Since the QViewer can hold many categories and events at the same time, the display can become cluttered with many short events, many small events, very large events, or events in categories far away from the center of the display . With too many events, the speed at which the display can be redrawn can slow down. The QViewer has a number of mechanisms to reduce clutter and confusion. These are an important part of the experience of using the QViewer and are known collectively as data hiding.
One of the hiding mechanisms is directly controlled by the user. The popup menu for an event may have a section called "level of detail" each name in the list is associated with a number. Each event in a category has a range defined by two numbers. When the level of detail number is between these, the event is shown. Often an event's range is 0-100, which means that the event is always displayed.
Although this was designed to allow more or less detail to be visible at the user's request, the author of a time line can assign any meaning to the level numbers. At level zero, the US Census timeline shows a graph of the change in national population. At each higher level it graphs some of the constituent data in various ways.
|The US Census timeline shows the rural/urban populations at level one and population by region at level three.|
It is up to the author of a timeline to inform the users if manual level of details ranges are being used and whether they have any special meanings.
Other hiding mechanisms are linked to the movement and scaling controls. Most of the mechanisms allow events to gradually fade away rather than have events abruptly vanish.
Each mechanism has a range between where fading starts and where events become invisible. These ranges are controlled by internal variables that have reasonable default values. These defaults can be modified by the user. However, there is no single setting that is ideal for all sets of data, so authors can set these values on each of the categories they define. Sometimes, thousands of tiny events are exactly what is required. In that case, an author can turn a hiding mechanism off completely.
The following hiding mechanisms are controlled by movement and scaling actions:
- Categories can be hidden as they move away from the center of the screen.
As the category moves off screen, it becomes fainter and then disappears entirely.
As the center of the display area moves away from a category, it is less likely that the category is providing useful information and it may gradually disappear.
- Categories can be hidden when they become too small.
The regiments and brigades in the first image gradually disappear. Then the divisions disappear, leaving only the entire army.
As categories are shrunk to occupy only a few pixels on the screen, they are not likely to provide much information to the user. They may also get in the way of features, such as images, that are attached to events in their parent categories.
- Categories can be hidden when they become too large.
The Civil War category gradually fades away as it becomes larger.
Sometimes, categories can become much larger than the screen size. When this happens, it is often nested categories that are interesting and the larger category gets in the way.
- The time scale can be mapped to a range of integers.
The USBaseball categories are designed to show the events for each game when the time scale reaches eight.
A mapping from time scale to an integer can be attached to a category. Events are shown or hidden based on this integer.
- As events with an audio file move away from the center of the screen, the audio volume can decrease.
Audio is also subject to a kind of data hiding. As the edges of the event with the audio moves away from the center of the screen, the volume of the sound can diminish at a given rate. This allows sound to be localized to part of the virtual space.
- Labels can be hidden as they get farther from the viewpoint.
Only a few of the labels nearest the viewpoint are shown.
Labels take up a lot of display space and are not scaled with distance, so their appearance should be regulated closely to avoid clutter. They may also need to be arranged when they are covered by labels of closer events. Unlike other hiding mechanisms, labels do not fade or get more transparent-- they are either drawn or not.
There are variables available to the author of timelines that control hiding behavior. The mechanisms described here are provided with reasonable default values. The user can change these defaults with the preference editor. Authors can tune the values and override the default values in their category definitions.