Magic video compression

[Megafox]

My video from DVD disc weighs 900 mb, it has excellent quality. Why, when I try to re-encode it in mpeg2 from a decoded source, it weighs in 2 times more? Or if the weight of the same quality deteriorates. How does DVD encode that no codec is able to compress video without loss? I'm looking for a way to do this.

[aedipuss]

dvd is already encoded to mpeg-2. it's just in a vob wrapper. use something like vobtompg to take the mpeg-2 out of the vob losslessly.

[poisondeathray]

My video from DVD disc weighs 900 mb, it has excellent quality. Why, when I try to re-encode it in mpeg2 from a decoded source, it weighs in 2 times more? Or if the weight of the same quality deteriorates. How does DVD encode that no codec is able to compress video without loss? I'm looking for a way to do this.

I noticed your other thread, video compression does not work like that

First, filesize = bitrate * running time . That's it. Irrespective of quailty. If you re-encode with the same bitrate, it will be the same filesize (apart from other small differences like closed captions, container overhead, if you compare 2 elementary video streams with the same bitrate, they will have the same filesize if they have the same duration if you look at the equation)

When you re-encode something, it gets decoded to uncompressed frames. Uncompressed video will typically be 5-20x larger. Then you recompress the uncompressed video . The compression is relative to the uncompressed state.

For example, the DVD master from the studio would typically be 5-20x larger than the version you have on DVD. So there is quality loss on the DVD compared to the DVD master

If you "rip" the DVD, that is a 1:1 copy of the DVD video . It should be the same filesize , same quality as the DVD. But there is major quality loss compared to the DVD master from which the original DVD was derived from

Each time you re-encode with a lossy format, you lose quality. It's know as "generation loss" . A lossless codec has no quality loss, but that is relative to the uncompressed state. So the compression is in terms of the uncompressed video. It will still be many times larger than the original compressed video. (But smaller than the uncompressed video)



Conversely, something like archival ZIP, RAR do not decode the video stream . It does not decode to uncompressed frames . Do you see the distinction ?

[usually_quiet]

My video from DVD disc weighs 900 mb, it has excellent quality. Why, when I try to re-encode it in mpeg2 from a decoded source, it weighs in 2 times more? Or if the weight of the same quality deteriorates. How does DVD encode that no codec is able to compress video without loss? I'm looking for a way to do this.

MPEG-2 is a type of lossy compression. "Lossy" means that some picture information is discarded when the video is encoded. Lower bitrate settings create smaller files, but using a low bitrate setting requires more information to be discarded. Using a high bitrate reduces the amount of picture information that is discarded, but creates a larger file.

When MPEG-2 video is decompressed, the discarded picture information cannot be restored, which causes the decompressed picture to exhibit flaws called "compression artifacts". There is no way to avoid more quality loss if the decompressed video is compressed again using MPEG-2 or some other lossy codec. Some of the existing picture information will be discarded, reducing the quality even more. Lossless codecs exist, but using them produces much larger files than lossy codecs produce.

Oops, poisondeathray already provided a better explanation.