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0 pointsepcoa2y ago0 comments

You mean Quake 3 and fast inverse square root? No. And it wasn’t Carmack. https://www.beyond3d.com/content/articles/15/

https://en.wikipedia.org/wiki/Fast_inverse_square_root

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dboreham2y ago· 2 in thread

I wondered about the name of that function: surely it isn't inverse square root? That would be "squared". It's "1 over square root" or something. So down the rabbit hole to see if it was always called that. Yup, in Wikipedia articles at least, but the first paper seems to be by Jim Blinn in 1997, without the term in the title. So let's read the paper... Frustratingly, although I am an IEEE member, and I did subscribe to Computer Graphics and Applications in 1997, it won't let me read the paper without paying again. So curious to hear from folks knowledgeable in the space if this was a mis-naming that stuck or I'm confused about the meaning of "inverse". In my universe we used inverse in the context of functions and their inverses, and "invert" in the context of binary values (1s compliment of x is also called x inverted). Never to describe reciprocal.

epcoaOP2y ago

It was called "inverse square root" as it is just another practical "layer" on top of "inverse" which is just meant to mean multiplicative inverse.

This is the original Blinn 97 BTW. https://web.archive.org/web/20130309073539/http://rufus.hack...

This seems to have been common usage. I never really thought about it as it was just so normal to refer to reciprocal as "inverse" in this context.

> In my universe we used inverse in the context of functions and their inverses

Yes but, the other type of inverse that is so fundamental to CS in general, and especially geometry is a matrix inverse, which is again a multiplicative inverse, so it's not too surprising how this usage became assumed in many contexts.

dahart2y ago

I’ve heard inverse used to mean reciprocal often enough. And it’s technically accurate - a reciprocal is a multiplicative inverse. The problem is mainly that “inverse” is ambiguous, especially in this particular case (an inverse square root is a square!), whereas “reciprocal” is clear and unambiguous. Online you can find lots of uses of inverse, and questions about inverse vs reciprocal. So yes reciprocal is the better, more clear term to use here, but “inverse” to mean reciprocal does happen.

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