On 9/4/07, Ian C. Blenke <icblenke@nks.net> wrote:
> Mario Lombardo wrote:
>
> >That won't work since I need to lug around discs with me. I just want
> >to have files to click on, specifically compressed. It's the 21st
> >century. I feel I should be able to do this. No more mucking around
> >with toxic DVDs that I'll just throw away someday. More waste really.
> >
> >
>
> MPEG2 files are already pretty well compressed. You _might_ get a small
> compression with bzip2, but it would take forever and you would save
> less than 5% of the original filesize (largely the repeating MPEG2
> overhead involved in framing).
>
> Compression is all about removing entropy. Entropy is about having
> structure and order.
>
> In a big file full of zeros, you can compress that almost entirely. This
> file doesn't have much entropy.
>
> In a big file full of nothing but pure random noise, you won't compress
> it much if at all. This file has no entropy.
>
> All compression is about minimizing repetition. This requires some
> knowledge of the content being compressed. The more specific a
> compression algorithm to a specific type of content, the better it will
> compress said content, as it will inherently know what things are most
> likely to repeat.
>
> There are two kinds of compression. Lossless and lossy.
>
> Lossless is like a ZIP file. It is meant to retain every precious bit in
> the origional file). Executables won't run if the bits aren't exactly
> the same after decompressing as they were before compressing.
>
> Lossy is like a JPEG file. It is meant to look generally the same (to
> the human eye) as the quality of said image is reduced to save space.
> This is why you specify a quality between 0% and 100% when compressing a
> JPEG file: to tell the compression algorithm how important the details
> and quality are to you for storing a given image.
>
> A movie is nothing more than a buch of photographs, taken at some
> resolution and framerate.
>
> Raw video files are typically stored as mjpeg (motion jpeg), with some
> level of acceptable lossiness. In this format, all of the individual
> frames of a movie are stored as individual compressed images. This makes
> it fairly painless to "splice" together video snippets and do frame by
> frame overlays and tricks on the fly. These files can be VERY large.
>
> Motion video files, like MPEG2 as included on a DVD, have "keyframes"
> every N number of frames (that are an entire refresh of the picture).
> These keyframes and successive frames are broken down into smaller
> squares and "delta" changes between them are recorded to the digital
> stream. The idea is that a picture changes very little between
> successive frames, and the deltas can minimize the redundant data
> between frames.
>
> Newer encodings like MPEG4 break the image down into even smaller
> squares, and use even more advanced (also, slower) predictive algorithms
> to minimize these repetitions even more.
>
> So, where a MPEG2 file with 480i video for a 2 hour movie might take up
> 4G of disk space and is playable on any DVD player, a MPEG4 transcoded
> version of that same digital movie might take up 1G of disk space but
> you will need a fast PC to be able to play it back.
>
> This is even more true when you step into the world of 720p or 1080i
> video, where videos may commonly be 20G in size on a blueray or HDDVD
> disc. The blueray/hddvd player has enough CPU to play it back largely
> due to hardware assistance. While you can transcode such video into
> I.264 video formats and save quite a bit of disk space (20G HD file
> might get you a 4G I.264 file), you likewise need quite a bit of CPU or
> hardware assistance to play back that transcoded file due to the
> additional overhead of the algorithms involved.
>
> That being said.. there's nothing wrong with losing quality in the
> transcoding, you will generally get a viewable movie as a result, but
> quite a bit of detail may be lost in your conversion.
>
> If you think you are going to convert a 480i 4G DVD (MPEG2) to a 480i
> 740M VCD (MPEG2) without some kind of loss, you're kidding yourself.
>
> The big tradeoff between MPEG2 and MPEG4 is horsepower. You need much
> more CPU power to compress video to an MPEG4, as well as more CPU power
> to decompress the video on the fly as you play it.
>
> So, given these constraints, you can trade off quality or you can make
> things harder to play back, but you always have to trade something off
> to save the additional space.
>
> A lossless compression algorithm simply doesn't buy you much.
>
> Hope this helps.
>
> - Ian C. Blenke <ian@blenke.com> http://ian.blenke.com
>
>
> -----------------------------------------------------------------------
I guess the correct term I should have used is transcoding a DVD. I
don't care for compression since, as you pointed out, MPEG2 is already
compressed. It would be pointless for me to bzip2 an ISO only to
extract what I might want to watch. I can't imagine much gain
(anybody try?), and the decompression time would only annoy me as I
wait.
What I was looking for was transcoding a DVD into a better, another
more-compressed and lossy format, like MPEG4 or H.264 (not sure what
I.264 is). I'm OK sparing the extra CPU cycles for the decoder since,
while watching a movie, I'm not doing anything else. It seems the
menu preservation is something altogether different :(
/mario
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