6M views · Nov 28, 2020 · 20:08 · 3 HN recommenders · 4 mentions · discussed 2021–2022
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.
Why Hacker News points people here: HN recommends it as an approachable explanation that makes the famous algorithm finally make sense.
“This video is approachable and explains it pretty well”HN comment, 2021
“Here's the very fun video if anyone wants to take a look”
More interesting video about the famous Quake code
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
“Once that coincidence(?) was explained, Q_rsqrt finally made sense.”
Explains float bits as logarithm, making Q_rsqrt make sense
> 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)
“This video is approachable and explains it pretty well”
Approachable explanation of why fast inverse square root works
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
“I've heard good things about the Fast Inverse Square Root Calculation algorithm”
Famous algorithm cited as great code
print("Hello World") /s But I've heard good things about the Fast Inverse Square Root Calculation algorithm in the Doom 3 source code. this video
