[FFmpeg-user] Why does the bit rate get reduced significantly when re-encoding to same format?

[David Varghese]

Hi ,

I have a media file having mpeg4 as video format and amrnb as audio format
. I wanted to apply some effects (brightness,contrast etc) to the video and
re-encode it to the same format and no changes to audio stream(only copying
the audio packets) ,  using ffmpeg.

The steps I followed were ,

1. Open the input file and initialize the AVCodecContext for the video
stream of input file ater finding the decoder .
2. Created an output file and added new streams to it for video and audio
streams.
3. Since I wanted same configurations for the new file I initialised the
AVCodecContext of the output streams using  avcodec_copy_context() .
4. Decoded the video frames and re-encoded the frames after applying some
changes in Y,U,V components and just copied and remuxed the audio frames to
the output file .

I was able to re-encode the video frames succesfully but I found a
significant decrease in bit rate of video streams in the output file .
Infact my input video  bitrate was 10032kb/s and output video bitrate
reduced to 675kb/s . Also the file size got reduced from 46MB to 3.1MB !!!

I also tried without applying any effects ie,just re-encoding video frames
after decoding .

Am I doing something seriously wrong here ??

Note : I followed
http://ffmpeg.org/doxygen/trunk/transcoding_8c-example.html for re-encoding
.







-- 
Thanks and Regards ,
David Varghese