Would it be possible and if yes, than how? Take a DVD5 that was previously a DVD9 but now its a DVD5 being it was compressed to DVD5 with DVD Shrink, and revert it back to the original uncompressed of DVD9?

# Revert a compressed DVD back to uncompressed?

**Kerry56**#2

No, this isn’t possible. Shrink discards data as it compresses. That is lost for good.

Here is an explanation from Dr. Shrink himself:

[I]There are two kinds of data in an MPEG video stream:

- Motion vectors
- Pixel “residual” data (in the form of DCT coefficients)

Each decoded picture is formed by combining parts of previous decoded pictures (using the motion vectors) with new pixel information (residual data).

The basic idea is, that since each decoded picture is very similar to the previous, it can be fairly accurately described using pixels from the previously decoded picture, offset by some motion vectors to compensate for camera movement, or movement of objects in the scene.

The purpose of residual data is then to compensate for any errors in this process, since you are unlikely to get an exact match for the new picture using only the previous picture + motion vectors.

DVD Shrink achieves compression by removing some of the residual data. This process is called requantization. Selected DCT coefficients are scaled down (thus reducing the number of bits required to store them) and a corresponding scale

value for these coefficients (quantizer scale) is scaled up. The result is a less accurate description of the same residual data, which takes up less space. Note motion vectors are left unmodified by this process.

[/I]