How to use FFmpeg for video transcoding? What are the important encoding parameters?

Video transcoding is one of the most commonly used features of FFmpeg, involving the conversion of encoders, decoders, and container formats.

Basic Transcoding Commands

bash
# Basic format conversion
ffmpeg -i input.mp4 output.avi

# Specify encoder
ffmpeg -i input.mp4 -c:v libx264 -c:a aac output.mp4

# Specify encoding parameters
ffmpeg -i input.mp4 -c:v libx264 -crf 23 -preset medium -c:a aac -b:a 128k output.mp4

Important Parameters

Video Encoding Parameters

• -c:v: Specify video encoder (e.g., libx264, libvpx-vp9)
• -crf: Constant Rate Factor (0-51, lower value means higher quality, default 23)
• -preset: Encoding speed preset (ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow)
• -b:v: Video bitrate (e.g., 2M, 1500k)

Audio Encoding Parameters

• -c:a: Specify audio encoder (e.g., aac, mp3, libopus)
• -b:a: Audio bitrate (e.g., 128k, 192k)
• -ar: Sample rate (e.g., 44100, 48000)

Hardware Accelerated Transcoding

bash
# NVIDIA GPU acceleration
ffmpeg -hwaccel cuda -i input.mp4 -c:v h264_nvenc output.mp4

# Intel QSV acceleration
ffmpeg -hwaccel qsv -i input.mp4 -c:v h264_qsv output.mp4

Common Scenarios

1. Reduce video quality to decrease file size

bash
ffmpeg -i input.mp4 -crf 28 -preset slow output.mp4

1. Extract video stream

bash
ffmpeg -i input.mp4 -an -c:v copy output.mp4

1. Adjust resolution

bash
ffmpeg -i input.mp4 -vf scale=1280:720 output.mp4

When transcoding, you need to select appropriate encoders and parameters based on the target platform and device to achieve a balance between quality and file size.