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

• About
  • Status
• Installation
• Dependencies
• Usage examples
• How it works
  • Restrictions & behavior controls
  • HDPI support
• SVG conversion
• Supported shape types
  • Group
  • Definition group
  • Circle
  • Rect
  • Line
  • Polyline / Polygon
  • Path
  • Points
  • Text
  • Image
  • Gradients
• Attributes
• Coordinate transformations
  • Transform matrix
  • Translation
  • Scaling
  • Rotation
• Authors
• License

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.

BETA - in active development, possibly breaking changes ahead...

yarn add @thi.ng/hdom-canvas

• @thi.ng/api
• @thi.ng/checks
• @thi.ng/diff
• @thi.ng/hdom

Please see these example projects for reference:

• hdom-canvas-clock
• hdom-canvas-shapes

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)]
    ];
});

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, however with the following...

• 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 }, ...]

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:

[canvas,
    {
        width: window.innerWidth,
        height: window.innerHeight
    },
    // shapes
    ...
]

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

serialize(
    svg({ width: 100, height: 100},
        convertTree(
            normalizeTree(
                {}, // default normalization options
                ["g",
                    {
                        fill: "red",
                        stroke: "none",
                        translate: [50, 50]
                    },
                    ["circle", {}, [0, 0], 25],
                    ["polygon", { fill: "white" },
                        [[-10,10],[10,10],[0,-10]]
                    ]
                ]
            )
        )
    )
);

<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"/>
    </g>
</svg>

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.

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

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

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

["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]]

["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.

["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", attribs, [x,y], "body..."]

["img", attribs, [x, y], img]

img MUST be an HTML image, canvas or video element.

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], ...]
]

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

Coordinate system transformations can be achieved via the following attributes. Nested transformations are supported.

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

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

See MDN docs for further details.

Also see the Mat23 type in the @thi.ng/vectors package for creating different kinds of transformation matrices, e.g.

{ transform: Mat23.skewX(Math.PI / 12) }

{ translate: [x, y] }

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

{ rotate: theta } // in radians

• Karsten Schmidt

© 2018 Karsten Schmidt // Apache Software License 2.0