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How to use html5 canvas to achieve uniform motion

一个新手Original: 2017-10-09 10:19:352092browse

Uniform motion: refers to an object moving in a straight line, and the displacement of the object in any equal time interval is equal. In fact, it is uniform linear motion. Its characteristic is that the acceleration is 0. From the definition, it can be seen that within any equal time interval, the speed magnitude and direction are the same.

<head>
    <meta charset=&#39;utf-8&#39; />
    <style>
        #canvas {
            border: 1px dashed #aaa;
        }
    </style>
    <script>
        window.onload = function () {
            var oCanvas = document.querySelector("#canvas"),
                oGc = oCanvas.getContext(&#39;2d&#39;),
                width = oCanvas.width, height = oCanvas.height,
                x = 0;
            function drawBall( x, y, cxt ){
                cxt.fillStyle = &#39;#09f&#39;;
                cxt.beginPath();
                cxt.arc( x, y, 20, 0, 2 * Math.PI );
                cxt.closePath();
                cxt.fill();
            }
            ( function linear(){
                oGc.clearRect( 0, 0, width, height );
                drawBall( x, height / 2, oGc );
                x += 2;
                console.log( x );
                requestAnimationFrame( linear );
            } )();
        }
    </script>
</head>
<body>
    <canvas id="canvas" width="1200" height="600"></canvas>
</body>

The above example allows a small ball with a radius of 20px to move uniformly to the right at a speed of 2px per frame from x=0, y=half the height of the canvas.

We can encapsulate the ball into an object:

ball.js file:

function Ball( x, y, r, color ){
    this.x = x || 0;
    this.y = y || 0;
    this.radius = r || 20;
    this.color = color || &#39;#09f&#39;;
}
Ball.prototype = {
    constructor : Ball,
    stroke : function( cxt ){
        cxt.strokeStyle = this.color;
        cxt.beginPath();
        cxt.arc( this.x, this.y, this.radius, 0, 2 * Math.PI );
        cxt.closePath();
        cxt.stroke();
    },
    fill : function( cxt ){
        cxt.fillStyle = this.color;
        cxt.beginPath();
        cxt.arc( this.x, this.y, this.radius, 0, 2 * Math.PI );
        cxt.closePath();
        cxt.fill();
    }
}

The ball object can customize the position radius and color. Supports two rendering methods (stroke and fill)

<head>
    <meta charset=&#39;utf-8&#39; />
    <style>
        #canvas {
            border: 1px dashed #aaa;
        }
    </style>
    <script src="./ball.js"></script>
    <script>
        window.onload = function () {
            var oCanvas = document.querySelector("#canvas"),
                oGc = oCanvas.getContext(&#39;2d&#39;),
                width = oCanvas.width, height = oCanvas.height,
                ball = new Ball( 0, height / 2 );
            (function linear() {
                oGc.clearRect(0, 0, width, height);
                ball.fill( oGc );
                ball.x += 2;
                requestAnimationFrame(linear);
            })();
        }
    </script>
</head>

<body>
    <canvas id="canvas" width="1200" height="600"></canvas>
</body>

Uniform slash movement:

<head>
    <meta charset=&#39;utf-8&#39; />
    <style>
        #canvas {
            border: 1px dashed #aaa;
        }
    </style>
    <script src="./ball.js"></script>
    <script>
        window.onload = function () {
            var oCanvas = document.querySelector("#canvas"),
                oGc = oCanvas.getContext(&#39;2d&#39;),
                width = oCanvas.width, height = oCanvas.height,
                ball = new Ball( 0, height );
            (function linear() {
                oGc.clearRect(0, 0, width, height);
                ball.fill( oGc );
                ball.x += 2;
                ball.y -= 1;
                requestAnimationFrame(linear);
            })();
        }
    </script>
</head>

<body>
    <canvas id="canvas" width="1200" height="600"></canvas>
</body>

Uniform motion in any direction (velocity decomposition)

<head>
    <meta charset=&#39;utf-8&#39; />
    <style>
        #canvas {
            border: 1px dashed #aaa;
        }
    </style>
    <script src="./ball.js"></script>
    <script>
        window.onload = function () {
            var oCanvas = document.querySelector("#canvas"),
                oGc = oCanvas.getContext(&#39;2d&#39;),
                width = oCanvas.width, height = oCanvas.height,
                ball = new Ball( 0, 0 ),
                speed = 5,
                vx = speed * Math.cos( 10 * Math.PI / 180 ),
                vy = speed * Math.sin( 10 * Math.PI / 180 );
                
            (function linear() {
                oGc.clearRect(0, 0, width, height);
                ball.fill( oGc );
                ball.x += vx;
                ball.y += vy;
                requestAnimationFrame(linear);
            })();
        }
    </script>
</head>
<body>
    <canvas id="canvas" width="1200" height="600"></canvas>
</body>

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