[saurik]

A trademark isn't "you don't get to use this word because I own it now", it is "you don't get to use this word in this context as it is confusing". If you want to name your restaurant Ludwig, go ahead: you just can't name your filter Ludwig. With the exception of Lo-Fi, where maybe you could make a defense, these names are non-obvious and have never been in common use to describe a set of modifications to photographs. Just because Clarendon isn't a unique word does NOT have ANYTHING to do with whether or not you could trademark it for something.

You can't just say "durrrr... I've heard this word before!" you have to actually show that that word has been connected to that context and isn't some otherwise unique usage, and I simply don't see how you are going to claim that for these words: if you show those filters to people and ask them to describe them, the only reason they would say "Clarendon" is because of Instagram's prior usage carefully associating that word with that filter behavior: if you believe otherwise you have to show THAT, not that the word itself has been uttered by someone in the past.

[jrockway]

Indeed. Microsoft Windows and those holes in your wall have coexisted for decades without any trouble. (Not so for Mike Rowe's software company, though: https://en.wikipedia.org/wiki/Microsoft_v._MikeRoweSoft)

[YPCrumble]

Why is it confusing to use “Ludwig” as a filter? Nobody is going to get confused what site they are on based on the name of a filter. In fact, it’s less confusing than having to rename filters for each product.

[kitsune_]

I get that it's about context. But to me this is meaningless, why should any company be able to restrict the usage of common words in a business context?

As a sidenote, Lo-fi has definitely been used in the context of photography before Meta decided to use it as a filter.