Release: liboggz 0.9.6

This release of Oggz 0.9.6 contains a new tool, oggz-comment, which can be used to edit the basic metadata (title, producer, copyright etc.) of Ogg Theora files. The library also has some pretty major improvements to the way it works out timestamps and does seeking, mostly the work of Shane Stephens.

In media files, timing and synchronization is extremely important. If the image and audio start to go out of sync, it is very noticeable and the video quickly becomes unwatchable. When you scan through a file you often need to decode a lot more data than you actually display. This is particularly the case when you jump backwards, which is common in a user interface that supports scrubbing. As video frames are stored as a difference relative to earlier (or later) frames, you end up needing to secretly jump further back in the file to the previous keyframe, and then decode many frames up to the one you actually want to show. For a smooth user experience you need to do this as quickly as possible.

Ogg has some interesting framing properties. Given that timing is so important, you might expect that every packet has its precise timing information associated with it. In Ogg, it turns out not to be so. Packets are stored in pages, and there is only one timestamp per page. It is common for many audio packets to be crammed onto one page; the timing information for all the rest is not stored in the file. On the other hand, the encoded data for video keyframes is usually much larger, and spans multiple pages. Only the last packet on a page has its timestamp recorded, so if the keyframe is followed by an a much smaller packet of frame data in the same page, the timestamp for the keyframe will be lost. For these reasons I tend to refer to Ogg as a "lossy" container.

In order to minimize these problems, liboggz now inspects the encoded data in order reconstruct the expected granulepos (corresponding to a timestamp) for every packet in an Ogg stream. This allows applications to use reliable timestamps, even though these are only sparsely recorded in most Ogg streams. This is not as easy as it sounds, particularly for Ogg Vorbis. To get a flavour of what's involved, read Shane's rant in the comments, explaining how to calculate Vorbis timestamps.

For an in-depth discussion, come to Ralph Giles' talk at linux.conf.au, Seeking is hard: Ogg design internals.