Configure libvpx-vp9 Speed in FFmpeg

This guide explains how to configure the speed and quality trade-off when encoding VP9 video using the libvpx-vp9 encoder in FFmpeg. You will learn about the crucial -cpu-used parameter, its recommended values for different encoding scenarios, and how to structure your FFmpeg commands to optimize encoding times without sacrificing visual quality.

The Speed Parameter: -cpu-used

In the libvpx-vp9 encoder, encoding speed is controlled primarily by the -cpu-used parameter. This parameter determines how much CPU effort is spent on compression efficiency.

• Range: Integer values from -8 to 8 (though the effective range depends on the deadline mode).
• Behavior: Lower numbers result in slower encoding speeds and higher visual quality (better compression). Higher numbers result in faster encoding speeds but lower compression efficiency (larger file size or lower quality at the same bitrate).

Quality and Speed Modes

The behavior of -cpu-used is tied to the -deadline parameter, which sets the overall quality/speed mode:

1. Good Quality Mode (-deadline good): This is the default mode and is recommended for most file-based transcodes.
   • The effective range for -cpu-used is 0 to 5.
   • -cpu-used 0: Slowest speed, highest quality.
   • -cpu-used 1 or 2: Recommended balance for high-quality archival.
   • -cpu-used 4: Recommended balance for general use and faster encoding.
   • -cpu-used 5: Fastest speed in this mode, with some quality loss.
2. Realtime Mode (-deadline realtime): Used for live streaming where low latency is critical.
   • The effective range for -cpu-used is -16 to 16, but values between 5 and 8 are most common.
   • Higher values (like 5 to 8) are necessary to achieve real-time frame rates.

Practical FFmpeg Command Examples

1. Constant Quality (CRF) Encoding

For standard video-on-demand (VOD) encoding where you want a balance of speed and quality, use a -cpu-used value of 1 to 4.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 -cpu-used 2 -c:a libopus output.webm

In this command, -cpu-used 2 is used to achieve excellent quality at a reasonable encoding speed.

2. High-Speed Encoding

If you need to encode quickly and are willing to accept slightly lower compression efficiency, set -cpu-used to 4 or 5.

ffmpeg -i input.mp4 -c:v libvpx-vp9 -crf 30 -b:v 0 -cpu-used 4 -c:a libopus output.webm

3. Recommended Two-Pass Encoding

For the best possible quality-to-file-size ratio, use two-pass encoding. You can speed up the first pass by using a higher -cpu-used value (like 4), and then use a lower value (like 1 or 2) for the second pass to ensure high quality.

Pass 1:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -speed 4 -pass 1 -f null /dev/null

(Note: -speed is an alias for -cpu-used)

Pass 2:

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 2M -speed 1 -pass 2 -c:a libopus output.webm