The argument from illusion against direct realism is almost always phrased in terms of something like a stick in a puddle appearing bent. I find this very unconvincing given that the misleading part of it is in the world, that is, refraction. Something like the Müller-Lyer illusion on the other hand seems to have all the ambiguity in the perceiver's mind.

The two horizontal lines are the exact same length, and it's only the arrows on the end that make you think otherwise, and yet, unlike refraction, these arrows don't actually change how the light moves or anything like that which would change the appearance of the horizontal lines to some kind of unbiased observer. So, how does the direct realist explain this? It seems impossible for them to explain it given that it's a bias purely in the brain affecting our perception.