    Absolute: 0,
    Relative: 1,
});

const VersineTolerance = 1e-6;

function formatNumber(value) {
    return Number.isFinite(value) ? value.toFixed(3) : String(value);
}

function createCurve(sections) {
    const builder = CurveBuilder.create().beginXY(sections[0].start.x, sections[0].start.y);
    for (const section of sections)
        builder.bulgeToXY(section.end.x, section.end.y, section.bulge);
    return builder.createCurve();
}

function createNodeDefinition(sections) {
    const polyCurve = PolyCurve.create();
    polyCurve.addCurve(createCurve(sections));

    const noFill = FillDescriptor.createNone();
    const lineFill = FillDescriptor.createSolid(SVG11.blue, BlendMode.Normal);
    const definition = PolyCurveNodeDefinition.create(
        polyCurve,
        noFill,
        lineFill,
        LineStyleDescriptor.createDefault(3),
        noFill
    );
    definition.userDescription = "Bulged Polyline";
    return definition;
}

function currentStart(dlg, sections) {
    if (sections.length > 0)
        return sections[sections.length - 1].end;
    return {x: dlg.startX.value, y: dlg.startY.value};
}

function readCurrentSection(dlg, sections) {
    const start = currentStart(dlg, sections);
    const argument = {x: dlg.argumentX.value, y: dlg.argumentY.value};
    const endpointMode = dlg.endpointMode.selectedIndex;
    const end = endpointMode == EndpointMode.Relative
        ? {x: start.x + argument.x, y: start.y + argument.y}
        : argument;
    const dx = end.x - start.x;
    const dy = end.y - start.y;
    const chordLength = Math.hypot(dx, dy);
    const bulge = dlg.bulgeValue.value;

    return {
        start: {x: start.x, y: start.y},
        end,
        dx,
        dy,
        chordLength,
        bulge,
        versine: bulge * chordLength * 0.5,
    };
}

function describeCurrentSection(section, addedSectionCount) {
    const lines = [
        `Added sections: ${addedSectionCount}`,
        `Current section: ${addedSectionCount + 1}`,
        `Start: (${formatNumber(section.start.x)}, ${formatNumber(section.start.y)})`,
        `End: (${formatNumber(section.end.x)}, ${formatNumber(section.end.y)})`,
        `Chord: ${formatNumber(section.chordLength)} px`,
        `Bulge: ${formatNumber(section.bulge)}`,
    ];

    if (section.chordLength == 0) {
        lines.push("Coincident endpoints: appends a zero-length line.");
        return lines.join('\n');
    }

    if (Math.abs(section.versine) <= VersineTolerance) {
        lines.push("Tolerance-zero versine: appends a straight line.");
        return lines.join('\n');
    }

    const sweepRadians = 4 * Math.atan(section.bulge);
    const absVersine = Math.abs(section.versine);
    const radius = absVersine * 0.5 + section.chordLength * section.chordLength / (8 * absVersine);
    const direction = sweepRadians < 0 ? "clockwise" : "counterclockwise";
    const arcKind = Math.abs(sweepRadians) > Math.PI ? "major" :
        Math.abs(sweepRadians) == Math.PI ? "semicircle" : "minor";

    lines.push(
        `Signed sagitta: ${formatNumber(section.versine)} px`,
        `Sweep: ${formatNumber(Math.abs(sweepRadians) * 180 / Math.PI)}° ${direction} (${arcKind} arc)`,
        `Radius: ${formatNumber(radius)} px`
    );
    return lines.join('\n');
}

function buildDialog(doc) {
    const dlg = Dialog.create("Create Bulged Polyline");
    const column = dlg.addColumn();

    const sectionLength = Math.min(doc.widthPixels * 0.25, 300);
    const startX = doc.widthPixels * 0.5 - sectionLength;
    const startY = doc.heightPixels * 0.5;

    const startGroup = column.addGroup("Start point");
    dlg.startX = startGroup
        .addUnitValueEditor("X", UnitType.Pixel, doc.units, startX)
        .setNoMinValue()
        .setNoMaxValue()
        .setPrecision(2);
    dlg.startY = startGroup
        .addUnitValueEditor("Y", UnitType.Pixel, doc.units, startY)
        .setNoMinValue()
        .setNoMaxValue()
        .setPrecision(2);

    const endGroup = column.addGroup("End point");
    dlg.endpointMode = endGroup.addButtonSet(
        "Endpoint",
        ["To (x, y)", "Relative (dx, dy)"],
        EndpointMode.Absolute
    );
    dlg.argumentX = endGroup
        .addUnitValueEditor("X / dx", UnitType.Pixel, doc.units, startX + sectionLength)
        .setNoMinValue()
        .setNoMaxValue()
        .setPrecision(2);
    dlg.argumentY = endGroup
        .addUnitValueEditor("Y / dy", UnitType.Pixel, doc.units, startY)
        .setNoMinValue()
        .setNoMaxValue()
        .setPrecision(2);

    const sectionGroup = column.addGroup("Section");
    dlg.bulgeValue = sectionGroup
        .addUnitValueEditor("Bulge", UnitType.Number, UnitType.Number, 0, -4, 4)
        .setNoMinValue()
        .setNoMaxValue()
        .setShowPopupSlider(true)
        .setPrecision(4);
    dlg.addSection = sectionGroup.addButton("Add section").setIsFullWidth();
    sectionGroup.addStaticText(
        null,
        "Add section keeps the current section and starts another. OK keeps the current section and creates the polyline."
    ).setIsFullWidth();

    const resultGroup = column.addGroup("Current geometry");
    dlg.readout = resultGroup.addStaticText(null, "").setIsFullWidth();

    dlg.initialWidth = 480;
    return dlg;
}

function main() {
    const doc = Document.current;
    if (!doc) {
        app.alert("Open a document before running Create Bulged Polyline.");
        return;
    }

    const dlg = buildDialog(doc);
    const sections = [];
    let updating = false;
    let previousEndpointMode = dlg.endpointMode.selectedIndex;

    function preserveEndpointAcrossModeChange() {
        const endpointMode = dlg.endpointMode.selectedIndex;
        if (endpointMode == previousEndpointMode)
            return;

        const start = currentStart(dlg, sections);
        if (endpointMode == EndpointMode.Relative) {
            dlg.argumentX.value -= start.x;
            dlg.argumentY.value -= start.y;
        }
        else {
            dlg.argumentX.value += start.x;
            dlg.argumentY.value += start.y;
        }
        previousEndpointMode = endpointMode;
    }

    function update(preview) {
        if (updating)
            return false;
        updating = true;
        try {
            preserveEndpointAcrossModeChange();
            const currentSection = readCurrentSection(dlg, sections);
            dlg.readout.text = describeCurrentSection(currentSection, sections.length);
            doc.addNode(createNodeDefinition(sections.concat(currentSection)), null, undefined, preview);
            return true;
        }
        catch (error) {
            dlg.readout.text = `CurveBuilder rejected this section:\n${String(error)}`;
            doc.clearPreviews();
            return false;
        }
        finally {
            updating = false;
        }
    }

    function addSection() {
        if (updating)
            return;
        updating = true;
        try {
            preserveEndpointAcrossModeChange();
            const section = readCurrentSection(dlg, sections);
            createCurve(sections.concat(section));
            sections.push(section);

            dlg.startX.value = section.end.x;
            dlg.startY.value = section.end.y;
            dlg.startX.isEnabled = false;
            dlg.startY.isEnabled = false;

            if (dlg.endpointMode.selectedIndex == EndpointMode.Absolute) {
                dlg.argumentX.value = section.end.x + section.dx;
                dlg.argumentY.value = section.end.y + section.dy;
            }
            dlg.bulgeValue.value = 0;
        }
        catch (error) {
            app.alert(`Cannot add this section:\n${String(error)}`);
        }
        finally {
            updating = false;
        }
        update(true);
    }

    for (const control of [
        dlg.startX,
        dlg.startY,
        dlg.endpointMode,
        dlg.argumentX,
        dlg.argumentY,
        dlg.bulgeValue,
    ]) {
        control.onValueChangedHandler = () => update(true);
    }
    dlg.addSection.onClickHandler = addSection;

    update(true);
    while (dlg.runModal() == DialogResult.Ok) {
        if (update(false))
            return;
        app.alert("Choose a section that CurveBuilder accepts before creating the polyline.");
    }
    doc.clearPreviews();
}

module.exports.main = main;