javascript - 如何使用 Canvas 动画绘制曲线？

Question

我有一堆要点想慢慢画。我尝试 setTimeOut 和这个 tutorial 的效果.但没有那么成功。

函数看起来像这样

函数:

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();
            }

围绕这个函数还有很多其他的绘图函数，但我只想制作一个动画。

如何用canvas慢慢画一个函数？

Answer 1

我想到了两种方法来完成此操作。一个基本上是绘制一个点并在绘制另一个点之前暂停一定时间。这是我提供的第一个示例。第二种方法涉及绘制部分线到当前目标，这提供了更平滑的绘图外观。作为旁注，我使用 requestAnimationFrame在这两个示例中，它都是执行任何类型的 Canvas 动画的推荐方法。

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

如果您注意到它有点不连贯，您可以执行以下操作以使绘制的线条更平滑

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