How to convert .mp4 file to .webm using avconv tool

Question

I have .mp4 file and I want to convert it to .webm with the same quality by avconv tool,

and vice from .webm to . mp4 with the same quality

document fuzzy to me.

Answer 1

That's not possible. WebM cannot contain MPEG media (video or audio.) So you can't repack ("remux") the media from mp4 to webm. You would need to re-encode them ("transcode" them), with VP8 or VP9 as the video format and Vorbis as the audio format. Thus you can't have the same quality, since transcoding from one lossy format to another always loses some quality.

So your only option is to transcode.