@K. In my course, as I said we have to study direct realism vs indirect realism vs idealism, and two imo very reasonable objections are made to direct realism... the arguments from the variation of perception and from illusion... but both are countered with imo a very weak response in the form of these 'relational' 'secondary' properties... that rather than being they just look that way and look is a relational property. Do you think direct's counter is weak? I think it is... I think it just looks like word play that would allow them to have an answer for every reasonable objection. But I've come up with an objection and I'd like your opinion on whether you think it's sound... so I can use it in my essays/exams 
Accepting one of the primary properties of an object... size ("extension")... I'd call my argument the argument from sensory limitation; we can't be perceiving it directly because we have two eyes; two eyes are necessary to create a 3D perception out of two 2D detectors... so 3D object out there > two 2D detectors > computation > 3D perception in here. In other words if we were perceiving it directly, why would we need two eyes? Is that a reasonable objection?
Accepting one of the primary properties of an object... size ("extension")... I'd call my argument the argument from sensory limitation; we can't be perceiving it directly because we have two eyes; two eyes are necessary to create a 3D perception out of two 2D detectors... so 3D object out there > two 2D detectors > computation > 3D perception in here. In other words if we were perceiving it directly, why would we need two eyes? Is that a reasonable objection?
