Teach a man to fish…

(Third installment of my “Movie Repair Guide“, where you’ll learn how a movie can be extracted from a larger file)

When you repair movies, you never know what you can find.
Yesterday I had an old shoe moment while looking for some mountain footage in a corrupt file.
Some obscure AVI Windows file containing this infamous “Download” animation surfaced when I least expected it:

Today I’ll teach you to fish, in other words I’ll explain how to rescue movies from inside larger files. It will only work if certain conditions are met, but it’s an exciting experience… that can end with an old shoe. You’ve been warned.

Container Structure Correction is a repair technique that acts upon the container structure data, leaving the media data and the index and tables untouched.
Unlike reindexing or other techniques that need to act on hundreds of audio or video frames, a structure correction is usually a question of few bytes to correct or a misplaced block of data. For this reason, it can be done manually.

If the same correction has to be applied on a collection of files, then maybe it’s worth spending time automating the structure correction.

Most common structure damages and how to fix them:

Full movie data, including container, is embedded in a larger file, for example after a data recovery.
In this case, the data can be extracted and saved with the container suffix and it usually works.

In this example, we have examined the file with an Hex editor and have found an intact RIFF structure in the middle of a file. Note that the length of the structure 0x49e4 is encoded just after RIFF in 32 bits, big-endian.
A quick check shows that after 0x49e4+8 bytes, there is suddenly no data, thus confirming that we are into something.

By copying this data to a new file and saving with an AVI suffix, we now have a valid movie.
Make sure you copy exactly the required bytes, starting from the R in RIFF, ending after idx structure, otherwise it won't work.

When you edit a movie in-place (ie without writing again all the file), some edition softwares just add a new moov container at the end of the file, without bothering to remove older ones. If the file has become corrupt, those old containers can still work.
The trick consists in finding the moov structures and redirect towards an older one.
You'll need an Hex editor and good knowledge of QuickTime file format to do that.

Files from a data recovery can present defects: contents not matching the file name, truncated files, mashed contents coming from several files, moved or missing blocks.
As data recovery acts at the lower filesystem level, the data is always presented in blocks, for example exactly 2048 bytes.
A movie can fail to open because some blocks have been moved, duplicated or missing. The fact have blocks always have a length of 2048 bytes helps to detect and correct those problems. It's like solving a puzzle and is usually complex.