How to animate the drawing of a Curve with Canvas?

Question

i have a bunch of points that i wanted to slowly draw. I try setTimeOut and the effect from this tutorial. But without so much success.

The Function looks like this

Function:

var myFunction = function(ctx, grid, points) {
                ctx.beginPath();
                ctx.moveTo(points[0].x, points[0].y);
                ctx.lineWidth = 2;
                ctx.strokeStyle = '#2068A8';
                ctx.fillStyle = '#2068A8';
                var count = 1;
                for (count = 1; count < points.length; count++) {
                    ctx.lineTo(points[count].x , points[count].y);
                }
                ctx.stroke();
            }

Around this function there a many other drawing functions but i just want to animate only one.

How can i slowly draw a function with canvas?

Answer 1

There's two ways you can go about doing this that I can think of off the top of my head. One is basically drawing a point and pausing for a certain amount of time before drawing the other point. That's the first example I provided.The second method involves drawing partial lines to the current target, which gives a much smoother drawing appearance. As a side note I use requestAnimationFrame in both examples, its the recommended way to do any types of canvas animations.

var canvas = document.getElementById("canvas"),
    ctx = canvas.getContext("2d");

canvas.width = 400;
canvas.height = 200;

var points = [],
    currentPoint = 1,
    nextTime = new Date().getTime()+500,
    pace = 200;

// make some points
for (var i = 0; i < 50; i++) {
    points.push({
        x: i * (canvas.width/50),
        y: 100+Math.sin(i) * 10
    });
}

function draw() {

    if(new Date().getTime() > nextTime){
        nextTime = new Date().getTime() + pace;

        currentPoint++;
        if(currentPoint > points.length){
            currentPoint = 0;
        }
    }
    ctx.clearRect(0,0,canvas.width, canvas.height);
    ctx.beginPath();
    ctx.moveTo(points[0].x, points[0].y);
    ctx.lineWidth = 2;
    ctx.strokeStyle = '#2068A8';
    ctx.fillStyle = '#2068A8';
    for (var p = 1, plen = currentPoint; p < plen; p++) {
        ctx.lineTo(points[p].x, points[p].y);
    }
    ctx.stroke();

    requestAnimFrame(draw);
}

draw();

Live Demo

If you notice that is a bit choppy, you can do the following to get smoother lines being drawn

var canvas = document.getElementById("canvas"),
    ctx = canvas.getContext("2d");

canvas.width = 400;
canvas.height = 200;

var points = [],
    currentPoint = 1,
    speed = 2,
    targetX = 0,
    targetY = 0,
    x = 0,
    y = 0;

// make some points
for (var i = 0; i < 50; i++) {
    points.push({
        x: i * (canvas.width/50),
        y: 100+Math.sin(i) * 10
    });
}

// set the initial target and starting point
targetX = points[1].x;
targetY = points[1].y;
x = points[0].x;
y = points[0].y;

function draw() {
    var tx = targetX - x,
        ty = targetY - y,
        dist = Math.sqrt(tx*tx+ty*ty),
        velX = (tx/dist)*speed,
        velY = (ty/dist)*speed;

        x += velX
        y += velY;

    if(dist < 1){
        currentPoint++;

        if(currentPoint >= points.length){
            currentPoint = 1;
            x = points[0].x;
            y = points[0].y;
        }

        targetX = points[currentPoint].x;
        targetY = points[currentPoint].y;
    }

    ctx.clearRect(0,0,canvas.width, canvas.height);
    ctx.beginPath();
    ctx.moveTo(points[0].x, points[0].y);
    ctx.lineWidth = 2;
    ctx.strokeStyle = '#2068A8';
    ctx.fillStyle = '#2068A8';

    for (var p = 0, plen = currentPoint-1; p < plen; p++) {
        ctx.lineTo(points[p].x, points[p].y);
    }
    ctx.lineTo(x, y);    
    ctx.stroke();

    requestAnimFrame(draw);
}

draw();

Live Demo