FFmpeg libvpx-vp9 Default Encoding Settings

This article provides a clear overview of the default encoding configurations applied when using the libvpx-vp9 encoder in FFmpeg without specifying any extra flags. It covers key parameters such as rate control, target bitrate, CPU usage, pixel format, and threading, helping you understand how FFmpeg processes VP9 video out of the box.

When you run FFmpeg with the basic command ffmpeg -i input.mp4 -c:v libvpx-vp9 output.webm without defining additional parameters, the encoder falls back on a set of built-in defaults. These defaults are generally optimized for compatibility rather than speed or visual quality, often resulting in slow encoding times and low-quality output if left unmodified.

Default Bitrate and Rate Control

By default, the libvpx-vp9 encoder uses One-Pass Average Bitrate (ABR) mode.

• Default Bitrate: If no bitrate (-b:v) is specified, FFmpeg defaults to 256 kbps.
• Quality Implications: A default of 256 kbps is extremely low for modern high-definition video (such as 1080p or 4K), which will result in heavy pixelation and compression artifacts.

Speed and Quality Settings (CPU-Used)

The -cpu-used parameter controls the trade-off between encoding speed and video quality. The scale ranges from 0 (slowest, highest quality) to 8 (fastest, lowest quality).

• Default Value: The default setting is 1.
• Impact: Because the default is set to 1, the encoding process will be exceptionally slow. For practical use, values between 2 and 4 are recommended for a better balance, or 5 to 8 for fast/real-time encoding.

Threading and Multi-Processing

VP9 encoding is highly resource-intensive, but its default threading settings do not utilize modern multi-core processors efficiently.

• Default Threads: The default is 1 thread.
• Row-Based Multithreading: Row-based multithreading (-row-mt 1) is disabled by default.
• Impact: Without manually setting the -threads and -row-mt flags, your CPU usage will remain low, and encoding will take significantly longer.

Pixel Format and Color Space

• Default Format: FFmpeg will attempt to match the input format, but it typically defaults to yuv420p (8-bit, 4:2:0 chroma subsampling) for maximum compatibility with web browsers and media players.

Keyframe Interval (GOP Size)

• Default GOP: The default maximum keyframe interval (Group of Pictures size) is 250 frames. This means the encoder will insert a keyframe at least once every 250 frames, or sooner if a scene change is detected.