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About

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This package provides a simple draw() function, which accepts a scene tree of different shape types in @thi.ng/hiccup syntax/format (i.e. nested arrays, IToHiccup implementations) and then translates these into canvas API draw calls.

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Installation

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Dependencies

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API

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The shape tree given to draw() MUST consist of well-formed, normalized hiccup syntax (incl. objects implementing the IToHiccup() interface, like the shape types provided by @thi.ng/geom).

SVG conversion

Even though the shape element names & syntax are intentionally very similar (largely the same) to SVG elements, for performance reasons all geometry data given to each shape remains un-stringified (only styling attributes can be strings). However, the @thi.ng/hiccup-svg package provides a convertTree() function which takes the arguably more "raw" shape format used by this package and converts an entire shape tree into SVG compatible & serializable format.

It's very likely (and recommended) you're using the shape type provided @thi.ng/geom, in which case these can be provided as-is to this package's draw() function and SVG conversion (from the same geometry) can be done like so:

import { asSvg, svgDoc, group, circle } from "@thi.ng/geom";
import { canvas2d } from "@thi.ng/canvas";
import { draw } from "@thi.ng/hiccup-canvas";

const dots = group({}, [
    circle([100, 100], 20, { fill: "red" }),
    circle([140, 100], 20, { fill: "green" }),
    circle([160, 100], 20, { fill: "blue" }),
]);

const { ctx } = canvas2d(200, 200, document.body);

// draw geometry group to canvas
draw(ctx, dots);

// convert to SVG
// (unless given, width, height and viewBox will be auto-computed)
asSvg(svgDoc({}, dots))

Supported shape types

Group

["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 / circular arc

["circle", attribs, [x, y], radius, startTheta?, endTheta?, ccw?]

Angles in radians. Please see note about SVG support.

Ellipse / elliptic arc

["ellipse", attribs, [x, y], [rx, ry], axisTheta?, startTheta?, endTheta?, ccw?]

Angles in radians. Please see note about SVG support.

Rect

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

If radii is given, creates a rounded rectangle. See Canvas API roundRect() for possible radius values.

Line

["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

["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.

Format Description
["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", rx, ry, theta, large-arc-flag, clockwise, [x,y]] Elliptic arc (SVG compatible, see below)
["R", [x1,y1], [x2, y2], r] Circular arc (not SVG compatible, see below)
["Z"] Close (sub)path

Important

Prior to v3.0.0, only circular arc segments were supported and used the A/a identifier, which actually should have been reserved for SVG-style elliptic arcs (as is the case now). In the unlikely event you've been using paths with circular arc segments, you'll need to update these to use R/r segment types instead.

SVG paths with arc segments

Reference about the params for arc segments:

Since v3.0.0 this package supports both circular and elliptic arc path segments, however only the latter segment type is compatible with SVG (circular arcs are only supported by the HTML Canvas API). We recommended to use one of the available path constructor functions in @thi.ng/geom to create individual arcs or paths which ensure SVG compatibility:

Arcs:

Paths:

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

// path w/ elliptic arc segments (for 2 of the corners)
const a = roundedRect([0, 0], [100, 100], [0, 40]);

console.log(asSvg(a));
// <path d="M0,0H60A40,40,0,0,1,100,40V100H40A40,40,0,0,1,0,60.000V0z"/>

// normalize path to only use cubic curves
const b = normalizedPath(a);

console.log(asSvg(b));
// <path d="M0,0C20,0,40,0,60,0C82.091,0,100,17.909,100,40C100,60,100,80,100,100C80,100,60,100,40,100C17.909,100,0.000,82.091,0,60.000C0,40,0,20,0,0z"/>

// convert/sample path as polyline
// (some paths have multiple boundaries, here we only want the first)
const c = asPolyline(a, { dist: 20 })[0];

console.log(asSvg(c));
// <polyline fill="none" points="0,0 20,0 40,0 60,0 79.168,4.924 93.644,18.410 99.889,37.186 100,40 100,60 100,80 100,100 80,100 60,100 40,100 20.832,95.076 6.356,81.590 0.111,62.814 0,60 0,40 0,20 0,0"/>

Points

["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

Text

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

Image

["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

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" }

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

Using flat, packed vertex buffers

The points, polyline and polygon shape types also provide alternative versions, each allowing the use of a single packed buffer (e.g. typed array) for all point coordinates instead of individual arrays/views per vertex. This much simplifies & speeds up WASM interop usecases, which can now skip creating vector views of a vertexbuffer memory region.

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

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

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

Optional start index, number of points, component & element stride lengths (i.e. the number of indices between each vector x/y component and/or each point respectively) can be given as attributes and thus these packaged shapes support both AOS and SOA memory layouts/arrangements.

Options & defaults:

  • start: start index = 0
  • num: number of vertices = (array_length - start) / estride
  • cstride: component stride = 1
  • estride: 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...]]

Attributes

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

Attribute Context 2D property
align textAlign
alpha globalAlpha
baseline textBaseline
compose globalCompositeOperation
dash setLineDash
dashOffset lineDashOffset
direction direction
fill fillStyle
filter filter
font font
lineCap lineCap
lineJoin lineJoin
miterLimit miterLimit
shadowBlur shadowBlur
shadowColor shadowColor
shadowX shadowOffsetX
shadowY shadowOffsetY
smooth imageSmoothingEnabled
stroke strokeStyle
weight lineWidth

Color attributes

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

String

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

Number

Interpreted as ARGB hex value:

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

Array

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) }

Translation

{ translate: [x, y] }

Scaling

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

Rotation

{ rotate: theta } // in radians

Special attributes

Background fill

The special __background attribute can be used to fill the entire canvas with a given background color. The attribute only makes sense if attached to the root group/shape and can take the same values as any other color attribs.

Background clear

The special __clear boolean attribute is used to force clearing of the canvas before drawing. This attrib takes priority over __background and it too only should be attached to the root group/shape. By default the canvas is not being cleared.