3

我有一个带有图像的画布，我想不断地淡入和淡出图像。我已经使用了上面的代码：

<!DOCTYPE HTML>
 <html>
<head>
    <script>
        var canvas;
        var context;
        var ga = 0.0;
        var timerId = 0;
        var timerId1 = 0;

        function init()
        {
            canvas = document.getElementById("myCanvas");
            context = canvas.getContext("2d");

            timerId = setInterval("fadeIn()", 300);
            console.log(timerId);

        }

        function fadeIn()
        {
            context.clearRect(0,0, canvas.width,canvas.height);
            context.globalAlpha = ga;
            var photo = new Image();
            photo .onload = function()
            {
                context.drawImage(photo , 0, 0, 450, 500);
            };
            photo .src = "photo .jpg";

            ga = ga + 0.1;

            if (ga > 1.0)
            {
                fadeOut();
                goingUp = false;
                clearInterval(timerId);

            }
        }

        function fadeOut()
        {
            context.clearRect(0,0, canvas.width,canvas.height);
            context.globalAlpha = ga;

            var photo = new Image();
            photo .onload = function()
            {
                context.drawImage(photo , 0, 0, 450, 500);
            };
            photo .src = "photo .jpg";

            ga = ga - 0.1;

            if (ga < 0)
            {

                goingUp = false;
                clearInterval(timerId);
            }
        }
    </script>
</head>
<body onload="init()">
    <canvas height="500" width="500" id="myCanvas"></canvas>
</body>

是否可以在 setIntervals 中插入两个函数？我想在fadeIn 函数之后触发fadeOut。这怎么可能？？

4

1 回答 1

7

这是执行此操作的一种方法：

var alpha = 0,   /// current alpha value
    delta = 0.1; /// delta = speed

然后在主循环中用当前的 alpha 增加 alpha。当 alpha 达到 0 或 1 反向 delta 时。这将创建乒乓渐变：

function loop() {

    alpha += delta;
    if (alpha <= 0 || alpha >= 1) delta = -delta;

    /// clear canvas, set alpha and re-draw image
    ctx.clearRect(0, 0, demo.width, demo.height);
    ctx.globalAlpha = alpha;
    ctx.drawImage(img, 0, 0);

    requestAnimationFrame(loop); // or use setTimeout(loop, 16) in older browsers
}
于 2013-10-08T22:19:26.660 回答