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”
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The arguments -c:v hevc_nvenc -qp 22 in the FFmpeg command you gave define the video codec and rate-control technique. Let’s analyze these possibilities:
The video codec to be used is specified by the -c:v hevc_nvenc option, which in this instance is set to hevc_nvenc. This means that you are utilizing the hardware-accelerated HEVC (H.265) encoder NVENC from NVIDIA. Hevc_nvenc is the abbreviation for the HEVC codec, while NVENC is the NVIDIA video encoding API.
-qp 22: This option establishes the video stream’s Quantization Parameter (QP) value. QP has to do with the encoder’s rate-control mechanism. However since you’re utilizing a constant QP technique in your command, the QP value is set for the duration of the video.
Higher quality but bigger file sizes are typically the outcome of a lower QP value.
Lower quality but smaller file sizes are typically the result of higher QP values.
The rate-control method is a constant quantization method as you’re employing a constant QP method (-qp 22). For all frames, the encoder will employ the given QP value in this case, 22 without dynamically changing it in response to the content. This approach is often used in situations when maintaining a constant quality level is desired.
It’s important to remember that not all situations will benefit from utilizing a constant QP. Using a rate-control technique like constant bitrate (CBR) or variable bitrate (VBR) may be more appropriate in some situations, depending on your unique needs for streaming capabilities, file size, and quality.
Consider options like -b:v for target bitrate setting or -crf for Constant Rate Factor, which is frequently used in the context of constant quality rate control, if you want to investigate other rate-control strategies.
Can you please elaborate it more, I am unable to get your point.
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.
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.
Thanks for your suggestions now all the doubts are cleared Thanks Gumlet community.