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WorldWind is a collection of components that interactively display 3D geographic information within Java
applications. Applications use WorldWind by placing one or more {@link gov.nasa.worldwind.WorldWindow} components
in their user interface. The WorldWind 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, in the future, SWT-Eclipse. See {@link
gov.nasa.worldwind.awt.WorldWindowGLCanvas}.
In addition to WorldWindow, there are five major WorldWind interfaces. They are:
- {@link gov.nasa.worldwind.globes.Globe} — represents a planet's shape and terrain.
- {@link gov.nasa.worldwind.layers.Layer} — applies imagery or information to a
Globe.
- {@link gov.nasa.worldwind.Model} — aggregates a
Globe and the Layers to apply to
it.
- {@link gov.nasa.worldwind.SceneController} — controls the rendering of a
Model.
- {@link gov.nasa.worldwind.View} — interactively controls the user's view of the model.
In typical usage, applications associate a Globe and several Layers 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 WorldWind or those created by application
developers. Objects implementing 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 WorldWind's components are defined by interfaces. This allows application developers to create their own
implementations and easily integrate them into WorldWind.
The WorldWind Class
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Multiple WorldWind Windows
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Data Retrieval
WorldWind works with enormous quantities 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 WorldWind. The classes that
implement retrieval are {@link gov.nasa.worldwind.retrieve.Retriever} and {@link
gov.nasa.worldwind.retrieve.RetrievalService}.
{@link gov.nasa.worldwind.retrieve.Retriever} encapsulates a single network retrieval request. It is an interface.
The most commonly used concrete Retriever is {@link gov.nasa.worldwind.retrieve.HTTPRetriever}, which
retrieves data via http. Retrievers are typically created by a {@link gov.nasa.worldwind.layers.Layer} to retrieve
the data the layer displays, and by an {@link gov.nasa.worldwind.globes.ElevationModel} to retrieve elevation data.
{@link gov.nasa.worldwind.retrieve.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 {@link gov.nasa.worldwind.WorldWind}, which holds a singleton
instance.
When a retriever's data arrives, the retrieval service calls the retriever's {@link
gov.nasa.worldwind.retrieve.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. {@link gov.nasa.worldwind.layers.TiledImageLayer},
for instance, can convert non-DDS formats to DDS, or simply store the data as-is in the file cache. {@link
gov.nasa.worldwind.terrain.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 WorldWind 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 {@link gov.nasa.worldwind.util.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 {@link
java.lang.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 WorldWind is cached in a {@link
gov.nasa.worldwind.cache.MemoryCache}. MemoryCache enable cached data to be shared among all
WorldWindWindow instantiations in an application. 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 {@link gov.nasa.worldwind.cache.FileStore}
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
WorldWind can determine the displayed objects at a given screen position 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.
WorldWind uses a method similar to drawing to detect objects in the pick region. During picking, the frame
controller invokes each layer's {@link gov.nasa.worldwind.layers.AbstractLayer#doPick(DrawContext, java.awt.Point)}.
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, WorldWind 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 visually. 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 application then
receives the full list of picked items, with the truly visible ones marked as such.
WorldWind provides utility classes to make it simple for layers to participate in this picking scheme. See {@link
gov.nasa.worldwind.pick.PickSupport}
Use of Proxies
A proxy is set by calling {@link gov.nasa.worldwind.Configuration#setValue} for each of the following keys:
- {@link gov.nasa.worldwind.avlist.AVKey#URL_PROXY_HOST} — indicates the proxy host address
- {@link gov.nasa.worldwind.avlist.AVKey#URL_PROXY_PORT} — indicates the port to use on that host
- {@link gov.nasa.worldwind.avlist.AVKey#URL_PROXY_TYPE} — One of the values defined by java.net.Proxy.Type
After these values are set, all retrievals from the network will go through the specified proxy.
Offline Mode
WorldWind's use of the network can be disabled by calling {@link gov.nasa.worldwind.WorldWind#setOfflineMode}. Prior
to attempting retrieval of a network resource — anything addressed by a URL — WorldWind checks the
offline-mode setting and does not attempt retrieval if the value is true.
Path Types
There is only one way to draw a straight line on a plane, but there are several ways to draw a straight line on the
surface of a globe. Most shapes support the following path types:
- {@link gov.nasa.worldwind.avlist.AVKey#LINEAR} — A line interpolated by treating latitude and longitude as
a rectangular grid. The result is a straight line in the Equirectangular
map projection. This type of path is not the shortest distance between points on a sphere, and does not follow a
constant compass bearing.
- {@link gov.nasa.worldwind.avlist.AVKey#RHUMB_LINE} or {@link gov.nasa.worldwind.avlist.AVKey#LOXODROME} —
A line of constant bearing. Such a path is a straight line in the Mercator map
projection. This type of path is not the shortest distance between points on a sphere.
- {@link gov.nasa.worldwind.avlist.AVKey#GREAT_CIRCLE} — A line that follows great circle arc. This is the
shortest path between two points on a sphere.
Set the {@code pathType} attribute to change how the lines of a shape are drawn (for example, {@link
gov.nasa.worldwind.render.Path#setPathType}). The {@link gov.nasa.worldwind.geom.LatLon} class provides utility
methods to calculate points along each type of path.