How to Configure FFmpeg ip_ratio and pb_ratio

This article explains how to configure the -ip_ratio and -pb_ratio parameters in FFmpeg to optimize video encoding quality and compression. You will learn what these ratios represent, how they impact I-frame, P-frame, and B-frame quality, and how to apply them using practical command-line examples.

Understanding the Parameters

In video compression, frames are divided into three main types: I-frames (intra-coded, full images), P-frames (predicted from previous frames), and B-frames (bi-directionally predicted).

The -ip_ratio and -pb_ratio parameters control the Quantization Parameter (QP) relationship between these frame types. QP determines how much lossy compression is applied; a lower QP means higher quality and larger file size, while a higher QP means lower quality and smaller file size.

• -ip_ratio (I-to-P Ratio): This parameter defines the target quality/quantizer ratio between I-frames and P-frames. It determines how much better the quality of an I-frame should be compared to a P-frame.
• -pb_ratio (P-to-B Ratio): This parameter defines the target quality/quantizer ratio between P-frames and B-frames. B-frames are highly compressed and rely heavily on neighboring frames, so they typically use a higher QP (lower quality) than P-frames.

Default Values

For standard encoders like libx264, the default values are generally optimized for a balance of visual quality and compression:

• -ip_ratio default: 1.40
• -pb_ratio default: 1.30

These defaults mean that P-frame QPs are calculated by multiplying the I-frame QP by 1.4, and B-frame QPs are calculated by multiplying the P-frame QP by 1.3.

FFmpeg Command Syntax

To configure these parameters, you pass them as video options in your FFmpeg command line.

ffmpeg -i input.mp4 -c:v libx264 -ip_ratio 1.40 -pb_ratio 1.30 output.mp4

When and How to Adjust the Ratios

Adjusting these ratios allows you to fine-tune how bits are allocated across different frame types depending on your specific use case.

1. Tuning for High-Motion Video

In high-motion videos (such as sports or action gameplay), B-frames can degrade quickly because references change rapidly. * Action: Decrease -pb_ratio (e.g., to 1.15 or 1.20) to allocate more bits to B-frames, keeping their quality closer to P-frames. * Example: bash ffmpeg -i input.mp4 -c:v libx264 -ip_ratio 1.40 -pb_ratio 1.15 output.mp4

2. Tuning for Maximum Compression (Low Bitrate)

If you need to compress a video as much as possible (such as talking-head videos or static presentations) and do not mind a slight drop in overall quality, you can increase the ratios. * Action: Increase -pb_ratio (e.g., to 1.40 or 1.50) so B-frames discard more redundant data. * Example: bash ffmpeg -i input.mp4 -c:v libx264 -ip_ratio 1.45 -pb_ratio 1.40 output.mp4

3. Tuning for Constant Quality (Uncompressed/Lossless Feel)

If you want almost uniform quality across all frame types, you can set the ratios closer to 1.0. Note that this will significantly increase the output file size. * Action: Set both ratios close to 1.0. * Example: bash ffmpeg -i input.mp4 -c:v libx264 -ip_ratio 1.10 -pb_ratio 1.10 output.mp4