Main difference between encoding and decoding of a video file

What is the main difference? Suppose i have a vide开发者_开发问答o in the format .flv and i want to convert it to format .h264 using avcodec.dll. Will I be encoding or decoding it? I am really confused. Any clear answer will be appreciated.

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there're two types of video formats - uncompressed (raw video formats like RGB or YUV or whatever else) and compressed (like H.264 or WebM etc.). typically there's no direct transcoding from one compressed format to another, so you need to use common denominator - raw video (not compressed). You have to decode (decompress) it and then encode (compress) to another format.

for analogy - think you have zip archive and you need to make it rar archive - first you'll unzip your file(s) and then compress by rar

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You will probably be transcoding it.

If not, then you will first decode it, and than encode it again.