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Recently I have received a very large 2 GB video file that was compressed to 200 MB inside a RAR file. How is this possible?

I have tried this instruction but it did not work.

The codec of the video is H264 - MPEG-4 AVC part10 avc1 Audio: mp4a Sample rate 48000 HZ and Bits per sample: 32 The following is an screenshot from the video statistics:

enter image description here

cool
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1 Answers1

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Video-files in general are NOT very compressible any further using RAR or ZIP. You may be able to shrink a 2% or 3%, but 90% as this seem to be the case here is normally impossible.
(Using a very large dictionary on the compression may help a bit, but there just is very little to gain.)

This is because a video-stream is normally already highly compressed internally and compression on top of compression doesn't gain much.
(Effectively compression removes redundant information. This is also known as removing entropy from the original data. After the compression there is very little entropy left. A second compression simply can't find any more entropy to remove.)

Now, in some cases, a video-file is stored in a form that has very little or no internal compression. In that specific case RAR or ZIP might be able to do a good job.

Screen-capture software is a prime example of software that writes its video-captures in such a low-compression method.
Screen-capture software needs to record and write to disk as quick as possible in order not to miss anything from the action on-screen and also has to do that using as little resources (CPU and RAM) as possible so as not to get in the way of what the user making the recording is doing.
That often means it sacrifices compression (which is very resource intensive and relatively slow) and it writes the video-capture with very little or no compression at all to the video-file.
Some screen-capture programs have an (optional) post-processing step to optimize (aka compress to reduce file-size), but if that is not done you have video-file that is very large, but can be compressed a great deal with RAR or ZIP, especially if you use a large dictionary as well. (Another thing that helps with such screen-captures is that often a lot of the screen-area in the video is static, with only the mouse-pointer moving. This leads to a video that has a lot of entropy that RAR can remove.)

But it is the exception to the rule: Normally you can't (much) compress a video-file.

Tonny
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