There's this cool site called http://www.thesixtyone.com/, great for discovering new music (if they only agree to ban Foo Fighters, the site will be perfect). Discovering music but also discovering interesting web design patterns, as it turns out.
TheSixtyOne.com shows a progress indicator when you click a song, as if the background shows how the song gets loaded. So I had to try implementing this myself and I also played with two other variants of the background animation.
Here's a video, a little screencast to demonstrate how the loading looks like:
Example #1 - filling the background
For this you get an image that is at least as long as the container you're about to animate. The height of the background image doesn't matter, could be just one pixel. Here's the image I used.
Ar the start of the animation, the background image is so to the left, it's invisible. At the end of the animation, the background fills out the whole container.
Example #2 - overflowing background
This is a modified version of the previous pattern. Once the background is filled the animation keeps going, so the background escapes from view. Then it's moved back at the beginning and the animation repeats.
Example #3 - dancing arrow
If you remember old radios, this pattern looks like you're playing with the dial. Here, the background image is different, just a small square basically. It gets repeated vertically. Here's the backgournd.