... because people are not frogs .
If we had been frogs with eyes sitting on the side of the head, the story might have been different, but humans have their eyes smack in the front of their face , looking straight out.
Evolution optimized homo sapiens for wandering the savannah - moving around a plane - and not swinging through the trees. Today this evolutionary bias shows in comparing the number of people who drive a car versus the number of helicopter pilots: 2D navigation (on the ground) vs. 3D navigation (in the air).
Using 3D on a computer adds a range of difficulties:
- The screen and the mouse are both 2D devices, so we don't get true 3D unless we strap on weird head-gear and buy expensive bats (flying mice)
- It is difficult to control a 3D space with the interaction techniques that are currently in common use since they were designed for 2D manipulation (e.g., dragging, scrolling)
- Users need to pay attention to the navigation of the 3D view in addition to the navigation of the underlying model: the extra controls for flying, zooming, etc. get in the way of the user's primary task
- Poor screen resolution makes it impossible to render remote objects in sufficient detail to be recognizable; any text that is in the background is unreadable
- The software needed for 3D is usually non-standard, crash-prone, and requires an extra download (which users don't want to wait for)
Bad Use of 3D
Most abstract information spaces work poorly in 3D because they are non-physical. If anything, they have at least a hundred dimensions, so visualizing an information space in 3D means throwing away 97 dimensions instead of 98: hardly a big enough improvement to justify the added interface complexity.
In particular, navigation through a hyperspace (such as a website) is often very confusing in 3D, and users frequently get lost. 3D navigation looks very cool in a demo, but that's because you are not flying through the hyperspace yourself. Thus, you don't have to remember what's behind you or worry about what remote objects are hidden by near-by objects. The person giving the demo knows where everything is (the first law of demos: never try to actually use the system for anything; simply step through a well-rehearsed script that does not touch anything that might cause a crash).
Avoid virtual reality gimmicks (say, a virtual shopping mall) that emulate the physical world. The goal of Web design is to be better than reality . If you ask users to "walk around the mall", you are putting your interface in the way of their goal . In the physical world, you need to schlepp between shops; on the Web you teleport through cyberspace directly to your destination using a navigational topology that conforms to user needs (assuming good information architecture, of course).
When to Use 3D
When you visualize physical objects that need to be understood in their solid form. Examples include:
- surgeons planning where to cut a patient: the body is 3D and the location of the tumor has a 3D location that is easier to understand from a 3D model than from a 2D X-ray
- mechanical engineers designing a widget that needs to fit into a gadget
- chemistry researchers trying to understand the shape of a molecule
- planning the layout of a trade-show booth
Sometimes physical objects work better in 2D. A help system explaining how to replace a harddisk in a computer chassis may be better off with a schematic drawing from exactly the perspective that highlights the correct spot. Or use a video of a repair tech who removes the old disk and inserts the new one. Video is 2D with respect to the images but uses sound to enhance the understanding of events (e.g., the satisfying snap when the disk is safely docked). Sounds provides additional dimensionality without navigational overhead because it's synched to the video.
Abstract data sets that have exactly three attributes are sometimes easier to understand in a 3D visualization. But first attempt to simplify the problem and experiment with 2D views - including the comic-strip-like layout of multiple charts that Tufte loves so much in The Visual Display of Quantitative Information .
Finally, entertainment applications and some educational interfaces can benefit from the fun and engaging nature of 3D, as evidenced by countless shoot-them-up games. Note that 3D works for games because the user does not want to accomplish any goals beyond being entertained. It would be trivial to design a better interface than DOOM if the goal was to kill the bad guys as quickly as possible: give me a 2D map of the area with icons for enemy troops and let me drop bombs on them by clicking the icons. Presto: game over in a few seconds and the good guys win every time. That's the design you want if you are the Pentagon, but it makes for a boring game.
See reader comments on this Alertbox: 3D may just be too new and waiting for good designs to be discovered.