What determines video bitrate and rate-control method if do not specify in FFmpeg command while transcoding a video?

Tech QnA
1

I have been transcoding videos with the following FFmpeg command, I am not passing any option related to rate-control methods, and i am wondering what determines the video and rate-control method for the following command.
ffmpeg -threads 1 -hwaccel auto -extra_hw_frames 3 -i [input] -map 0:a? -map 0:s? -map 0:v -c:v hevc_nvenc -qp 22 -movflags +faststart “[output]_HEVC.mkv”

2

Hey, Encoding options differ from codec to codec but FFmpeg uses information provided in terms of options to determine the rate control method.

3

Can you please elaborate it more, I am unable to get your point.

4

When you use -qp quantization parameter with the transcoding command FFmpeg assumes Constant QP (CQP) as the rate control method. I hope now your confusion is resolved.

5

Here, the -crf parameter denotes the use of the Constant Rate Factor (CRF) rate control method, and when you specify video bitrate with -b:v parameter it assumes the Variable Bitrate (VBR) or Constant Bitrate (CBR) rate control method depending on other options used with it.

6

Thanks for your suggestions now all the doubts are cleared Thanks Gumlet community.