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Converting Between Frames

Having seen how clever SkyFrames are (§12.1 and §12.4), we will next examine how dumb a basic FrameFrame can be in comparison. For example, if we create two 2-dimensional Frames and use astConvertastConvert to derive a conversion between them, as follows:

196#196

then the coordinate transformation which the “cvt” FrameSetFrameSet performs will be as follows:

103#103

This is an identity transformation, exactly the same as a UnitMapUnitMap (§5.10). Even if we permute the axis order of our Frames, as we did above (§12.4), 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 SkyFrameSkyFrame, 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.