Image decoding and elaboration

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image capturing and elaboration

What is elaboration? Do you mean processing?

I know that arduino’s atmega are not able to do that,

Good.

Video comes in various formats, what you have to do is to digitize that video and read it into a computer as fast as it comes from the video source. You then store it in memory and you can then do image processing of some sort on that.
A video digitiser can feed into it’s own memory and then that is called a frame grabber. These are normally expensive.

The simplest way to do some image processing is to get a WebCam and run a language called Processing on your computer. If you want to do this on a small board then the Raspberry Pi will allow you do do this for not much money.