`transformImageStream` is a WinJS (Windows 8) utility to transform an image stream. The sample code to get a snapshot from a WebView control requires Windows 8.1.

getThumbnailFromWebView.js

// Takes a snapshot of WebView control (Windows 8.1) and returns a thumbnail URL.
function getThumbnailFromWebView(webviewControl, width, height) {
    return capturePreviewToBlobAsync(webviewControl).then(function (completeEvent) {
        var blob = completeEvent.target.result;
        var streamIn = blob.msDetachStream();

        var Imaging = Windows.Graphics.Imaging;
        var transform = new Imaging.BitmapTransform();
        transform.scaledHeight = height;
        transform.scaledWidth = width;
        transform.interpolationMode = Imaging.BitmapInterpolationMode.fant;

        return transformImageStream(streamIn, transform).then(function (memoryStream) {
            streamIn.close();
            blob.msClose();
            var imageblob = MSApp.createBlobFromRandomAccessStream('image/png', memoryStream);
            return URL.createObjectURL(imageblob);
        });
    });
}

// wrap `webviewControl.capturePreviewToBlobAsync` in a promise helper function.
function capturePreviewToBlobAsync(webviewControl) {
    return new WinJS.Promise(function (complete, error) {
        var captureOperation = webviewControl.capturePreviewToBlobAsync();
        captureOperation.oncomplete = function (args) {
            complete(args);
        };
        captureOperation.onerror = function (e) {
            error(e);
        };
        captureOperation.start();
    });
}

transformImageStream.js

// Here is where the magic happens.
// Takes an image stream and transforms it into another image stream.
// Uses? Create a thumbnail of an image, flip an image, grab just a section of an image.
// Use the output imgage stream to create a data URL, upload to a server, or write the image to disk.
function transformImageStream(streamIn, transform, encoderId) {
    var Imaging = Windows.Graphics.Imaging;
    encoderId = encoderId || Imaging.BitmapEncoder.pngEncoderId;
    var memoryStream = new Windows.Storage.Streams.InMemoryRandomAccessStream();
    return Imaging.BitmapDecoder.createAsync(streamIn).then(function (decoder) {
        return decoder.getPixelDataAsync(Imaging.BitmapPixelFormat.rgba8, Imaging.BitmapAlphaMode.ignore,
            transform, Imaging.ExifOrientationMode.ignoreExifOrientation,
            Imaging.ColorManagementMode.doNotColorManage).then(function (pixelDataProvider) {
            var pixelData = pixelDataProvider.detachPixelData();
            return Imaging.BitmapEncoder.createAsync(encoderId, memoryStream).then(function (encoder) {
                encoder.setPixelData(Imaging.BitmapPixelFormat.rgba8, Imaging.BitmapAlphaMode.ignore,
                    transform.scaledWidth, transform.scaledHeight, decoder.dpiX, decoder.dpiY, pixelData);
                return encoder.flushAsync().then(function () {
                    return memoryStream;
                });
            });
        });
    });
}

webview-thumbnail-sample.js

// Sample code that calls `getThumbnailFromWebView`
// Creates a PNG <img> in the DOM that is proportional to the webview.

var THUMB_HEIGHT = 120;
var scale = THUMB_HEIGHT / webviewControl.offsetHeight;
var width = Math.round(webviewControl.offsetWidth * scale);
var height = THUMB_HEIGHT;

getThumbnailFromWebView(webviewControl, width, height).done(function (thumbUrl) {
    var img = document.createElement("img");
    img.setAttribute("src", thumbUrl);
    img.setAttribute("alt", webviewControl.documentTitle);
    thumbs.appendChild(img);
});