user30076: I want to compress a video to store it for backup purpose.
By that, I mean that I don't need to play video when it is compress format. So I don't want to lose quality or something like that, just compress it.
I have tried to compress with 7zip, but size not decrease. Is it possible to compress a video ? 
For example, I use 7zip on a 3.92Go mkv video. I try :

zip format : 3.92Go
gz format : 3.88Go


user29988: Generally no. Compression algorithms search for repetitive data and replace it with a shorter code and keep a dictionary with all codes. Compressing text works excellent because in longer texts a lot of words repeat. 
With video this work has been done most of the time already. Usually the encoder is  performing a similar operation: looking for repeating patterns and replacing them with a shorter code. Most video files you see are already compressed, most repetitive patterns already encoded and therefore any further compression won't do much.
Sometimes re-encoding videos with a much newer encoder saves a lot of space, but this process is always a bit lossy and requires more work.
You could check if the files have any additional unwanted audio tracks and remove them, like tracks in languages you don't speak anyway.

30076: <p>I want to compress a video to store it for backup purpose.</p>

<p>By that, I mean that I don't need to play video when it is compress format. So I don't want to lose quality or something like that, just compress it.</p>

<p>I have tried to compress with 7zip, but size not decrease. Is it possible to compress a video ? </p>

<p>For example, I use 7zip on a 3.92Go mkv video. I try :</p>

<ul>
<li>zip format : 3.92Go</li>
<li>gz format : 3.88Go</li>
</ul>
 29988: <p>Generally no. Compression algorithms search for repetitive data and replace it with a shorter code and keep a dictionary with all codes. Compressing text works excellent because in longer texts a lot of words repeat. </p>

<p>With video this work has been done most of the time already. Usually the encoder is  performing a similar operation: looking for repeating patterns and replacing them with a shorter code. Most video files you see are already compressed, most repetitive patterns already encoded and therefore any further compression won't do much.</p>

<p>Sometimes re-encoding videos with a much newer encoder saves a lot of space, but this process is always a bit lossy and requires more work.</p>

<p>You could check if the files have any additional unwanted audio tracks and remove them, like tracks in languages you don't speak anyway.</p>
