javascript - canvas怎么写刮开

Question

没想到思路。

懂的大神附上代码。

Answer 1

http://runjs.cn/detail/uk9ho4ad
代码在上面自己看下

Answer 2

<p class="view">
    <p class="box" id="bb">
        <canvas id="cas" ></canvas>
    </p>
</p>

    var canvas = document.getElementById("cas"), ctx = canvas.getContext("2d");
    var x1, y1, a = 30, timeout, totimes = 100, distance = 30;
    var saveDot = [];

    var canvasBox = document.getElementById("bb");

    canvas.width = canvasBox.clientWidth;
    canvas.height = canvasBox.clientHeight;

    var img = new Image();
    img.src = "pic1.JPG";

    img.onload = function () {
        var w = canvas.height*img.width/img.height;
        ctx.drawImage(img, (canvas.width-w)/2, 0, w, canvas.height);

        // var box = document.querySelector('.box');
        // box.style.backgroundImage='pic2.JPG';

        tapClip()
    };

    function getClipArea(e, hastouch){
        var x = hastouch ? e.targetTouches[0].pageX : e.clientX;
        var y = hastouch ? e.targetTouches[0].pageY : e.clientY;
        var ndom = canvas;

        while(ndom.tagName!=="BODY"){
            x -= ndom.offsetLeft;
            y -= ndom.offsetTop;
            ndom = ndom.parentNode;
        }

        return {
            x: x,
            y: y
        }
    }

    //通过修改globalCompositeOperation来达到擦除的效果
    function tapClip() {
        var hastouch = "ontouchstart" in window ? true : false,
            tapstart = hastouch ? "touchstart" : "mousedown",
            tapmove = hastouch ? "touchmove" : "mousemove",
            tapend = hastouch ? "touchend" : "mouseup";

        var area;
        var x2,y2;

        ctx.lineCap = "round";
        ctx.lineJoin = "round";
        ctx.lineWidth = a * 2;
        ctx.globalCompositeOperation = "destination-out";

        window.addEventListener(tapstart, function (e) {
            clearTimeout(timeout);
            e.preventDefault();

            area = getClipArea(e, hastouch);

            x1 = area.x;
            y1 = area.y;

            drawLine(x1, y1);

            this.addEventListener(tapmove, tapmoveHandler);

            this.addEventListener(tapend, function () {
                this.removeEventListener(tapmove, tapmoveHandler);

                //检测擦除状态
                timeout = setTimeout(function () {
                    var imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
                    var dd = 0;
                    for (var x = 0; x < imgData.width; x += distance) {
                        for (var y = 0; y < imgData.height; y += distance) {
                            var i = (y * imgData.width + x) * 4;
                            if (imgData.data[i + 3] > 0) { dd++ }
                        }
                    }
                    if (dd / (imgData.width * imgData.height / (distance * distance)) < 0.4) {
                        canvas.className = "noOp";
                    }
                }, totimes)
            });

            function tapmoveHandler(e) {
                clearTimeout(timeout);

                e.preventDefault();

                area = getClipArea(e, hastouch);

                x2 = area.x;
                y2 = area.y;

                drawLine(x1, y1, x2, y2);

                x1 = x2;
                y1 = y2;
            }
        })
    }

    function drawLine(x1, y1, x2, y2){
//        ctx.save();
//        if(arguments.length==2){
//            saveDot = [[x1, y1]];
//            ctx.beginPath();
//            ctx.arc(x1, y1, a, 0, 2 * Math.PI);
//            ctx.fill();
//        }else {
//            saveDot.push([x2,y2]);
//            if(saveDot.length >= 3){
//                ctx.beginPath();
//                ctx.moveTo(saveDot[0][0], saveDot[0][1]);
//                ctx.quadraticCurveTo(saveDot[1][0], saveDot[1][1], saveDot[2][0], saveDot[2][1]);
//                ctx.stroke();
//                saveDot = [saveDot.pop()];
//            }
//        }
//        ctx.restore();

        ctx.save();
        ctx.beginPath();
        if(arguments.length==2){
            ctx.arc(x1, y1, a, 0, 2 * Math.PI);
            ctx.fill();
        }else {
            ctx.moveTo(x1, y1);
            ctx.lineTo(x2, y2);
            ctx.stroke();
        }
        ctx.restore();
    }

    //使用clip来达到擦除效果
    function otherClip() {
        var hastouch = "ontouchstart" in window ? true : false,
            tapstart = hastouch ? "touchstart" : "mousedown",
            tapmove = hastouch ? "touchmove" : "mousemove",
            tapend = hastouch ? "touchend" : "mouseup";

        var area;

        canvas.addEventListener(tapstart, function (e) {
            clearTimeout(timeout);
            e.preventDefault();

            area = getClipArea(e, hastouch);

            x1 = area.x;
            y1 = area.y;

            ctx.save();
            ctx.beginPath();
            ctx.arc(x1, y1, a, 0, 2 * Math.PI);
            ctx.clip();
            ctx.clearRect(0, 0, canvas.width, canvas.height);
            ctx.restore();

            canvas.addEventListener(tapmove, tapmoveHandler);
            canvas.addEventListener(tapend, function () {
                canvas.removeEventListener(tapmove, tapmoveHandler);

                timeout = setTimeout(function () {
                    var imgData = ctx.getImageData(0, 0, canvas.width, canvas.height);
                    var dd = 0;
                    for (var x = 0; x < imgData.width; x += distance) {
                        for (var y = 0; y < imgData.height; y += distance) {
                            var i = (y * imgData.width + x) * 4;
                            if (imgData.data[i + 3] > 0) {
                                dd++
                            }
                        }
                    }
                    if (dd / (imgData.width * imgData.height / (distance * distance)) < 0.4) {
                        canvas.className = "noOp";
                    }
                }, totimes)

            });

            function tapmoveHandler(e) {
                e.preventDefault();
                area = getClipArea(e, hastouch);
                x2 = area.x;
                y2 = area.y;

                var asin = a * Math.sin(Math.atan((y2 - y1) / (x2 - x1)));
                var acos = a * Math.cos(Math.atan((y2 - y1) / (x2 - x1)));
                var x3 = x1 + asin;
                var y3 = y1 - acos;
                var x4 = x1 - asin;
                var y4 = y1 + acos;
                var x5 = x2 + asin;
                var y5 = y2 - acos;
                var x6 = x2 - asin;
                var y6 = y2 + acos;

                ctx.save();
                ctx.beginPath();
                ctx.arc(x2, y2, a, 0, 2 * Math.PI);
                ctx.clip();
                ctx.clearRect(0, 0, canvas.width, canvas.height);
                ctx.restore();

                ctx.save();
                ctx.beginPath();
                ctx.moveTo(x3, y3);
                ctx.lineTo(x5, y5);
                ctx.lineTo(x6, y6);
                ctx.lineTo(x4, y4);
                ctx.closePath();
                ctx.clip();
                ctx.clearRect(0, 0, canvas.width, canvas.height);
                ctx.restore();

                x1 = x2;
                y1 = y2;
            }
        })
    }