---
tags: rav1e, tuning
title: Per-speed rdo-lookahead-frames optimization
---

This is the third story about how we made rav1e up to 2x fast in 3 weeks. For the first one go [here](https://dev.to/barrbrain/video-encoder-rollback-optimization-in-rav1e-4d5k) for the second go [here](https://dev.to/luzero/temporal-rdo-update-optimization-2pf1).

## Speed levels

**rav1e** have 11 speed levels, 0 to 10, each tuned to give reasonable quality to speed trade-off for each use-case.

Previously, by default all speed levels had `--rdo-lookahed-frames` set to 40, which at time was a reasonable decision as it gave encoder plenty of lookahead to base coding decision on.

In recent benchmarking of encoder settings we discovered that high and constant value of lookahead is excessive for higher speeds. Relative time spend on rdo lookahead increase with increasing encoding speed preset and gains decrease.

## Speed, Quality, BD-rate

In changing encoder presets we need to balance 2 things: Speed Gain and Quality Decrease.

As speed gain we took average difference of encoding time beetween baseline and different `--rdo-lookahed-frames` settings, in a range from 30 to 10.

Quality difference was measured by BD-rate (Bjontegaard rate). It allows measurent of bitrate difference beetween codecs or features of codec at same quality, for used metric. Rate change is computed as the average percent difference in rate over a range of qualities.

In simple terms, how much more bits encoder need to achieve same quality, on average.
- **Negative** values of BD-rate: bit-rate saves, **better**
- **Positive** values do BD-rate: bit-rate expenses, **worse**

## Per-speed level rdo lookahead

This graph shows what average speed difference is espected by changing `--rdo-lookahed-frames` from default 40.
As we can see, biggest change occurs at speed 10, around 12% for `--rdo-lookahed-frames` 10, after that speed up is steady for speeds 9 to 5, and after that decrease for speeds 4 to 1.

Now, let's look at quality trade-off.


As we can see, speeds 10 and 9 have lowest impact on quality, for speed 9 data even show that `--rdo-lookahed-frames 30` have negative BD-rate, which is of course just noise of data. Given this, it's reasonable to set `--rdo-lookahed-frames 10` for speeds 10 and 9.

After speed 9 BD-rate difference increase, especially for lowest value of 10, and ~1% of BD-rate difference is sensible trade-off, we found reasonable to set `--rdo-lookahed-frames 20` for speeds 8,7,6. Speeds lower than 6 are high quality presets and it's important to make them faster but also make them good, so we decided to set `--rdo-lookahed-frames 30` for speeds 5,4,3 as it gives small speed up with marginal difference in BD-rate, and left highest quality presets unchanged.

Final values:
For speeds 10,9 -- `--rdo-lookahed-frames 10`
For speeds 8,7,6 - `--rdo-lookahed-frames 20`
For speeds 5,4,3 - `--rdo-lookahed-frames 30`
For speeds 2,1,0 - `--rdo-lookahed-frames 40`