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Fast Inverse Square Root — A Quake III Algorithm
A 20-minute video by Nemean about algorithms and data structures, on the Mathematics shelf. It came up in r/math, in a thread called “What are some of your favorite math explainer videos that aren’t from mathologer, numberphile, or 3b1b?”. It is pitched at intermediate level.
Nemean, Video, 20 min, 2020, on the Mathematics shelf
In this video we will take an in depth look at the fast inverse square root and see where the mysterious number 0x5f3759df comes from. This algorithm became famous after id Software open sourced the engine for Quake III. On the way we will also learn about floating point numbers and newton's method.
Study notes
Intermediate, Derivation. Step 8 of 10 in How a computer works
Helpful first
- Basic programming
- Scientific notation
What it builds
- Recognize why inverse square roots matter in graphics.
- Use an approximate starting answer.
- Refine the approximation with Newton’s method.
Useful moments
Check yourself: Why can an approximate initial answer become much better after one refinement step?
Notes are based on metadata, Reddit context and selected English caption passages. They are not a review of every claim in the video.
