How to exclude shapes in svg or canvas? - svg

Question

Forgive my English
I am using svg with filters, but faced the following problem.
This is the base for svg. The result is expected:
<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100" viewBox="0 0 100 100">
<g>
<circle id="2" cx="50" cy="50" r="50"/>
<g id="1">
<rect x="0" y="0" width="50" height="50" fill="#ccc"/>
<rect x="50" y="50" width="50" height="50" fill="#ccc"/>
</g>
</g>
</svg>
But with the filter feComposite:
<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100" viewBox="0 0 100 100">
<defs>
<filter id="myFilter1">
<feImage href="#1" result="1"/>
<feImage href="#2" result="2"/>
<feComposite in="1" in2="2" operator="xor"/>
</filter>
</defs>
<g filter="url(#myFilter1)">
<circle id="2" cx="50" cy="50" r="50"/>
<g id="1">
<rect x="0" y="0" width="50" height="50" fill="#ccc"/>
<rect x="50" y="50" width="50" height="50" fill="#ccc"/>
</g>
</g>
</svg>
As you can see, the image is shifted. If you inspect the code, the blocks will not match the visible image:
Here with the addition of interactivity:
const value = (max = 100000000, min = 0) => Math.round(Math.random() * (max - min)) + min;
const createCircle = (size) => {
const r = value(10, 3);
const cx = value(size - r - 10, r + 10);
const cy = value(size - r - 10, r + 10);
return {
r,
cx,
cy
}
};
const createCircles = (counts, size) => Array(counts).fill().map(() => createCircle(size));
class App extends React.Component {
constructor(props) {
super(props);
this.state = {
position: {
x: 0,
y: 0,
}
};
this.size = 300;
this.circlesData = createCircles(100, this.size);
const getCoords = (c, i) => c + (this.state.position.x * 0.002 * c * (i % 2 ? 1 : -1));
this.circles = () => this.circlesData.map((item, i) => <circle key = {`circles_12dew1_${i}`} cx={getCoords(item.cx, i)} cy={getCoords(item.cy, i)} r={item.r}/>);
}
onMouseMove = e => {
const position = {
x: e.pageX,
y: e.pageY,
};
this.setState({position});
}
render() {
return (
<div className = "App" >
<svg onMouseMove={this.onMouseMove} ref = {elem => this.svg = elem} xmlns = "http://www.w3.org/2000/svg" width = {this.size} height = {this.size} viewBox={`0 0 ${this.size} ${this.size}`}>
<defs>
<filter id="myFilter1">
<feImage href="#1" result="1"/>
<feImage href="#2" result="2"/>
<feComposite in ="1" in2="2" operator="xor"/>
</filter>
</defs>
<g id = "3" filter = "url(#myFilter1)" >
<circle id = "2" cx={this.size / 2 + 100} cy={this.size / 2 + 100} r={this.size / 3}/>
<g id="1"> {this.circles()} </g>
</g>
</svg>
</div>
);
}
}
ReactDOM.render( < App / > , document.getElementById('root'));
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
As you can see, a big circle is moving, but it shouldn't be doing that.
How to fix?
Or how to make the exception of figures as in the interactive example without svg? For example using canvas, thank you!

Answer 1

The image placement behaviour you are seeing is because <feImage> elements are positioned using the filter region, or filter primitive region.
https://www.w3.org/TR/SVG11/single-page.html#filters-feImageElement
By default the filter region is an area that is 10% bigger than the original object on all sides.
x="-10%" y="-10%" width="120%" height="120%"
This is done to cater for filter primitives, such as <feGuassianBlur>, which extend outside the original size and would otherwise get clipped.
To get the images to position how you want, change the filter region, or filter primitive region, to be the same size as the original object. For example:
<feImage href="#1" x="0" y="0" width="100%" height="100%" result="1"/>
Updated demo:
<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100" viewBox="0 0 100 100">
<defs>
<filter id="myFilter1">
<feImage href="#1" x="0" y="0" width="100%" height="100%" result="1"/>
<feImage href="#2" x="0" y="0" width="100%" height="100%" result="2"/>
<feComposite in="1" in2="2" operator="xor"/>
</filter>
</defs>
<g filter="url(#myFilter1)">
<circle id="2" cx="50" cy="50" r="50"/>
<g id="1">
<rect x="0" y="0" width="50" height="50" fill="#ccc"/>
<rect x="50" y="50" width="50" height="50" fill="#ccc"/>
</g>
</g>
</svg>