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This project is part of the @thi.ng/umbrella monorepo.


Declarative canvas scenegraph & visualization for @thi.ng/hdom.

This package provides a re-usable canvas component, which accepts child nodes defining a scene tree of different shape types in standard @thi.ng/hiccup syntax/format (i.e. nested arrays) and then translates these into canvas API draw calls during the hdom update process / cycle.


STABLE - used in production

Related packages


yarn add @thi.ng/hdom-canvas
// ES module
<script type="module" src="https://unpkg.com/@thi.ng/hdom-canvas?module" crossorigin></script>

// UMD
<script src="https://unpkg.com/@thi.ng/hdom-canvas/lib/index.umd.js" crossorigin></script>

Package sizes (gzipped, pre-treeshake): ESM: 3.19 KB / CJS: 3.33 KB / UMD: 3.31 KB


Usage examples

Several demos in this repo's /examples directory are using this package.

A selection:

ScreenshotDescriptionLive demoSource
Interactive inverse FFT toy synthDemoSource
Doodle w/ K-nearest neighbor search result visualizationDemoSource
Poisson-disk shape-aware sampling, Voronoi & Minimum Spanning Tree visualizationDemoSource
Realtime analog clock demoDemoSource
Interactive pattern drawing demo using transducersDemoSource
2D Bezier curve-guided particle systemDemoSource
Various hdom-canvas shape drawing examples & SVG conversion / exportDemoSource
Canvas based Immediate Mode GUI componentsDemoSource
Animated sine plasma effect visualized using contour linesDemoSource
Basic rstream-gestures multi-touch demoDemoSource
Unison wavetable synth with waveform editorDemoSource
Animated Voronoi diagram, cubic splines & SVG downloadDemoSource
2D scenegraph & shape pickingDemoSource
2D scenegraph & image map based geometry manipulationDemoSource
Fork-join worker-based raymarch rendererDemoSource


Generated API docs

import { start } from "@thi.ng/hdom";
import { canvas } from "@thi.ng/hdom-canvas";

start(() => {
    const t = Date.now() * 0.001;
    return [canvas, { width: 100, height: 100 },
        ["circle", { fill: "red", stroke: "black" }, [50, 50], 25 + 25 * Math.sin(t)]

Usage with @thi.ng/geom shape primitives:

import { start } from "@thi.ng/hdom";
import { canvas } from "@thi.ng/hdom-canvas";
import * as g from "@thi.ng/geom";

start(() => {
    const t = Date.now() * 0.001;
    return [canvas, { width: 100, height: 100 },
                    g.asPolygon(g.circle(50), 6),
                    { rotate: t % Math.PI, stroke: "red" }
                g.star(25 + 25 * Math.sin(t), 6, [0.5, 1], { stroke: "blue" }),
            { translate: [50, 50], fill: "none" }

How it works

The package provides a canvas component which uses the branch-local behavior implementation feature of @thi.ng/hdom v5.0.0 to support virtual SVG-like shape elements / components. These are defined as part of the main UI component tree just like any other component, but are then translated into canvas API draw commands during the hdom update process. Any embedded shape component functions receive the user context object as first arg, just like normal hdom components.

Shape components are expressed in standard hiccup syntax (or as objects implementing the IToHiccup() interface, like the shape types provided by @thi.ng/geom), and with the following...

Restrictions & behavior controls

  • Shape component objects with life cycle methods are only partially supported, i.e. only the render & release methods are used.
  • For performance reasons release methods are disabled by default. If your shape tree contains stateful components which use the release life cycle method, you'll need to explicitly enable the canvas component's __release control attribute by setting it to true.
  • Currently no event listeners can be assigned to shapes (ignored), though this is planned for a future version. The canvas element itself can of course have event handlers as usual.

For best performance it's recommended to ensure all resulting shapes elements are provided in already normalized hiccup format, i.e.

[tag, {attribs}, ...] // or
[tag, null, ...]

That way the __normalize: false control attribute can be added either to the canvas component itself (or to individual shapes / groups), and if present, will skip normalization of that element's children.

Likewise, for animated scenes, the __diff control attribute should be set to false to skip unnecessary diffing and force redraws.

To disable the automatic background clearing of the canvas, set the __clear attribute to false.

[canvas, { width: 100, height: 100, __clear: false }, ...]

HDPI support

The canvas component automatically adjusts its size for HDPI displays by adding CSS width & height properties and pre-scaling the drawing context accordingly before any shapes are processed. For fullscreen canvases simply set the width & height attribs to:

        width: window.innerWidth,
        height: window.innerHeight
    // shapes

SVG conversion

Even though the element names & syntax are very similar to SVG elements, for performance reasons all geometry data given to each shape remains un-stringified (only styling attributes are). However, the @thi.ng/hiccup-svg package provides a convertTree() function which takes the arguably more "raw" shape format used by hdom-canvas and converts an entire shape tree into SVG compatible & serializable format. Note: the tree MUST first be normalized (if not already) using hdom-canvas' normalizeTree().

import { serialize } from "@thi.ng/hiccup";
import { convertTree, svg } from "@thi.ng/hiccup-svg";
import { normalizeTree } from "@thi.ng/hdom-canvas";

    svg({ width: 100, height: 100},
                {}, // default normalization options
                        fill: "red",
                        stroke: "none",
                        translate: [50, 50]
                    ["circle", {}, [0, 0], 25],
                    ["polygon", { fill: "white" },
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="100" height="100">
    <g transform="translate(50.00 50.00)" fill="red" stroke="none">
        <circle cx="0.00" cy="0.00" r="25.00"/>
        <polygon points="-10.00,10.00 10.00,10.00 0.00,-10.00" fill="white"/>

Supported shape types

In the near future, factory functions for these shape types will be provided...


["g", attribs, child1, child2, ...]

Attributes defined at group level are inherited by child elements.

Definition group

["defs", {}, def1, def2, ...]

Special group / container for gradient definitions. If used, should always come first in a scene tree.


["circle", attribs, [x, y], radius]

Circular arc

["arc", attribs, [x, y], radius, startAngle, endAngle, anticlockwise?]

Only circular arcs are supported in this format. Please see note about differences to SVG.

Ellipse / elliptic arc

["ellipse", attribs, [x, y], [rx, ry], axisTheta?, start?, end?, ccw?]


["rect", attribs, [x, y], w, h, radius?]

If radius is given, creates a rounded rectangle. radius will be clamped to Math.min(w, h)/2.


["line", attribs, [x1, y1], [x2, y2]]

Horizontal Line

["hline", attribs, y]

Vertical Line

["vline", attribs, x]

Polyline / Polygon

["polyline", attribs, [[x1, y1], [x2, y2], [x3, y3]...]]

Always non-filled (even if fill attrib is given or inherited)

["polygon", attribs, [[x1, y1], [x2, y2], [x3, y3]...]]

Always closed, can be filled and/or stroked.


["path", attribs, [seg1, seg2, ...]]

Path segments are tuples of [type, [x,y]...]. The following segment types are supported and (as with SVG), absolute and relative versions can be used. Relative versions use lowercase letters and are always relative to the end point of the previous segment. The first segment (usually of type "M") must be absolute.

["M", [x, y]]Move
["L", [x, y]]Line
["H", x]Horizontal line
["V", y]Vertical line
["C", [x1,y1], [x2, y2], [x3, y3]]Cubic / bezier curve
["Q", [x1,y1], [x2, y2]]Quadratic curve
["A", [x1,y1], [x2, y2], r]Circular arc (see below)
["Z"]Close (sub)path

SVG paths with arc segments

IMPORTANT: Currently, due to differences between SVG and canvas API arc handling, SVG paths containing arc segments are NOT compatible with the above format. This issue is being worked on, but in the meantime, to use such paths, these should first be converted to use cubics or polygon / polyline. E.g. here using @thi.ng/geom:

import { normalizedPath, pathFromSVG, asPolyline } from "@thi.ng/geom";

// path w/ arc segments (*not* usable by hdom-canvas)
const a = pathFromSvg("M0,0H80A20,20,0,0,1,100,20V30A20,20,0,0,1,80,50")[0];

// normalized to only use cubic curves (usable by hdom-canvas)
const b = normalizedPath(a);

// converted to polyline (usable by hdom-canvas)
const c = asPolyline(a);

// <path d="M0.00,0.00C26.67,0.00,53.33,0.00,80.00,0.00C..."/>

// <polyline fill="none" points="0.00,0.00 80.00,0.00 81.57,0.06..."/>


["points", attribs, [[x1,y1], [x2,y2],...]]

The following shape specific attributes are used:

  • shape: circle or rect (default)
  • size: point size (radius for circles, width for rects) - default: 1

Packed points

Similar to points, but uses a single packed buffer for all point coordinates.

["packedPoints", attribs, [x1,y1, x2,y2,...]]

Optional start index, number of points, component & point stride lengths (number of indices between each vector component and each point respectively) can be given as attributes.


  • start index: 0
  • number of points: (array_length - start) / estride
  • component stride: 1
  • element stride: 2
["packedPoints", { cstride: 1, estride: 4 },
    [x1, y1, 0, 0, x2, y2, 0, 0, ...]]

["packedPoints", { offset: 8, num: 3, cstride: 4, estride: 1 },
    [0, 0, 0, 0, 0, 0, 0, 0, x1, x2, x3, 0, y1, y2, y3, 0...]]


["text", attribs, [x,y], "body...", maxWidth?]


["img", { width?, height? }, img, dpos, spos?, ssize?]

IMPORTANT: Since v2.0.0 this element has new/changed args...

img MUST be an HTML image, canvas or video element. dpos, spos, ssize are 2D vectors. The latter two are optional, as are width and height attribs. Defaults:

  • width - original image width
  • height - original image height
  • spos - [0,0]
  • ssize - [width, height]

Note: For SVG conversion spos & ssize will be ignored. Sub-image blitting is not supported in SVG.


Gradients MUST be defined within a root-level defs group, which itself MUST be given prior to any other shapes. Use the $ prefix to refer to a gradient in a fill or stroke attribute, e.g. {stroke: "$foo" }

    {id: "foo", from: [x1,y1], to: [x2, y2]},
    [[offset1, color1], [offset2, color2], ...]
    {id: "foo", from: [x1,y1], to: [x2, y2], r1: r1, r2: r2 },
    [[offset1, color1], [offset2, color2], ...]


Some attributes use different names than their actual names in the CanvasRenderingContext2D:

AttributeContext 2D property

Color attributes

Color conversions are only applied to fill, stroke, shadowColor attributes and color stops provided to gradient definitions.


String color attribs prefixed with $ are replaced with url(#...) refs (e.g. to refer to gradients), else used as is (untransformed)


Interpreted as ARGB hex value:

{ fill: 0xffaabbcc } => { fill: "#aabbcc" }


Interpreted as float RGB(A):

{ fill: [1, 0.8, 0.6, 0.4] } => { fill: "rgba(255,204,153,0.40)" }

@thi.ng/color values

Colors defined via the @thi.ng/color package can be automatically converted to CSS color strings:

{ fill: hcya(0.1666, 1, 0.8859) } => { fill: "#ffff00" }

Coordinate transformations

Coordinate system transformations can be achieved via the following attributes (for groups and individual shapes). Nested transformations are supported.

If using a combination of translate, scale and/or rotate attribs, the order of application is always TRS.

Transform matrix

{ transform: [xx, xy, yx, yy, ox, oy] }

Override transform

{ setTransform: [xx, xy, yx, yy, ox, oy] }

Similar to transform but completely overrides transformation matrix, rather than concatenating with existing one.

See MDN docs for further details.

Also see the 2x3 matrix functions in the @thi.ng/matrices package for creating different kinds of transformation matrices, e.g.

{ transform: skewX23([], Math.PI / 12) }


{ translate: [x, y] }


{ scale: [x, y] } // non-uniform
{ scale: x } // uniform


{ rotate: theta } // in radians





© 2018 - 2020 Karsten Schmidt // Apache Software License 2.0

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