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Transform contexts

The JCanvas roots a tree of graphical items, starting with a CanvasPane, then layers, figures, and then additional figures and panes to arbitrary depth. Some nodes of this tree define a new transform context or coordinate context. These nodes contain an instance of the TransformContext class, which in turn contains an instance of java.awt.geom.AffineTransform. A canvas pane always defines a new transform context; canvas layers never do, as they use they use the transform context of their containing pane; individual figure classes can choose whether or not to define a new context.

The CanvasComponent interface defines the getTransformContext() method. Any component which defines a transform context should return its instance of TransformContext; components that do not define a new context must call their parent's getTransformContext() method.

The transform context defines the coordinate system in which operations on figures take place. If a figure is contained directly in a layer, geometric operations on the figure are in the transform context of that layer. (Although we are talking about layers, the transform context is in fact defined by the layer's enclosing pane.) Most interactive applications will use this transform context by default, as it can be thought of as the "world coordinates" of the application. If the figure is a descendent of another figure that has a transform context, geometric operations on the figure are in the enclosing transform context -- that is, the transform context of its parent -- and must therefore be transformed to and from layer coordinates when necessary.

The diagram below illustrates a sample of the display tree. If a client takes the bounds of figures A, B, C or F, then the result will be in layer coordinates -- that is, transform context X. However, if the client takes the bounds of D or E, the result will be in the transform context Y, and will need to be transformed by the client to layer coordinates if necessary.

The primary function of a TransformContext is to keep an instance of an AffineTransform and to provide translation between its coordinate space and the containing layer and the screen. In addition, because Java2D does not implement a transform stack, this class provides push() and pop() methods. Components that define a transform context must call these methods when entering and leaving their context during a paint() operation.

A secondary function of TransformContext objects is to cache computed transforms to reduce the overhead of computing new transforms. To support caching, the transform context contains a flag, cacheValid, that indicates the state of cached transforms. If a request for a layer or screen transform is made, and the cache is not valid. the TransformContext recomputes the transform and caches it. There is also a version number in the context, which is incremented whenever the transform changes. This can be used by client components to figure out when to update cached data that depends on the transform.

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