Alfheim is a fork (or a rewrite depending on your views on the ship of Theseus) of Hesperus which is a fork of Phosphor
Alfheim aims to be compatible in all scenarios, and if for any reasons a something is fundamentally incompatible, a warning will tell you what is happening instead of crashing with hardly any information.
Alfheim also has a lot of optimizations of all sizes over Phosphor/Hesperus it uses slightly less memory overall.
Alfheim also improves client side performance unlike Phosphor/Hesperus, by reducing by lighting related block updates by a factor of 27. To improve client side performance, Alfheim also limits how much lighting updates are performed each frame. In addition, Phosphor/Hesperus unnecessarily forces lights to be updates before each frame which Alfheim doesn't.
Alfheim also fixes more vanilla lighting issues than Phosphor/Hesperus, here is a list of all fixes and where they come from:
Fixed in Alfheim:
Fair question, the reason is simple, Alfheim compared to Hesperus isn't just a fork to fix bugs, it is my continuation of it. And as seen in the past, my continuations of mods aren't just modifying some things, they are pretty invasive and basically change everything internally.
It wasn't this way at first, I wanted to PR some stuff to Hesperus but after doing parts of it, it became clear that it had grown bigger than a PR. So then I decided to do Alfheim, for now it includes optimizations, bug fixes and compatibility fixes, but later down the line I have more plans.
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