[FFmpeg-user] How to call ffmpeg code without quality loss for video conversion?

[seaguest]

Hello,


I am using ffmpeg library to convert the video format, I want to have 
exactly same output video as done by this following command line:

*ffmpeg -i mv.webm -c:a aac -c:v libx264 mv.mp4*

My first question is how are the bitrates of audio/video determined in 
case they are not specified in command line?

Second question, In the way of calling ffmpeg code, how should I set the 
AVCodecContext properly to make sure that I have same setting as above 
command line?

Because I am having an audio quality loss problem when I call the ffmpeg 
library, whilst the command line works fine.

Please see my detailed question on stackoverflow:
http://stackoverflow.com/questions/37085959/how-to-call-ffmpeg-code-without-losing-quality

best regards