Fast Inverse Square Root — A Quake III Algorithm

Why HN readers recommended it

HN recommends it as an approachable explanation that makes the famous algorithm finally make sense.

This is a summary from the source dataset, not an endorsement by Hacker News or the video creator.

Video & recommendation details

Creator / channel

Nemean

Topic

fast inverse square root

Format

explainer

Level

intermediate

HN recommenders in dataset

3

HN mentions in dataset

4

Recommendation years in dataset

2021–2022

These counts and years describe the included Hacker News records; they are not YouTube views or the video’s publication date.

From the original HN conversations

Excerpts from the recorded comments. Follow each source for the full comment and surrounding discussion.

• Very interesting that this was posted as I literally JUST watched an even MORE interesting youtube upload about this very bit of code just last weekend. Here's the very fun video if anyone wants to take a look: [VIDEO]

• > The bits of a float as an integer literal are proportional to its log2 value (plus a constant), … I watched a video[0] a while back about that. Once that coincidence(?) was explained, Q_rsqrt finally made sense. [0]: [VIDEO] (~20 minutes)

• Apparently the bit pattern of a float interpreted as an int is sorta-kinda it's own logarithm, which is why the famous fast inverse square root from quake 3 works: log(x^-½) = -½log(x) =~ -log(x) >> 1. This video is approachable and explains it pretty well: …