My last post was all about how good mobile video is now using 4G. I start off with that because I want to make sure you know I’m not contradicting myself with this post. Really, I’m not.
Here’s what I’m talking about … blocky video. Have you ever watched a video when it all of a sudden burst into a collection of little squares? Usually it’s not the whole screen, but parts of the video in fast motion or sometimes what appears to be a random area. You can see an example in the photo.
For a long time I assumed that the blocks were due to poor network connections – that the video data didn’t have enough bandwidth to get through all of it. But that isn’t what’s going on.
Those pesky blocks are called artefacts. And they occur, oddly enough, from something called artifacting.
So why does it happen?
Just about every video we see has under gone some form of compression. When a lossy compression is used, parts of the video are cut out in order to reduce the overall size. That makes it easier to stream over the Internet since it requires less bandwidth but the price is a loss of quality and blocky video is the result.
Compression and encoding algorithms try to make up for the lost data by being smart about what data is lost. But the algorithms aren’t perfect. For example they will check frame by frame and drop data that is the same in each frame (so it doesn’t store the redundant data). This works great if you have a fairly static scene of two people talking next to a car. But when the frames suddenly change as the car speeds off like a rocket, the algorithms don’t always catch up. They try to “predict” what should be in the next frame. When those predictions are wrong we see those blocks pop up on the screen.
Can artifacting be avoided? According to this blog on Streamingmedia.com it can. The author, Tim Siglin, says artifacting is primarily a result of poor encoding. “It turns out the majority of high resolution content is just poorly encoded, with limited quality control to check those high-action or complexly composited scenes,” he says. Future codecs will alleviate some of the issues, but Mr. Siglin isn’t hopeful technology alone will fix the problem.
I guess we’ll all have to wait and see how the new codecs will improve the problem. But if you’re creating video with fast action or a “complexly composited scenes” you might want to pay more attention to your encoding options from now on.