Compress and send image over network in java?

Question

Hi.

I have a server which waits for a connection from a client then sends an image to that client using the Socket class.

Several clients will be connecting over a short time, so I would like to compress the image before sending it.

The images are 1000 by 1000 pixel BufferedImages, and my current way of sending them is to iterate over all pixels and send that value, then reconstruct it on the other side. I suspect this is not the best way to do things.

Can anyone give any advice on compression and a better method for sending images over a network?

Thanks.

Answer 1

It's been a long time since I did any image processing in Java, but you can save the image on the server as a JPEG, and then send them a URI and let them retrieve it themselves.

Answer 2

JPEG is lossy, so if you need the exact same image on the other side, you can use a GZIPOutputStream on top of the socket's OutputStream to send the compressed data, and receive it on the other side through a GZIPInputStream on top of the socket's InputStream.

Answer 3

If you are using the getInputStream and getOutputStream methods on Socket, try wrapping the streams with java.util.net.GZIPInputStream and java.util.net.GZIPOutputStream.

Answer 4

Compression is very much horses for courses: which method will actually work better depends on where your image came from in the first place, and what your requirements are (principally, whether you allow lossy compression, and if so, what constraints you place on it).

To get started, try using ImageIO.write() to write the image in JPEG or PNG format to a ByteArrayOutputStream, whose resulting byte array you can then send down the socket[1]. If that gives you an acceptable result, then the advantage is it'll involve next-to-no development time.

If they don't give an acceptable result (either because you can't use lossy compression or because PNG compression doesn't give an acceptable compression ratio), then you may have to come up with something custom to suit your data; at that point. Only you know your data at the end of the day, but a general technique is to try and get your data in to a form where it works well with a Deflater or some other standard algorithm. So with a deflater, for example, you transform/re-order your data so that repeating patterns and runs of similar bytes are likely to occur close to one another. That might mean sending all of the top bits of pixels, then all the next-top bits, etc, and actually not sending the bottom few bits of each component if they're effectively just noise.

Hopefully the JPEG/PNG option will get you the result you need, though, and you won't have to worry much further.

[1] Sorry, should have said -- you can obviously make the socket output stream the one that the image data is writte into if you don't need to first query it for length, take a hash code...