获取到像素点，遇到255红色rgb的改为黑色0
ps:红色的rgb为255 0 0

        function test(base64) {// 创建一个Canvas元素var canvas = document.createElement("canvas");var context = canvas.getContext("2d");var redStamp = new Image();redStamp.src = base64redStamp.onload = function () {canvas.width = redStamp.width;canvas.height = redStamp.height;context.drawImage(redStamp, 0, 0);var imageData = context.getImageData(0, 0, canvas.width, canvas.height);var data = imageData.data;for (var i = 0; i < data.length; i += 4) {if (data[i] === 255) {data[i] = 0;data[i + 1] = 0;data[i + 2] = 0;}}context.putImageData(imageData, 0, 0);var base64 = canvas.toDataURL();console.log(base64);};}