HTML5 canvas Custom Shape Tutorial

To create a custom shape with Konva, we can instantiate a Konva.Shape() object.

When creating a custom shape, we need to define a drawing function that is passed a Konva.Context renderer and a shape instance:

var rect = new Konva.Shape({
x: 10,
y: 20,
fill: '#00D2FF',
width: 100,
height: 50,
sceneFunc: function (context, shape) {
context.beginPath();
// don't need to set position of rect, Konva will handle it
context.rect(0, 0, shape.getAttr('width'), shape.getAttr('height'));
// (!) Konva specific method, it is very important
// it will apply are required styles
context.fillStrokeShape(shape);
}
});

Konva.Context is a wrapper around native 2d canvas context that have the same properties and methods with some additional API.

We can use the renderer to access the HTML5 Canvas context, and to use special methods like context.fillStrokeShape(shape) which automatically handles filling, stroking, and applying shadows.

There are two properties that can be used for drawing custom shapes: sceneFunc and hitFunc.

sceneFunc should be used to define visual appearance of a shape. Konva will use sceneFunc for drawing its hit graph for events detecting. So in many cases you just need to define sceneFunc only. If you want to overwrite hit testing, take a look into Custom Hit Region demo.

Some best practices for writing sceneFunc and hitFunc:

  1. Make it as optimal, as possible because that function can be called many times per second. It is not ok to create images (document.createElement('image') or new window.Image()) or other large objects here.

  2. The function should not have any side effects like moving shapes, attaching events or changing state of your app.

  3. If you want to apply complex styles to the canvas manually or draw images, remember to define custom hitFunc.

  4. Do not apply position and scaling in sceneFunc manually. Konva can handle it automatically if you set that properties into shape instance directly. Example: shape.x(10).

  5. If possible do not apply styles in sceneFunc manually. Just draw a shape with some paths. context.fillStrokeShape(shape) function at the and will do all styling work.

  6. If you need more demos take a look into Konva core shapes implementations in the repo.

For a full list of attributes and methods, check out the Konva.Shape documentation

Konva Custom Demoview raw
<!DOCTYPE html>
<html>

<head>
<script src="https://unpkg.com/[email protected]/konva.min.js"></script>
<meta charset="utf-8">
<title>Konva Custom Shape Demo</title>
<style>
body {
margin: 0;
padding: 0;
overflow: hidden;
background-color: #F0F0F0;
}
</style>
</head>

<body>
<div id="container"></div>
<script>
var stage = new Konva.Stage({
container: 'container',
width: window.innerWidth,
height: window.innerHeight
});

var layer = new Konva.Layer();

/*
* create a triangle shape by defining a
* drawing function which draws a triangle
*/
var triangle = new Konva.Shape({
sceneFunc: function (context, shape) {
context.beginPath();
context.moveTo(20, 50);
context.lineTo(220, 80);
context.quadraticCurveTo(150, 100, 260, 170);
context.closePath();

// (!) Konva specific method, it is very important
context.fillStrokeShape(shape);
},
fill: '#00D2FF',
stroke: 'black',
strokeWidth: 4
});

// add the triangle shape to the layer
layer.add(triangle);

// add the layer to the stage
stage.add(layer);
</script>

</body>

</html>