Question

我有一个带有图像的画布，我想要不断淡化和淡出图像。我使用了上面的代码：

<!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函数后触发fadeOut。怎么可能？

Answer 1

以下是执行此操作的一种方法：

<强> ONLINE DEMO HERE

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

在主循环中，然后使用当前alpha增加alpha。当alpha达到0或1反向delta时。这将创建乒乓淡出：

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
}

在Html5画布中淡化FadeOut

1 个答案: