Loading images from URL partially, just like what is implemented in WhatsApp

Question

WhatsApp developers recently improved the image loading in which immediately loading some portion of the image (getting its dimension and some pixels of the images) and then after loading the entire image, replace the placeholder with the full image:

My question is, how did they implement it? Do they read the dimension of the image by reading its header (meta-data)? How about the image content? Or do they have two versions of the image at the server-side, a smaller one with low-quality which is loaded first and a bigger one which is the full image? Note that if it's the second approach then they still need to extract the smaller version of the image at the server side once receiving the image from the sender. Any other approaches?

Answer 1

There is another alternative to the colored placeholder solution, which is to show a thumbnail image (may be only a 100 X 100 px) as the placeholder until loading the real image, which is away more cooler than having only a colored placeholder :) .

I did it in Zingoo and made a blog post about it. Using Picasso, you can do it like this:

Transformation blurTransformation = new Transformation() {
    @Override
    public Bitmap transform(Bitmap source) {
        Bitmap blurred = Blur.fastblur(LiveImageView.this.context, source, 10);
        source.recycle();
        return blurred;
    }

    @Override
    public String key() {
        return "blur()";
    }
};

Picasso.with(context)
    .load(thumbUrl) // thumbnail url goes here
    .placeholder(R.drawable.placeholder)
    .resize(imageViewWidth, imageViewHeight)
    .transform(blurTransformation)
    .into(imageView, new Callback() {
        @Override
        public void onSuccess() {
            Picasso.with(context)
                    .load(url) // image url goes here
                    .resize(imageViewWidth, imageViewHeight)
                    .placeholder(imageView.getDrawable())
                    .into(imageView);
        }

        @Override
        public void onError() {
        }
    });

more details in the post itself including the Blur class.