Re: compressing compressed video
- From: Chris Johnson <nospam05.2.krishaven@xxxxxxxxxxxxxxx>
- Date: Mon, 22 Aug 2005 08:43:24 +0800
Thus one time, at band camp, James Sungjin Kim wrote:
copx 쓴 글:
Is it possible to noticeably compress already compressed video (DivX/MPEG) even futher using a loseless compression program?
The noticeble compression is not possible in that case,
True.
since the basic principle of the loseless compression is the same as the lossy compression.
Not true.
The reason why DivX/XviD/MPEG/MP4, et al, (also JPEG) don't compress further using a standard archival package is not due to the lossly compression, but due to the final layer of lossless compression applied to the lossy data stream. The results from the lossy stage are typically quite compressible, so running something like LZH over the output before it's turned into a file is built into the standards.
Trick is, this lossless compression has to be decoded in real time in order for the video to play smoothly, so there's a practical limit as to how compressed it can be. This decidedly average compression layer effectively blocks further lossless compression.
Not wanting to sound too much like an ad for Stuffit, but the latest version of the Stuffit archiver can roll back that final lossless compression stage in JPEGs, then compress the original lossy stream much more effectively. However, to view the JPEG you need to unpack it first and recreate the original file.
This "recompression" technique should be able to be applied to all manner of lossy compression formats, but it does add a costly (in time) extra stage to both the creation of the file and any attempt to view it. As such, it's really only useful if the viewing of a video can occur well after transmission (not great for live stuff).
Unfortunately Allume, the company that produces Stuffit, hasn't been able to release support for any new file formats since their JPEG compression technique was released at the beginning of the year, about six months ago. I fear also that they've patented the technique and may be in a position to stop other programmers or companies from developing a solution for popular video formats.
.
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