扫描像素点法获取图形轮廓

参考：

http://runjs.cn/code/k09vtlq7

 var _getContourData = function(contourData,canvas){        var w = canvas.width;        var h = canvas.height;        _draft = canvas.getContext('2d').createImageData(w, h);        var pixelsData = contourData;        var pixels = pixelsData.data;        //遍历像素并标记        for (var r = 0; r < h - 1; r++) {            for (var c = 0; c < w - 1; c++) {                var rIndexCurrent = 4 * (c + r * w);                var gIndexCurrent = 4 * (c + r * w) + 1;                var bIndexCurrent = 4 * (c + r * w) + 2;                var aIndexCurrent = 4 * (c + r * w) + 3;                var rIndexRight = 4 * (c + 1 + r * w);                var gIndexRight = 4 * (c + 1 + r * w) + 1;                var bIndexRight = 4 * (c + 1 + r * w) + 2;                var aIndexRight = 4 * (c + 1 + r * w) + 3;                var rIndexDown = 4 * (c + (r + 1) * w);                var gIndexDown = 4 * (c + (r + 1) * w) + 1;                var bIndexDown = 4 * (c + (r + 1) * w) + 2;                var aIndexDown = 4 * (c + (r + 1) * w) + 3;                var currentPixel = new pixel(pixels[rIndexCurrent], pixels[gIndexCurrent], pixels[bIndexCurrent], pixels[aIndexCurrent]);                var rightPixel = new pixel(pixels[rIndexRight], pixels[gIndexRight], pixels[bIndexRight], pixels[aIndexRight]);                var downPixel = new pixel(pixels[rIndexDown], pixels[gIndexDown], pixels[bIndexDown], pixels[aIndexDown]);                var horizonDis = distance(currentPixel, rightPixel);                var verticalDis = distance(currentPixel, downPixel);                if (horizonDis >= 200 || verticalDis >= 200) {                    _draft.data[rIndexCurrent] = 255;                    _draft.data[gIndexCurrent] = 0;                    _draft.data[bIndexCurrent] = 0;                    _draft.data[aIndexCurrent] = 255;                } else {                    _draft.data[rIndexCurrent] = 0;                    _draft.data[gIndexCurrent] = 0;                    _draft.data[bIndexCurrent] = 0;                    _draft.data[aIndexCurrent] = 0;                }            }        }        //计算距离        function distance(pa, pb) {            var sum = 0;            sum += Math.abs(Number(pa.r) - Number(pb.r));            sum += Math.abs(Number(pa.g) - Number(pb.g));            sum += Math.abs(Number(pa.b) - Number(pb.b));////            var sum =  Math.abs(Number(pa.a)-Number(pb.a));            return sum;        }        //自定义像素对象        function pixel(red, green, blue, aval) {            this.r = red;            this.g = green;            this.b = blue;            this.a = aval;        }    }