Overview | Package | Class | Use | Tree | Index | Help
| NASA World Wind
PREV NEXT | FRAMES NO FRAMES

# NASA World Wind API

World Wind is a collection of components that interactively display 3D geographic information within applications using Swing, AWT, SWT or other user interface toolkits.

See:
Description

Packages
gov.nasa.worldwind |
gov.nasa.worldwind.awt |
gov.nasa.worldwind.geom |
gov.nasa.worldwind.globes |
gov.nasa.worldwind.layers |
gov.nasa.worldwind.layers.Earth |

World Wind is a collection of components that interactively display 3D geographic information within applications using Swing, AWT, SWT or other user interface toolkits. Applications use World Wind by placing one or more `WorldWindow` components in their user interface. The World Wind components are extensible. The API is defined primarily by interfaces, so components can be selectively replaced by alternative components.

`WorldWindow` is an interface. Toolkit-specific implementations of the interface are provided for Swing/AWT and SWT-Eclipse (SWT is future). See `gov.nasa.worldwind.awt.WorldWindowAWT`.

In addition to `WorldWindow`, there are five major World Wind interfaces. They are:

- `Globe` &mdash represents a planet's shape and terrain.
- `Layer` &mdash applies imagery or information to a `Globe`.
- `Model` &mdash aggregates a `Globe` and the `Layer`s to apply to it.
- `SceneController` &mdash controls the rendering of a `Model`.
- `View` &mdash interactively control's the user's view of the model.

In typical usage, applications associate a `Globe` and several `Layer`s with a `Model` They then pass that model to a `SceneController` that displays the globe and its layers in a `WorldWindow`. The scene controller subsequently manages the display of the globe and its layers in conjunction with an interactive `View` that defines the user's view of the planet.

The objects implementing the above interfaces may be those provided by World Wind or those created by application developers. Objects implementinng a particular interface may be used wherever that interface is called for. World Wind provides several `Globe` objects representing Earth, Mars and the Earth's moon, and provides basic implementations of `Model`, `SceneController` and `View`.

Most of World Wind's components are defined by interfaces. This allows application developers to create their own implementations and easily integrate them into World Wind.

## The `WorldWind` Class

TODO

## Data Retrieval

World Wind works with enormous quantites of data and information, all of which exist primarily on remote data servers. Retrieval and local caching of that data is therefore a primary feature of World Wind. The classes that implement retrieval are `Retriever` and `RetrievalService`.

`Retriever` encapsulates a single network retrieval request. It is an interface. The most commonly used concrete `Retriever` is `HTTPRetriever`, which retrieves data via http. Retrievers are typically created by a `Layer` to retrieve the data the layer displays, and by an `ElevationModel` to retrieve elevation data.

`RetrievalService` manages a thread pool for retrieval tasks. Objects retrieve data by passing the retrieval service a `Retriever`. The service runs each retriever in an individual thread. Access to the retrieval service is through `WorldWind`, which holds a singleton instance.

When a retriever's data arrives, the retrieval service calls the retriever's `RetrievalPostProcessor`, which was specified to the retriever's constructor. The `RetrievalPostProcessor` is passed the data immediately upon download and determines how to persist it. Persistence and any processing prior to it is object specific. `gov.nasa.worldwind.layers.TextureLayer`, for instance, can convert non-DDS formats to DDS, or simply store the data as-is in the file cache. `BasicElevationModel` just persists the raw data. The post processor runs in the same thread as the retriever, which is neither the event-dispatching (UI) thread nor the rendering thread, but the one created by the retrieval service for that retriever.

Data that has been previously retrieved or is otherwise local (on disk) is brought into memory in a thread separate from the event-dispatching thread or the rendering thread. One of the World Wind conventions is that no code may access the computer's disk in any way during rendering. Therefore loading the data from disk is dispatched to another thread pool, the `ThreadedTaskService`. This service has a similar interface to RetrievalService. Tasks it runs typically read the data from disk and add it to the global memory cache (described below).

One consequence of the disk-access restriction is that determining whether needed data is on disk and can be loaded directly, or is not local and therefore must be retrieved, must not be done in the rendering thread. (A disk access is necessary to determine whether the data exists locally.) Objects that load data therefore follow the convention of first checking the memory cache for the desired data, and if it's not there create a `Runnable` to determine in a separate thread where the data must be drawn from, disk or network. If it's on the disk then the task can simply read it and cache it right away. If it's remote then the task creates a `Retriever` and requests retrieval. Later, after retrieval has placed the data on disk, the situation will be the local case and data can be loaded into memory within the `Runnable`.

## Memory Cache

So that data can be shared among caching objects, most cached data used within World Wind is cached in the `MemoryCache`. `MemoryCache` is a singleton so that cached data can be shared among all WorldWindWindow instantiations in an application, and so that memory usage can be governed. Thus two Earth globes each displayed in a separate window will share any image or elevation tiles that they are using simultaneously. The same would be true of any place name collections. The constraint this imposes is that cached data that is to be shared must base `equals()` and `hashCode()` on fields that are not instance specific to the caching object.

## File Cache

All data persisted to or drawn from the local computer is done so by the `FileCache` No object manages its own storage. The file cache cache manages multiple disk storage locations and unifies access to them. The file cache is a singleton, accessible through the `WorldWind` singleton.

## Picking and Selection

World Wind can determine the displayed objects at a given screen postion in a `WorldWindow`. When the application wants to know what's displayed at a particular point, say the cursor position, it calls a method on `WorldWindow` that accepts the point and returns a description of what's drawn there. In general the application specifies a pick region rather than a single point, with the region a few pixels wide and high and centered on the point. This provides a pick tolerance and allows the user to indicate something close to but not exactly at the screen position. Since several objects may intersect the pick region, descriptions of all these objects are returned to the application. Which of these objects are meaningful is determined by the application.

World Wind uses a method similar to drawing to detect objects in the pick region. During picking, the frame controller invokes each layer's `gov.nasa.worldwind.Layer.toPick(DrawContext)`. As in drawing, the methods are invoked in turn, according to the layer's position in the model's layer list. During the call, each layer is responsible for determining which of its items, if any, are picked. Prior to traversing the layer list, the frame controller sets the current view's viewport to the pick region specified by the application. When a layer identifies an object that intersects that pick region, it adds a description of that object to the draw context's pick list. Once all layers are traversed, the list of picked items is returned to the application.

It's typically not straightforward for a layer to determine which of its contents intersect a screen-space pick region. To do that usually requires transforming the screen point into model coordinates and determining intersection in that coordinate system. But depth values are ambiguous with only a two-dimensional screen point as input, complicating transformation to model coordinates, and geometric intersection determination can be very difficult and time consuming. To overcome this, World Wind implements a widely used method of sampling the window's color buffer to detect intersection, and makes this method easy for layers to use.

The method works as follows: The frame controller precedes a pick traversal by first setting the current view's viewport to the specified pick region and clearing the color buffer in that region. This clearing occurs in the window's back buffer and is therefore not visible to the user. During traversal, each layer draws itself not in its normal colors but in a set of colors that serve as pick identifiers. Since the result of pick traversal is never displayed, the specific colors used don't matter viusally. Each individual pickable item within a layer is drawn with a unique color that makes the item individually identifiable in the color buffer. By reading the region of the color buffer corresponding to the pick region, the specific items intersecting the region can be determined. The layer performs this read and makes this determination after drawing its pickable items.

Since one layer does not know how subsequently traversed layers might overwrite or otherwise affect it once drawn, items it determines have been picked could end up obscured by other layers. The items that intersect the pick region and are visible can be determined only after all layers are drawn. The frame controller therefore reads the final colors from the pick region of the color buffer and passes them to the list of picked items so that those items can compare their pick identifiers with the final colors and mark themselves as "on top." The appplication then recieves the full list of picked items, with the truly visible ones marked as such.

World Wind provides utility classes to make it simple for layers to participate in this picking scheme. See // `PickSupport`

Overview | Package | Class | Use | Tree | Index | Help
| NASA World Wind
PREV NEXT | FRAMES NO FRAMES