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<h4><a name="layer-healpix">8.3.27 <code>healpix</code></a></h4>
<p>Plots a table representing HEALPix pixels on the sky.
Each row represents a single HEALPix tile,
and a value from that row is used to colour
the corresponding region of the sky plot.
</p>
<p>The way that data values are mapped
to colours is usually controlled by options
at the level of the plot itself,
rather than by per-layer configuration.
</p>
<p>
<strong>Usage Overview:</strong>
<pre>
layerN=healpix datalevelN=<int-value> datasysN=<value>
viewsysN=equatorial|galactic|supergalactic|ecliptic
degradeN=<int-value> combinerN=<value> transparencyN=0..1
healpixN=<num-expr> valueN=<num-expr> inN=<table>
ifmtN=<in-format> istreamN=true|false icmdN=<cmds>
</pre>
</p>
<p>All the parameters listed here
affect only the relevant layer,
identified by the suffix
<code>N</code>.
</p>
<p>
<strong>Example:</strong>
</p>
<div align="center"><img src="plot2-layer-healpix.png" alt="" align="middle"></div>
<p><pre> stilts plot2sky <strong>layer1=healpix</strong> <strong>in1=simbad-hpx8.fits</strong> <strong>healpix1=HPX8</strong> <strong>value1=NBREF</strong>
<strong>datalevel1=8</strong> <strong>degrade1=2</strong>
projection=aitoff <strong>datasys1=equatorial</strong> viewsys=galactic labelpos=none
auxfunc=log auxmap=cold auxflip=true auxvisible=false auxclip=0,1
xpix=500 ypix=250</pre></p>
<p>
<dl>
<dt><strong><code>combinerN = <value></code> <em>(<a href="http://andromeda.star.bris.ac.uk/starjavadocs/uk/ac/starlink/ttools/plot2/layer/Combiner.html">Combiner</a>)</em></strong></dt>
<dd>Defines how pixel values will be combined if they are
degraded to a lower resolution than the data HEALPix level.
This only has any effect if
<code>degrade</code>>0.
<p>[Default: <code>sum</code>]
</p>
</dd>
<dt><strong><code>datalevelN = <int-value></code> <em>(Integer)</em></strong></dt>
<dd>HEALPix level of the (implicit or explicit) tile indices.
Legal values are between 0 (12 pixels) and
13
(805306368 pixels).
If a negative value is supplied (the default),
then an attempt is made to determine the correct level
from the data.
<p>[Default: <code>-1</code>]
</p>
</dd>
<dt><strong><code>datasysN = <value></code> <em>(<a href="http://andromeda.star.bris.ac.uk/starjavadocs/uk/ac/starlink/ttools/plot2/geom/SkySys.html">SkySys</a>)</em></strong></dt>
<dd>The sky coordinate system to which the HEALPix grid
used by the input pixel file refers.
<p>[Default: <code>equatorial</code>]
</p>
</dd>
<dt><strong><code>degradeN = <int-value></code> <em>(Integer)</em></strong></dt>
<dd>Allows the HEALPix grid to be drawn at a less detailed
level than the level at which the input data are supplied.
A value of zero (the default) means that the HEALPix tiles
are painted with the same resolution as the input data,
but a higher value will degrade resolution of the plot tiles;
each plotted tile will correspond to
4^<code>degrade</code> input tiles.
The way that values are combined within each painted tile
is controlled by the
<code>combiner</code> value.
<p>[Default: <code>0</code>]
</p>
</dd>
<dt><strong><code>healpixN = <num-expr></code> <em>(String)</em></strong></dt>
<dd>HEALPix index indicating the sky position of the tile
whose value is plotted.
If not supplied, the assumption is that the supplied table
contains one row for each HEALPix tile at a given level,
in ascending order.
<p>The value is a numeric algebraic expression based on column names
as described in <a href="jel.html">Section 10</a>.
</p>
</dd>
<dt><strong><code>icmdN = <cmds></code> <em>(<a href="http://andromeda.star.bris.ac.uk/starjavadocs/uk/ac/starlink/ttools/filter/ProcessingStep.html">ProcessingStep[]</a>)</em></strong></dt>
<dd>Specifies processing to be performed on
the layer N input table as specified by parameter <code>inN</code>.
The value of this parameter is one or more of the filter
commands described in <a href="filterSteps.html">Section 6.1</a>.
If more than one is given, they must be separated by
semicolon characters (";").
This parameter can be repeated multiple times on the same
command line to build up a list of processing steps.
The sequence of commands given in this way
defines the processing pipeline which is performed on the table.
<p>Commands may alteratively be supplied in an external file,
by using the indirection character '@'.
Thus a value of "<code>@filename</code>"
causes the file <code>filename</code> to be read for a list
of filter commands to execute. The commands in the file
may be separated by newline characters and/or semicolons.
</p>
</dd>
<dt><strong><code>ifmtN = <in-format></code> <em>(String)</em></strong></dt>
<dd>Specifies the format of the input table as specified by parameter <code>inN</code>.
The known formats are listed in <a href="inFormats.html">Section 5.2.1</a>.
This flag can be used if you know what format your
table is in.
If it has the special value
<code>(auto)</code> (the default),
then an attempt will be
made to detect the format of the table automatically.
This cannot always be done correctly however, in which case
the program will exit with an error explaining which
formats were attempted.
<p>[Default: <code>(auto)</code>]
</p>
</dd>
<dt><strong><code>inN = <table></code> <em>(<a href="http://www.starlink.ac.uk/stil/javadocs/uk/ac/starlink/table/StarTable.html">StarTable</a>)</em></strong></dt>
<dd>The location of the input table.
This may take one of the following forms:
<ul>
<li>A filename.</li>
<li>A URL.</li>
<li>The special value "<code>-</code>",
meaning standard input.
In this case the input format must be given explicitly
using the <code>ifmtN</code>
parameter.
Note that not all formats can be streamed in this way.
</li>
<li>A system command line with
either a "<code><</code>" character at the start,
or a "<code>|</code>" character at the end
("<code><syscmd</code>" or
"<code>syscmd|</code>").
This executes the given pipeline and reads from its
standard output.
This will probably only work on unix-like systems.
</li>
</ul>
In any case, compressed data in one of the supported compression
formats (gzip, Unix compress or bzip2) will be decompressed
transparently.
</dd>
<dt><strong><code>istreamN = true|false</code> <em>(Boolean)</em></strong></dt>
<dd>If set true, the input table
specified by the <code>inN</code> parameter
will be read as a stream.
It is necessary to give the
<code>ifmtN</code> parameter
in this case.
Depending on the required operations and processing mode,
this may cause the read to fail (sometimes it is necessary
to read the table more than once).
It is not normally necessary to set this flag;
in most cases the data will be streamed automatically
if that is the best thing to do.
However it can sometimes result in less resource usage when
processing large files in certain formats (such as VOTable).
<p>[Default: <code>false</code>]
</p>
</dd>
<dt><strong><code>transparencyN = 0..1</code> <em>(Double)</em></strong></dt>
<dd>Transparency with which components are plotted,
in the range 0 (opaque) to 1 (invisible).
The value is 1-alpha.
<p>[Default: <code>0</code>]
</p>
</dd>
<dt><strong><code>valueN = <num-expr></code> <em>(String)</em></strong></dt>
<dd>Value of HEALPix tile, determining the colour
which will be plotted.
<p>The value is a numeric algebraic expression based on column names
as described in <a href="jel.html">Section 10</a>.
</p>
</dd>
<dt><strong><code>viewsysN = equatorial|galactic|supergalactic|ecliptic</code> <em>(<a href="http://andromeda.star.bris.ac.uk/starjavadocs/uk/ac/starlink/ttools/plot2/geom/SkySys.html">SkySys</a>)</em></strong></dt>
<dd>The sky coordinate system used for the generated plot.
<p>Choice of this value goes along with the data coordinate
system that may be specified for plot layers.
If unspecified, a generic longitude/latitude system is used,
and all lon/lat coordinates in the plotted data layers
are assumed to be in the same system.
If a value is supplied for this parameter,
then a sky system must (implicitly or explicitly)
be supplied for each data layer,
and the coordinates are converted from data to view system
before being plotted.
</p>
<p>The available options are:
<ul>
<li><code>equatorial</code>: J2000 equatorial system
</li>
<li><code>galactic</code>: IAU 1958 galactic system
</li>
<li><code>supergalactic</code>: De Vaucouleurs supergalactic system
</li>
<li><code>ecliptic</code>: ecliptic system based on conversion at 2000.0
</li>
</ul>
</p>
<p>[Default: <code>equatorial</code>]
</p>
</dd>
</dl>
</p>
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<hr><i>STILTS - Starlink Tables Infrastructure Library Tool Set<br>Starlink User Note256<br>STILTS web page:
<a href="http://www.starlink.ac.uk/stilts/">http://www.starlink.ac.uk/stilts/</a><br>Author email:
<a href="mailto:m.b.taylor@bristol.ac.uk">m.b.taylor@bristol.ac.uk</a><br>Mailing list:
<a href="mailto:topcat-user@jiscmail.ac.uk">topcat-user@jiscmail.ac.uk</a><br></i></body>
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