/usr/share/doc/libstarlink-ast-doc/node118.html

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">

<!--Converted with LaTeX2HTML 2008 (1.71)
original version by:  Nikos Drakos, CBLU, University of Leeds
* revised and updated by:  Marcus Hennecke, Ross Moore, Herb Swan
* with significant contributions from:
  Jens Lippmann, Marek Rouchal, Martin Wilck and others -->
<HTML>
<HEAD>
<TITLE>Converting Between Frames</TITLE>
<META NAME="description" CONTENT="Converting Between Frames">
<META NAME="keywords" CONTENT="sun211">
<META NAME="resource-type" CONTENT="document">
<META NAME="distribution" CONTENT="global">

<META NAME="Generator" CONTENT="LaTeX2HTML v2008">
<META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css">

<LINK REL="STYLESHEET" HREF="sun211.css">

<LINK REL="next" HREF="node119.html">
<LINK REL="previous" HREF="node117.html">
<LINK REL="up" HREF="node113.html">
<LINK REL="next" HREF="node119.html">
</HEAD>

<BODY >

<DIV CLASS="navigation"><!--Navigation Panel-->
<A NAME="tex2html1654"
  HREF="node119.html">
<IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next"
 SRC="/usr/share/latex2html/icons/next.png"></A> 
<A NAME="tex2html1652"
  HREF="node113.html">
<IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up"
 SRC="/usr/share/latex2html/icons/up.png"></A> 
<A NAME="tex2html1646"
  HREF="node117.html">
<IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous"
 SRC="/usr/share/latex2html/icons/prev.png"></A>   
<BR>
<B> Next:</B> <A NAME="tex2html1655"
  HREF="node119.html">The Choice of Alignment</A>
<B> Up:</B> <A NAME="tex2html1653"
  HREF="node113.html">An Introduction to Coordinate</A>
<B> Previous:</B> <A NAME="tex2html1647"
  HREF="node117.html">Handling SkyFrame Axis Permutations</A>
<BR>
<BR></DIV>
<!--End of Navigation Panel-->

<H2><A NAME="SECTION000125000000000000000"></A><A NAME="ss:convertingframes"></A>
<BR>
Converting Between Frames
</H2>

<P>
Having seen how clever SkyFrames are (&#167;<A HREF="node114.html#ss:convertingskyframes">12.1</A>
and &#167;<A HREF="node117.html#ss:convertingpermutedaxes">12.4</A>), 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:

<P>
<SMALL CLASS="SMALL">
<BR>
<IMG
 WIDTH="323" HEIGHT="128" ALIGN="BOTTOM" BORDER="0"
 SRC="img198.png"
 ALT="\begin{terminalv}
AstFrame *frame1, *frame2;
\par
...
\par
frame1 = astFrame( 2,...
...= astFrame( 2, '''' );
cvt = astConvert( frame1, frame2, '''' );
\end{terminalv}">
<BR>
</SMALL>
<P>
then the coordinate transformation which the ``cvt'' FrameSetFrameSet performs
will be as follows:

<P>
<BR>
<IMG
 WIDTH="154" HEIGHT="90" ALIGN="BOTTOM" BORDER="0"
 SRC="img103.png"
 ALT="\begin{terminalv}
(1, 2) --&gt; (1, 2)
(2, 4) --&gt; (2, 4)
(3, 6) --&gt; (3, 6)
(4, 8) --&gt; (4, 8)
(5, 10) --&gt; (5, 10)
\end{terminalv}">
<BR>

<P>
This is an identity transformation, exactly the same as a UnitMapUnitMap
(&#167;<A HREF="node61.html#ss:unitmapexample">5.10</A>). Even if we permute the axis order of our
Frames, as we did above (&#167;<A HREF="node117.html#ss:convertingpermutedaxes">12.4</A>), we will
fare no better. The conversion between our two basic Frames will
always be an identity transformation.

<P>
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.
<P>
<BR><HR>

</BODY>
</HTML>
libstarlink-ast-doc 8.6.2+dfsg-2 / usr / share / doc / libstarlink-ast-doc / node118.html
