![stereogram pictures stereogram pictures](https://i.ytimg.com/vi/FCSr5lxwtR0/maxresdefault.jpg)
As a general rule, if something moves fast across your field of view, then it's closer to you than something that is moving slower. This is another secondary cue that the human eye uses. The brain doesn't have to perceive it this way, but with parallax scrolling, it's nearly impossible to persuade your brain to perceive it in any other way.Īlso notice that the pillars are moving faster across the field of view than the wall behind it. In this example, it's obvious that the pillars are in front of the wall behind it. The following animated GIF will demonstrate (it may run slow on some browsers but you get the idea). A common flavour of overlaying is called parallax scrolling which typically are seen in horizontal and vertical scrolling shoot-em-ups and platform games. Sometimes you see these sort of anomalities happening if a game messes up its z-buffer or you have your graphics card clocked too high. If a wall was overlaid on top of a person who is standing in front of it, things would appear very strange. Illustrated below is an example where one man appears to be standing behind the other even though both are drawn the same height so there is no perspective. When one picture overlays another, the eye assumes the picture doing the overlay is on top or in front. Size and perspective alone can be effective at representing a three dimensional object, although they are usually used in conjunction with secondary methods such as shading. Here is a Quake screen shot where I have highlighted the main vanishing point. Your brain assumes that the road, the fence, and the lights are all uniform in size, so it interprets the narrowing of them towards the center of the picture as an effect of distance. You probably did pictures like these at school.
![stereogram pictures stereogram pictures](https://i.ytimg.com/vi/6Hc4KBBuzOs/maxresdefault.jpg)
This point is called a vanishing point and is labeled "vp" on the drawing. The lines on the road fill more of your field of view closer to you and grow narrower and narrower until they reach a single point in the distance. Your brain is confused because it can interpret the above image in two different ways: There is one very large monitor behind a much smaller monitor that is closer to you or, there are still two monitors of the same size but they are hanging from the ceiling, or the larger one is floating in the air in front of us. We now see that the image doesn't look right. Now if we do something to break these assumptions by changing the assumed viewing angle. Second, we assume that they are standing the right way up, and not upside down, and that they are being viewed from above. The above example causes our brain to make a couple of assumptions: First, we assume that both monitors are the same size. This example works well because we have two instances of the same object, which your brain assumes are the same size. As shown below, one monitor looks closer than the other because it is larger. This particularly applies if you have two of the same object. The more field of view that an object takes up, the closer it is.
![stereogram pictures stereogram pictures](http://2.bp.blogspot.com/-P0PZXYuyxqY/TyWr92n6krI/AAAAAAAABHI/xZBgbvwqBqc/s1600/stereogram.jpg)
I will cover the secondary depth cues first as these are ones already commonly used in games and in the movies.Īs a very general and often inaccurate rule, larger objects are closer. There are primary and secondary depth cues, or indicators, that the human visual system uses. By depth I am referring to how far away an object is perceived. In order to understand how current 3D imagery methods work, and why they are limited, it is important to have some basic knowledge of how the human visual system perceives depth.