#### Description:

A FrameSet consists of a set of one or more Frames (which describe coordinate systems), connected together by Mappings (which describe how the coordinate systems are inter-related). A FrameSet makes it possible to obtain a Mapping between any pair of these Frames (i.e. to convert between any of the coordinate systems which it describes). The individual Frames are identified within the FrameSet by an integer index, with Frames being numbered consecutively from one as they are added to the FrameSet.

Every FrameSet has a `"` base`"` Frame and a `"` current`"` Frame (which are allowed to be the same). Any of the Frames may be nominated to hold these positions, and the choice is determined by the values of the FrameSet`’` s Base and Current attributes, which hold the indices of the relevant Frames. By default, the first Frame added to a FrameSet is its base Frame, and the last one added is its current Frame.

The base Frame describes the `"` native`"` coordinate system of whatever the FrameSet is used to calibrate (e.g. the pixel coordinates of an image) and the current Frame describes the `"` apparent`"` coordinate system in which it should be viewed (e.g. displayed, etc.). Any further Frames represent a library of alternative coordinate systems, which may be selected by making them current.

When a FrameSet is used in a context that requires a Frame, (e.g. obtaining its Title value, or number of axes), the current Frame is used. A FrameSet may therefore be used in place of its current Frame in most situations.

When a FrameSet is used in a context that requires a Mapping, the Mapping used is the one between its base Frame and its current Frame. Thus, a FrameSet may be used to convert `"` native`"` coordinates into `"` apparent`"` ones, and vice versa. Like any Mapping, a FrameSet may also be inverted (see astInvert), which has the effect of interchanging its base and current Frames and hence of reversing the Mapping between them.

Regions may be added into a FrameSet (since a Region is a type of Frame), either explicitly or as components within CmpFrames. In this case the Mapping between a pair of Frames within a FrameSet will include the masking effects produced by any Regions included in the path between the Frames.

#### Constructor Function

astFrameSet

#### Inheritance

The FrameSet class inherits from the Frame class.

#### Attributes

In addition to those attributes common to all Frames, every FrameSet also has the following attributes:

- AllVariants: List of all variant mappings stored with current Frame

- Base: FrameSet base Frame index

- Current: FrameSet current Frame index

- Nframe: Number of Frames in a FrameSet

- Variant: Name of variant mapping in use by current Frame

Every FrameSet also inherits any further attributes that belong to its current Frame, regardless of that Frame`’` s class. (For example, the Equinox attribute, defined by the SkyFrame class, is inherited by any FrameSet which has a SkyFrame as its current Frame.) The set of attributes belonging to a FrameSet may therefore change when a new current Frame is selected.

#### Functions

In addition to those functions applicable to all Frames, the following functions may also be applied to all FrameSets:

- astAddFrame: Add a Frame to a FrameSet to define a new coordinate system

- astAddVariant: Add a variant Mapping to the current Frame

- astGetFrame: Obtain a pointer to a specified Frame in a FrameSet

- astGetMapping: Obtain a Mapping between two Frames in a FrameSet

- astMirrorVariants: Make the current Frame mirror variant Mappings in another Frame

- astRemapFrame: Modify a Frame`’` s relationship to the other Frames in a FrameSet

- astRemoveFrame: Remove a Frame from a FrameSet

Copyright (C) 2021 East Asian Observatory