^Don't read that.^
Let's talk about perception filters. This seems to be a very common occurrence in the Slender-afflicted community, I have even noticed several specific instances of it I have been directly involved with. Despite their obvious uses and purposes, perception filters should not be confused with either invisibility, or the actual removal of information. In fact, from a purely theoretical standpoint, invisibility would actually be considered an easier accomplishment, to make something invisible you simply need to bend light around it, or perhaps project something on top of it; but finding a way to censor data so that only specific people can see, or fail to see, it, without using some kind of identifier like a password should be far more difficult. From what I can tell, there seems to be two kinds of perception filters (well obviously you could argue this several different ways, but for explanatory purposes let's assume that the most important thing is how many people are filtered): individual filters, and group filters. As I believe the former is easier to explain I'll begin there..
Individual perception filters are filters that are placed over a single individual that result in them not being able to see something, or altering their perception of it. It's relatively simple to explain, because the filter can easily be said to originate with the individual. Individual perception filters deal with the mind of the afflicted person, they set up barriers created by an external, or sometimes internal, entity to block or alter something. These can be very broad alterations, like not being able to see anything on a specific subject, to very specific changes, such as not being able to locate a door right in front of you. However, there is likely little variation from instance to instance, each encounter with whatever was being filtered should be the same as the past encounter, unless the filter has been cracked or destroyed.
Group filters are more complicated, in that they likely stem from the thing being filtered, rather than being a case of multiple individual filters (though such possibilities are not out of the question). A great example to use when talking about this is how nobody perceives the Tall Guy quite the same (if at all). While he has appeared in "plain view" of several non-hunted people before, most instances show him not being perceived at all by them, though in some cases witnesses have apparently seen him as a regular person. This shows how the filter changes a bit from person to person, and sometimes from encounter to encounter by the same people. I believe this is because the filter doesn't take place in the affected's heads, but rather comes from Slender himself. This also explains why the effect fails when viewing it back on a video tape, without the source there to exert the actual filter, people can perceive what was actually there. This becomes tricky, however, when filters take on a more layered approach, with the sharp dressed man there is only really the hunted and the ignorant, and, assuming he marks his victims in some kind of physical or psychological way, there could be a variety of ways the filter could just pass over the proper group. But when it comes to other things, where the differences between those subject to the filter and those who are not aren't so clear, things get a bit more confusing. Because I'm lazy, and I haven't slept in a good 20 hours now, I'll save you my highly speculative thoughts on how that works, looking back it's not like my better grounded ideas have had the best wording thus far.
Well that's all I've got for you on perception filters, I actually typed out another paragraph, but then I read back over it and realized it was virtually unreadable. Anyways, I'm sure none of you can see this post, except for Gargoyle, who's probably mad I made a post in this topic a full 12 hours before he makes one on the exact same subject (please be true, I feel like we're going for some kind of record here).
See you around