DivX/Xvid Index?

[ZeroAquaduct]

I played around with a program called "DivFix" and I noticed there are buttons for "Rebuild Index" and "Strip Index."

What exactly is an index file for DivXs and Xvids? I noticed that once I strip an index from a Xvid, it no longer works, but if I rebuild it, it works again. I just wanted to find out what this index thing is all about and what role it plays in a DivX or Xvid file.

[junkmalle]

Divx files consist of two basic types of frames. One type encodes an entire picture, much like a JPEG image, called I frames. The other type encodes only the differences between one picture and the next, called B and P frames. You typically have a hundred or more B and P frames for every I frame. A typical sequence might look like IPPPPPPPPPPPPPPPPPPPPIPPPPPPPPPPPP... This is one of the reasons Divx can get so much compression -- for example, all those scenes with talking heads on static backgrounds only have to have the talking heads encoded most of the time.

This is great for simple playback where you start at the begining and play through to the end. But to seek to a random location a playback program can't just go to that P frame and decode it, it has to first decode the preceding I frame to get an entire picture, then add the changes for each P frame until it has reached the desired one.

The Divx index is a list of all the I frames in the file. This makes it much easier for a program to seek to random locations. Without the index a program has to start at the very beginning of the file and walk through everything looking for I frames. With an index the program can quickly find the nearest I frame and start decoding.

[redwudz]

If I recall correctly the index for DIVX files is at the end of the file. If the file is corrupted or missing frames, then the index does not match the video file and seeking or FF or even locking up can happen.

The index can be stripped out and recreated to repair some of those problems. But you should probably edit out any corrupted parts before reindexing. VirtualDub does about the same thing when you resave a DIVX file.

EDIT: What junkmalle said.

[ZeroAquaduct]

Thanks for the replies! So I guess the index file doesn't affect the actual content (quality, etc.) of the video, but it acts as a "table of contents" kind of thing for the player?

redwudz, thanks for the tips. I'm actually just trying to learn more about DivXs and Xvids. I'm not trying to fix anything.

[celtic_druid]

The index is to do with the avi container not specifically divx or XviD and yes for an avi it is at the end.

.divx is basically an extension of the avi container to add subs and menus.

DivX and XviD video can be stored in various containers, not just avi.

[ZeroAquaduct]

OK, thanks for clearing that up celtic_druid!