3 HN users recommended this explainer3 months2021–2022score 3.5
Years with an HN recommendation, 2007–2026
HN recommends it as an approachable explanation that makes the famous algorithm finally make sense.
A walkthrough of the Quake III fast inverse square root, explaining where the constant 0x5f3759df comes from. It covers IEEE 754 floats, the bit hack that approximates a logarithm, and a Newton's method refinement.
“This video is approachable and explains it pretty well” (HN, 2021)
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: [this video]
More interesting video about the famous Quake code
> 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]: [this video] (~20 minutes)
Explains float bits as logarithm, making Q_rsqrt make sense
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: [this video]
Approachable explanation of why fast inverse square root works
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