HTML5 Canvas Text Animating around circle

问题:

Updated what is wrong with the code? I know it doesnt rotate but why is the text screwy.

Does anyone know why I am tearing my hair out trying to figure this out

function showCircularNameRotating(string, startAngle, endAngle){
    //context.clearRect(0, 0, canvas.width, canvas.height);
    context.font = '32pt Sans-Serif';
    context.fillStyle = '#1826B0';
    circle = {
        x: canvas.width/2,
        y: canvas.height/2,
        radius: 200
    };

    var radius = circle.radius,
        angleDecrement = (startAngle - endAngle/string.length-1),
        angle = parseFloat(startAngle),
        index = 0,
        character;

    context.save();
    while(index <string.length){
    character = string.charAt(index);
    context.save();
    context.beginPath();
    context.translate(circle.x + Math.cos(angle) * radius,
                      circle.y - Math.sin(angle) * radius);
    context.rotate(Math.PI/2 - angle);

    context.fillText(character, 0,0);
    context.strokeText(character,0,0);
    angle -= angleDecrement;
    index++;
    context.restore();
    }
    context.restore();

    }

回答1:

Yes, it's possible.

Here is a simple approach which you can build upon (I made it right now so it can certainly be optimized and tweaked in various ways).

• This uses two objects, one for the text itself and one for each char.
• The string is split into char objects in the text object's constructor
• The canvas is rotated
• The chars are each drawn relative to each other in a circular pattern

Live demo

Text object

function Text(ctx, cx, cy, txt, font, radius) {

    this.radius = radius;               // expose so we can alter it live

    ctx.textBaseline = 'bottom';        // use base of char for rotation
    ctx.textAlign = 'center';           // center char around pivot
    ctx.font = font;

    var charsSplit = txt.split(''),     // split string to chars
        chars = [],                     // holds Char objects (see below)
        scale = 0.01,                   // scales the space between the chars
        step = 0.05,                    // speed in steps
        i = 0, ch;

    for(; ch = charsSplit[i++];)       // create Char objects for each char
        chars.push(new Char(ctx, ch));

    // render the chars
    this.render = function() {

        var i = 0, ch, w = 0;

        ctx.translate(cx, cy);         // rotate the canvas creates the movement
        ctx.rotate(-step);
        ctx.translate(-cx, -cy);

        for(; ch = chars[i++];) {      // calc each char's position
            ch.x = cx + this.radius * Math.cos(w);
            ch.y = cy + this.radius * Math.sin(w);

            ctx.save();                // locally rotate the char
            ctx.translate(ch.x, ch.y);
            ctx.rotate(w + 0.5 * Math.PI);
            ctx.translate(-ch.x, -ch.y);
            ctx.fillText(ch.char, ch.x, ch.y);
            ctx.restore();

            w += ch.width * scale;
        }
    };
}

The Char object

function Char(ctx, ch) {
    this.char = ch;                    // current char
    this.width = ctx.measureText('W').width;  // width of char or widest char
    this.x = 0;                        // logistics
    this.y = 0;
}

Now all we need to do is to create a Text object and then call the render method in a loop:

var text = new Text(ctx, cx, cy, 'CIRCULAR TEXT', '32px sans-serif', 170);

(function loop() {
    ctx.clearRect(0, 0, w, h);
    text.render();
    requestAnimationFrame(loop);
})();

As said, there is plenty of room for optimizations here. The most expensive parts are:

• Text rendering (render text to images first)
• Local rotation for each char using save/restore
• Minor things

I'll leave that as an exercise for OP though :)

回答2:

In Canvas, not so sure. But would be trivial in SVG if you can use that?

• Place the text along a path representing the circle.
• Use an animateTransform to spin the path