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Converting Between Frames  

Having seen how clever SkyFrames are ([*] and [*]), we will next examine how dumb a basic Frame can be in comparison. For example, if we create two 2-dimensional Frames and use astConvert to derive a conversion between them, as follows:

AstFrame *frame1, *frame2;

...

frame1 = astFrame( 2, "" );
frame2 = astFrame( 2, "" );
cvt = astConvert( frame1, frame2, "" );

then the coordinate transformation which the ``cvt'' FrameSet performs will be as follows:

(1, 2) --> (1, 2)
(2, 4) --> (2, 4)
(3, 6) --> (3, 6)
(4, 8) --> (4, 8)
(5, 10) --> (5, 10)

This is an identity transformation, exactly the same as a UnitMap ([*]). Even if we permute the axis order of our Frames, as we did above ([*]), we will fare no better. The conversion between our two basic Frames will always be an identity transformation.

The reason for this is that, unlike a SkyFrame, all basic Frames start life the same and have axes that are indistinguishable. Therefore, permuting their axes doesn't make them look any different--they still represent the same coordinate system.



next up previous
Next: The Choice of Alignment System
Up: An Introduction to Coordinate System Conversions
Previous: Handling SkyFrame Axis Permutations

AST A Library for Handling World Coordinate Systems in Astronomy
Starlink User Note 211
R.F. Warren-Smith & D.S. Berry
30th April 2003
E-mail:ussc@star.rl.ac.uk

Copyright (C) 2003 Central Laboratory of the Research Councils