An extension over google's servingUrl "cdn" features: https://developers.google.com/appengine/docs/java/images/ It links to the Media datastore object and also provides rectangular cropping.

gistfile1.java

/**
 * An extension over google's servingUrl "cdn" features: https://developers.google.com/appengine/docs/java/images/
 *
 * It links to the Media datastore object and also provides rectangular cropping.
 *
 * Examples:
 * /c/i/69589306
 * /c/i/69589306=s300
 * /c/i/69589306=s300-c
 * /c/i/69589306=s300-c=h20
 *
 * TODO: add watermark functionality.
 *
 * TODO: Compare mediaId against PRODUCTION_SERVING_URLS to check if it's possible.
 * TODO: That way it's safer to detect someone trying to download larges portion
 * TODO: of the site with a simple  sequential script.
 * 
 * @author mufumbo
 */
public class ImagesExtendedApiServlet extends HttpServlet {
    final static Logger logger = Logger.getLogger(ImagesExtendedApiServlet.class.getName());
    final static int MAX_SIZE = 1500;


    static ImagesService imagesService = ImagesServiceFactory.getImagesService();
    static BlobstoreService blobstoreService = BlobstoreServiceFactory.getBlobstoreService();

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String uri = req.getRequestURI();
        logger.warning("processing " + uri + " host " + req.getHeader("Host"));

        try {
            String[] spl = uri.split("/");
            String type = spl[2];
            if ("i".equals(type)) {
                String content = spl[3];

                String[] splContent = content.split("=");
                long mediaId = Long.parseLong(splContent[0]);


                DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
                Key key = KeyFactory.createKey("Media", mediaId);
                Entity media = null;
                try {
                    media = datastore.get(key);
                }
                catch (EntityNotFoundException enfe) {
                    logger.log(Level.WARNING, "couldn't find media with id " + mediaId);
                    resp.setStatus(HttpServletResponse.SC_NOT_FOUND);
                }

                if (media != null) {
                    BlobKey oldBlobKey = (BlobKey) media.getProperty("image");

                    BlobKey newBlobKey = oldBlobKey;
                    /*BlobKey newBlobKey = BlobMigrationRecord.getNewBlobKey(oldBlobKey);
                    if (!newBlobKey.equals(oldBlobKey)) {
                        // this shouldn't ever be happening as we should migrate everything in Media!
                        logger.warning("blobKey changed from " + oldBlobKey + " to " + newBlobKey);
                    } */
                    Image newImage = ImagesServiceFactory.makeImageFromBlob(newBlobKey);

                    OutputSettings settings = new OutputSettings(ImagesService.OutputEncoding.JPEG);
                    settings.setQuality(83);

                    if (splContent.length > 1) {
                        int width = 0, height = 0;
                        for (int i = 1; i < splContent.length; i++) {
                            String cur = splContent[i];
                            if (cur.startsWith("s") && cur.endsWith("-c")) {
                                String w = cur.substring(1, cur.length() - 2);
                                width = height = Integer.parseInt(w);
                                logger.info("Cropping width[" + width + "]");
                            } else if (cur.startsWith("s")) {
                                String w = cur.substring(1, cur.length());
                                width = Integer.parseInt(w);
                                logger.info("Resizing image width[" + width + "]");
                            } else if (cur.startsWith("h")) {
                                String h = cur.substring(1, cur.length());
                                height = Integer.parseInt(h);
                                logger.info("Cropping height[" + height + "]");
                            } else {
                                logger.warning("unrecognized transformation " + cur);
                            }
                        }

                        if (width > MAX_SIZE || height > MAX_SIZE)
                            throw new Exception(MAX_SIZE + " is too big");

                        if (width > 0 && height == 0) {
                            Transform resize = ImagesServiceFactory.makeResize(width, width, false);
                            newImage = imagesService.applyTransform(resize, newImage, settings);

                            //ensureImageFetch(newImage);
                            //float scale = (float) width / (float) newImage.getWidth();
                            //height = (int) (newImage.getHeight() * scale);
                            //logger.info("Scaling[" + scale + "][" + width + "] height to " + height + " from " + newImage.getHeight());
                        }
                        else {
                            Transform transform = ImagesServiceFactory.makeResize(width, height, 0.5, 0.5);
                            newImage = imagesService.applyTransform(transform, newImage, settings);
                        }
                    } else {
                        ensureImageFetch(newImage, settings);
                        logger.info("full content download width[" + newImage.getWidth() + "] height[" + newImage.getHeight() + "]");
                    }

                    /*
                    ImagesServiceFactory.makeImageFromFilename();
                    InputStream stream = ImagesExtendedApiServlet.class.getResourceAsStream("default_logo_nopadding.png");
                    Image wm = ImagesServiceFactory.makeImage(asByteArray(stream));
                    Image wmComposite = ImagesServiceFactory.makeComposite(wm, width - , 30, 1.0f, Composite.Anchor.TOP_LEFT);
                    composites.add(wmComposite);
                    */

                    byte[] imageData = newImage.getImageData();

                    // int beforeStrip = imageData.length;
                    // No need to strip EXIF data off as appengine seems to do this already.
                    // ByteArrayOutputStream baos = new ByteArrayOutputStream(imageData.length);
                    // new ExifRewriter().removeExifMetadata(imageData, baos);//.removeExifMetadata(jpegImageFile, os);
                    // imageData = baos.toByteArray();
                    // logger.info("streaming nostrip[" + beforeStrip + "] after[" + imageData.length + "]");

                    resp.getOutputStream().write(imageData);

                    resp.setHeader("Content-Type", "image/jpeg");
                }
                else {
                    resp.setStatus(HttpServletResponse.SC_NOT_ACCEPTABLE);
                }
            }
            else if ("v".equals(type)) {
            	String content = spl[3];

            	String[] splContent = content.split("=");
            	long mediaId = Long.parseLong(splContent[0].indexOf('.') < 0 ? splContent[0] : splContent[0].substring(0, splContent[0].indexOf('.')));
            	DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
                Key key = KeyFactory.createKey("Media", mediaId);
                Entity media = null;
                try {
                    media = datastore.get(key);
                }
                catch (EntityNotFoundException enfe) {
                    logger.log(Level.WARNING, "couldn't find media with id " + mediaId);
                    resp.setStatus(HttpServletResponse.SC_NOT_FOUND);
                }

                if (media != null) {
                	BlobKey video = (BlobKey) media.getProperty("video");
                	BlobInfoFactory bif = new BlobInfoFactory();
                	BlobInfo info = bif.loadBlobInfo(video);
                	resp.setContentType(info.getContentType());
                	resp.setHeader("Accept-Ranges", "bytes");
                	if (req.getHeader("Range") != null) {
                		try {
                			ByteRange range = ByteRange.parse(req.getHeader("Range"));
                			blobstoreService.serve(video, range, resp);
                		}
                		catch (Exception ex){
                			logger.fine("Error parsing byte range: " + String.valueOf(req.getHeader("Range")));
                			blobstoreService.serve(video, resp);
                		}
                	}
                	else
                		blobstoreService.serve(video, resp);
                }
                else {
                    resp.setStatus(HttpServletResponse.SC_NOT_ACCEPTABLE);
                }
            }
        } catch (Exception e) {
            logger.log(Level.WARNING, "couldn't provision " + uri, e);
            resp.setStatus(HttpServletResponse.SC_INTERNAL_SERVER_ERROR);
        }
    }

    private static Image ensureImageFetch(Image img, OutputSettings settings) {
        Transform resize = ImagesServiceFactory.makeResize(800, 800, false);
        Image newImage = imagesService.applyTransform(resize, img, settings);
        logger.info("Original image[" + newImage.getImageData().length + "] width[" + newImage.getWidth() + "] height[" + newImage.getHeight() + "]");
        return newImage;
    }

    public static byte[] asByteArray(InputStream stream) throws IOException {
        final ByteArrayOutputStream buf = new ByteArrayOutputStream();
        final byte[] buffer = new byte[32768];
        int cbRead;
        while ((cbRead = stream.read(buffer, 0, buffer.length)) != -1)
        {
            buf.write(buffer, 0, cbRead);
        }
        return buf.toByteArray();
    }
}