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How to animate and erase an arc in HTML5

Here is my question: I have an animation that builds a circle. See: http://jsfiddle.net/2TUnE/

JavaScript:

var currentEndAngle = 0
var currentStartAngle = 0;
var currentColor = 'black';

setInterval(draw, 50);


function draw() { /***************/

    var can = document.getElementById('canvas1'); // GET LE CANVAS
    var canvas = document.getElementById("canvas1");
    var context = canvas.getContext("2d");
    var x = canvas.width / 2;
    var y = canvas.height / 2;
    var radius = 75;

    var startAngle = currentStartAngle * Math.PI;
    var endAngle = (currentEndAngle) * Math.PI;

    currentEndAngle = currentEndAngle + 0.01;

    var counterClockwise = false;

    context.beginPath();
    context.arc(x, y, radius, startAngle, endAngle, counterClockwise);
    context.lineWidth = 15;
    // line color
    context.strokeStyle = currentColor;
    context.stroke();

    /************************************************/
}

When the circle is completely drawn, I would like it to start to fade, just as it was created (it removes black so slowly). As soon as the whole circle is erased, I will again create a black circle, creating the effect of "waiting / loading".

What I tried to do is check if currentEndAngle is 2 (so the circle is complete), then move startAngle, but it doesn't work.

Any idea?

Thank!

EDIT: Forgot to say the animation will be above the image, so it should be "transparent", not white.

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2 answers

, JSFiddle: http://jsfiddle.net/fNTsA/

- , modulo . , 2, , 2. (0..2/2)% 2 == 0, (2..4/2)% 2 == 1 ..

, , , , , , , . , , ( , ). - .

, :

var lineRadius = 75;
var lineWidth = 15;

, :

currentStartAngle = currentEndAngle - 0.01;
currentEndAngle = currentEndAngle + 0.01;

if (Math.floor(currentStartAngle / 2) % 2) {
  currentColor = "white";
  radius = lineRadius - 1;
  width = lineWidth + 3;
} else {
  currentColor = "black";
  radius = lineRadius;
  width = lineWidth;
}
+4

! ( ). , .

    // Moved these to global scope as you don't want to re-declare 
    // them in your draw method each time your animation loop runs
    var canvas = document.getElementById("canvas1");
    var context = canvas.getContext("2d");
    var x = canvas.width / 2;
    var y = canvas.height / 2;
    var radius = 75;

    // Use objects to hold our draw and erase props
    var drawProps = {
      startAngle: 0,
      speed: 2,
      color: 'black',
      counterClockwise: false,
      globalCompositeOperation: context.globalCompositeOperation,
      lineWidth: 15            
    };

    var eraseProps = {
      startAngle: 360,
      speed: -2,
      color: 'white',
      counterClockwise: true,
      globalCompositeOperation: "destination-out",
      lineWidth: 17 // artefacts appear unless we increase lineWidth for erase
    };

    // Let work in degrees as they're easier for humans to understand
    var degrees = 0; 
    var props = drawProps;

    // start the animation loop
    setInterval(draw, 50);

    function draw() { /***************/

        degrees += props.speed;

        context.beginPath();
        context.arc(
            x, 
            y, 
            radius, 
            getRadians(props.startAngle), 
            getRadians(degrees), 
            props.counterClockwise
        );
        context.lineWidth = props.lineWidth;
        context.strokeStyle = props.color;
        context.stroke();

        // Start erasing when we hit 360 degrees
        if (degrees >= 360) {
            context.closePath();
            props = eraseProps;
            context.globalCompositeOperation = props.globalCompositeOperation;
        }

        // Start drawing again when we get back to 0 degrees
        if (degrees <= 0) {
            canvas.width = canvas.width; // Clear the canvas for better performance (I think)
            context.closePath();
            props = drawProps;
            context.globalCompositeOperation = props.globalCompositeOperation;
        }
        /************************************************/
    }  

    // Helper method to convert degrees to radians
    function getRadians(degrees) {
        return degrees * (Math.PI / 180);
    }
+1

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