Configure limit-modes in FFmpeg libx265

This article explains how to configure the limit-modes option in the FFmpeg libx265 encoder. You will learn what the limit-modes parameter does, how to structure the FFmpeg command line syntax, and how this setting affects your video encoding speed and efficiency.

What is limit-modes in x265?

The limit-modes option is an efficiency optimization feature in the HEVC/H.265 encoder (libx265). When enabled, it instructs the encoder to skip the evaluation of complex rectangular and asymmetric motion partition shapes if the simpler symmetric partitions (like square blocks) already yield a good prediction.

Enabling this feature reduces encoding time with a negligible impact on visual quality.

FFmpeg Syntax for limit-modes

To configure x265-specific settings in FFmpeg, you must pass them through the -x265-params flag. Multiple parameters within this flag are separated by colons.

The basic syntax to enable limit-modes is:

-x265-params limit-modes=1

To disable it, use:

-x265-params limit-modes=0

Example FFmpeg Command

Below is a practical command line example that transcodes an input video using libx265 with limit-modes enabled:

ffmpeg -i input.mp4 -c:v libx265 -crf 23 -preset medium -x265-params limit-modes=1 output.mp4

Combining with Other Parameters

If you want to configure limit-modes alongside other x265 parameters (such as no-info or aq-mode), separate them with colons inside the -x265-params argument:

ffmpeg -i input.mp4 -c:v libx265 -crf 22 -x265-params limit-modes=1:aq-mode=3:no-info=1 output.mp4

When to Use limit-modes

• Enable (limit-modes=1): Use this when you are encoding with slower presets (like slow, slower, or veryslow) and want to speed up the encoding process without noticeably degrading the output quality.
• Disable (limit-modes=0): Use this (or leave it at default) if you require absolute maximum compression efficiency and do not mind the extra processing time required to evaluate every partition shape.