印刷用のクリア画像を生成するhtml2canvasのサンプルコード

window.html2canvas(dom, {
        scale: scale,
        width: dom.offsetWidth,
        height: dom.offsetHeight
    }).then(function (canvas) {
        var context = canvas.getContext('2d');
        context.mozImageSmoothingEnabled = false;
        context.webkitImageSmoothingEnabled = false;
        context.msImageSmoothingEnabled = false;
        context.imageSmoothingEnabled = false;
        var src64 = canvas.toDataURL()
}

var context = canvas.getContext('2d');
context.mozImageSmoothingEnabled = false;
context.webkitImageSmoothingEnabled = false;
context.msImageSmoothingEnabled = false;
context.imageSmoothingEnabled = false;

getImgSize: function (str) {
    //Get the base64 image size and return the KB number
    var str = str.replace('data:image/jpeg;base64,', '');//modify here according to the format of your uploaded image

    var strLength = str.length;
    var fileLength = parseInt(strLength - (strLength / 8) * 2);

    // convert from bytes to KB
    var size = "";
    size = (fileLength / 1024).toFixed(2);

    return parseInt(size);
}

compress: function (base64String, w, quality) {
    var getMimeType = function (urlData) {
        var arr = urlData.split(',');
        var mime = arr[0].match(/:(. *?) ;/)[1];
        // return mime.replace("image/", "");
        return mime;
    };
    var newImage = new Image();
    var imgWidth, imgHeight;

    var promise = new Promise(function (resolve) {
        newImage.onload = resolve;
    });
    newImage.src = base64String;
    return promise.then(function () {


        imgWidth = newImage.width;
        imgHeight = newImage.height;
        var canvas = document.createElement("canvas");
        var ctx = canvas.getContext("2d");
        if (Math.max(imgWidth, imgHeight) > w) {
            if (imgWidth > imgHeight) {
                canvas.width = w;
                canvas.height = w * imgHeight / imgWidth;
            } else {
                canvas.height = w;
                canvas.width = w * imgWidth / imgHeight;
            }
        } else {
            canvas.width = imgWidth;
            canvas.height = imgHeight;
        }
        ctx.clearRect(0, 0, canvas.width, canvas.height);
        ctx.drawImage(newImage, 0, 0, canvas.width, canvas.height);
        var base64 = canvas.toDataURL(getMimeType(base64String), quality);   
        return base64;
    })
}

self.compress(src64,width,1).then(function(base){
    src64 = base
    src64 = src64.replace(/data:image\/. *;base64,/, '')
    // call the interface to save the image
}).catch(function(err){
    dialog.tip(err.message, dialog.MESSAGE.WARN);
})