Display Conditions

Display conditions are used to limit when a primitive, globe overlay, or screen overlay is rendered. This can be based on the distance from the camera to an object, such as a primitive or globe overlay, the distance from the camera to a position, the camera's altitude, pixel size, the current time, or other conditions. These fine grain conditions can be combined using Boolean logic with a composite - that is, a condition of conditions. For example, several time intervals can be or-ed together.

For efficient evaluation, primitives that share the same condition can be combined using a composite primitive. For example, 1,000 models with the same condition can be combined in a composite primitive so that they are quickly rejected. See the composite primitive overview.

Several fine grain conditions are available, including:

Distance from camera to object (DistanceDisplayCondition). The object is rendered if the distance is within [Minimum Distance, Maximum Distance].
Distance from camera to position (DistanceToPositionDisplayCondition). This allows multiple objects to show/hide based on the camera's distance to a position. For example, a filled country and surrounding wall may have different center positions, so the distance condition would result in each object turning on/off at a different camera position.
Camera Altitude (AltitudeDisplayCondition). The object is rendered if the camera's altitude on the given central body is within [Minimum Altitude, Maximum Altitude].
Pixel Size (PixelSizeDisplayCondition). The object is rendered if its bounding sphere's diameter, projected onto the screen, (or in the case of screen overlays, the area of its bounding rectangle) is within [Minimum Pixel Size, Maximum Pixel Size].
To improve performance, this can be used to implement a level of detail algorithm that takes into account the field of view. Several primitives with varying degrees of detail (e.g. number of vertices) are created, each with a pixel size display condition. The most detailed primitive has a pixel interval with the highest pixel sizes. Less detailed primitives have display conditions with smaller pixel sizes. When the primitive occupies many pixels, a detailed primitive is rendered. When a primitive occupies few pixels, a less detailed primitive is rendered since a user is unlikely to notice the difference between a high detail and low detail primitive when it only occupies few pixels.

Time Interval (TimeIntervalDisplayCondition). The object is rendered if the current animation time is within [Minimum Time, Maximum Time].

A display condition is applied to an object as shown in the following code example:

[C#] Copy Code

IAgStkGraphicsSceneManager manager =
 ((IAgScenario)root.CurrentScenario).SceneManager;
              IAgStkGraphicsAltitudeDisplayCondition condition =
 manager.Initializers.AltitudeDisplayCondition.InitializeWithAltitudes(0,
 1000); // Minimum Altitude, Maximum Altitude
              primitive.DisplayCondition = condition as
 IAgStkGraphicsDisplayCondition;

When an object's DisplayCondition property is not null, the condition is evaluated before the object is rendered. The object is rendered only if the condition is met. In this case, the primitive will only be rendered if the camera's altitude is within [0, 1000] meters from the Earth's surface.

The same display condition instance can be assigned to multiple objects.

Display Condition: Composite

Display conditions can be combined with Boolean logic using composites, CompositeDisplayCondition. Hierarchies of composites can represent expressions with parentheses, but 9 times out of 10, simple and / or combinations of conditions will do the trick.

The following example creates a composite of conditions that allows the primitive to be rendered during two time intervals.

[C#] Copy Code

IAgStkGraphicsSceneManager manager =
 ((IAgScenario)root.CurrentScenario).SceneManager;
                IAgDate start1 = root.ConversionUtility.NewDate("UTCG", "30 May 2008
 14:00:00.000");
                IAgDate end1 = root.ConversionUtility.NewDate("UTCG", "30 May 2008
 14:30:00.000");
                IAgDate start2 = root.ConversionUtility.NewDate("UTCG", "30 May 2008
 15:00:00.000");
                IAgDate end2 = root.ConversionUtility.NewDate("UTCG", "30 May 2008
 15:30:00.000");

                IAgStkGraphicsTimeIntervalDisplayCondition time1 =
 manager.Initializers.TimeIntervalDisplayCondition.InitializeWithTimes(start1,
 end1);
                IAgStkGraphicsTimeIntervalDisplayCondition time2 =
 manager.Initializers.TimeIntervalDisplayCondition.InitializeWithTimes(start2,
 end2);
                IAgStkGraphicsCompositeDisplayCondition composite =
 manager.Initializers.CompositeDisplayCondition.Initialize();

                composite.Add((IAgStkGraphicsDisplayCondition)time1);
                composite.Add((IAgStkGraphicsDisplayCondition)time2);
                composite.LogicOperation =
 AgEStkGraphicsBinaryLogicOperation.eStkGraphicsBinaryLogicOperationOr;

                primitive.DisplayCondition = composite as
 IAgStkGraphicsDisplayCondition;

If a large number of conditions are added to a composite, the composite can become expensive to compute, especially if a large number of primitives have a reference to the same composite. In this case, it is recommended to put the primitives into a composite of primitives and assign the composite condition to the composite primitive.