Uploaded April 2026 | Updated September 2026, 2 weeks ago

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How did a 1999 shooter pull off high-end lighting on hardware that shouldn’t have been able to handle it? The answer lies in 0x5f3759df. In this video, we tear down the Fast Inverse Square Root, a piece of programming wizardry that uses bit-shifting and a "magic constant" to bypass heavy division and square root calculations. We will explore the Newton's Method approximation, the genius behind Id Software, and why this 25-year-old hack is still legendary today.

Repo: github.com/BitsUndBolts/RSQRT

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▬▬▬▬ *Timestamps* ▬▬▬▬▬▬▬▬▬▬▬▬▬▬
00:00 Quake III Source Code
02:18 History
03:04 Why Does It Exist?
04:00 Floats in Memory
05:56 Fast Inverse Square Root
08:52 Newton Iteration
10:10 386DX-33 testing
17:05 486DX2-66 testing
20:45 Conclusion