People do not expect sudden changes

One of the downsides of Virtual Reality is that people sometimes get motion sickness. This happens when the vestibular system feels movement but the eyes do not see this. Alternatively, of course, when the reverse happens, the eyes see that you are moving but your vestibular system says otherwise. The body will react to this by thinking that something's wrong, probably some kind of poisoning, and will try to resolve this by getting rid of this poison. This is not the kind of reaction you want your users to have when using your app.

When you limit the amount of movement in a virtual environment, users will probably be alright, but still there's a level of discomfort when scenery suddenly changes or when items move about.

In augmented reality, this does not happen that much. After all, the user still sees the physical world, so the motion sickness will not happen--the movements the user makes match what they see.

However, we have other issues that we need to take into account. The level of realism in HoloLens apps is quite high. This means that people can get startled by what they see. Of course, this could be intentional; if you are writing an app that is supposed to scare users, this can be really powerful, but most apps are not written for this kind of audience, and you need to be careful not to startle your users.

In the real world, objects never materialize out of thin air. It never happens that one moment a table is empty and the next moment, there's something there. Things can appear fast, but it is never instantaneous. There is always a transition, no matter how short the time period in which this may occur. Your apps should do that as well. This guideline is not only valid for HoloLens apps, but also for any other platform you might write for, and it is so much more important for the HoloLens. After all, we want to have the user feel as if the virtual world is an actual part of his universe and thus we need to ensure that things feel natural. Yes, we can have things happening that are not possible in real life, but they still need to follow some of the rules in our universe; not immediately appearing without a transition is one of those rules.

You can choose many ways to have a new object appear in your scene:

• You can have it move in from the left, right, top, or bottom. Do not use this too much; when objects suddenly appear from apparently behind the user, they will feel as if there is something or someone behind them who makes this happen. This will result in a paranoid-like feeling. Again, if this is your goal, this can be a strong effect, but it's better to not use this most of the time.
• Have an object appear by starting as a bright dot somewhere and grow from that dot into the object. This transition can be slow (several seconds) or it can be fast (under half a second), but this gives a more relaxed feeling to the user.
• Have the object change its transparency from completely see-through to completely opaque. Again, you decide on the speed this is done at.

When you want objects to disappear, you can reverse this, but ensure that you use the same technique you used when you had it appear. Do not mix this. Again, we want realism!

Another thing to consider is sounds. Things in the real world make a noise when they move about, so why not do the same thing in your virtual world? Having a very faint 'pwop' sound when an object appears makes it more realistic, especially when you use spatial sound. This has the advantage that you automatically have you users' attention drawn to the object when it appears at the edge of his field of view or even outside it.

There are, of course, several other ways you can simulate the objects appearing, and you should, by all means, invent your own, but these three are the most used.