tcube constructs an N-dimensional histogram, or density
map, from N columns of an input table, and writes it out as an N-dimensional
data cube. The parameters you supply define which N numeric columns of the
input table you want to use and the dimensions (bounds and pixel sizes) of
the output grid, as well as any weighting to be applied to each point and
how the weighted quantities in a single bin are to be aggregated together.
Each table row then defines a point in N-dimensional space. The program goes
through each row, and if the point that row defines falls within the bounds
of the output grid you have defined, associates the weight value with the
relevant pixel in the output grid. When all the input values have been
processed, the weights in each pixel are aggregated according to the
requested combination method.
The resulting N-dimensional array, whose pixel values represent an
aggregation of the rows associated with that region of the N-dimensional
space, is then written out as a FITS file. In one dimension, this gives you
a normal histogram of a given variable. In two dimensions it might typically
be used to plot the density or weighted density on the sky of objects from a
catalogue.
As with some of the other generic table commands, you can perform
extensive pre-processing on the input table by use of the icmd
parameter before the actual cube counts are calculated.
See also tgridmap, which does a similar job to this command
but writes the output in table format.
- cols=<expr>
... ...
Columns to use for this task. One or more
<expr> elements, separated by spaces, should be given. Each one
represents a numeric value from the table, provided as a column name or
algebraic expression.
The number of columns listed in the value of this parameter
defines the dimensionality of the output data cube.
- ifmt=<in-format>
Specifies the format of the input table as specified by
parameter
in. The known formats are listed in SUN/256. This flag can be
used if you know what format your table is in. If it has the special value
(auto) (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. This parameter is ignored for scheme-specified tables.
- istream=true|false
If set true, the input table specified by the
in
parameter will be read as a stream. It is necessary to give the
ifmt
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). This parameter is ignored for
scheme-specified tables.
- in=<table>
The location of the input table. This may take one of the
following forms:
- A filename.
- A URL.
- The special value "-", meaning standard input. In this
case the input format must be given explicitly using the ifmt
parameter. Note that not all formats can be streamed in this way.
- A scheme specification of the form
:<scheme-name>:<scheme-args>.
- A system command line with either a "<" character at
the start, or a "|" character at the end
("<syscmd" or "syscmd|"). This
executes the given pipeline and reads from its standard output. This will
probably only work on unix-like systems.
In any case, compressed data in one of the supported compression formats (gzip,
Unix compress or bzip2) will be decompressed transparently.
- icmd=<cmds>
Specifies processing to be performed on the input table
as specified by parameter
in, before any other processing has taken
place. The value of this parameter is one or more of the filter commands
described in SUN/256. 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.
Commands may alteratively be supplied in an external file, by
using the indirection character '@'. Thus a value of
"@filename" causes the file filename 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, and lines which are blank
or which start with a '#' character are ignored.
- bounds=[<lo>]:[<hi>]
...
Gives the bounds for each dimension of the cube in data
coordinates. The form of the value is a space-separated list of words, each
giving an optional lower bound, then a colon, then an optional upper bound,
for instance "1:100 0:20" to represent a range for two-dimensional
output between 1 and 100 of the first coordinate (table column) and between 0
and 20 for the second. Either or both numbers may be omitted to indicate that
the bounds should be determined automatically by assessing the range of the
data in the table. A null value for the parameter indicates that all bounds
should be determined automatically for all the dimensions.
If any of the bounds need to be determined automatically in this
way, two passes through the data will be required, the first to determine
bounds and the second to populate the cube.
- binsizes=<size>
...
Gives the extent of of the data bins (cube pixels) in
each dimension in data coordinates. The form of the value is a space-separated
list of values, giving a list of extents for the first, second, ... dimension.
Either this parameter or the
nbins parameter must be supplied.
- nbins=<num>
...
Gives the number of bins (cube pixels) in each dimension.
The form of the value is a space-separated list of integers, giving the number
of pixels for the output cube in the first, second, ... dimension. Either this
parameter or the
binsizes parameter must be supplied.
- combine=sum|sum-per-unit|count|count-per-unit|mean|median|Q1|Q3|min|max|stdev|stdev_pop|hit
Defines how values contributing to the same density map
bin are combined together to produce the value assigned to that bin. Possible
values are:
- sum: the sum of all the combined values per bin
- sum-per-unit: the sum of all the combined values per unit of bin
size
- count: the number of non-blank values per bin (weight is
ignored)
- count-per-unit: the number of non-blank values per unit of bin size
(weight is ignored)
- mean: the mean of the combined values
- median: the median
- Q1: first quartile
- Q3: third quartile
- min: the minimum of all the combined values
- max: the maximum of all the combined values
- stdev: the sample standard deviation of the combined values
- stdev_pop: the population standard deviation of the combined
values
- hit: 1 if any values present, NaN otherwise (weight is
ignored)
- out=<out-file>
The location of the output file. This is usually a
filename to write to. If it is equal to the special value "-" the
output will be written to standard output.
The output cube is currently written as a single-HDU FITS
file.
- otype=byte|short|int|long|float|double
The type of numeric value which will fill the output
array. If no selection is made, the output type will be determined
automatically as the shortest type required to hold all the values in the
array. Currently, integers are always signed (no BSCALE/BZERO), so for
instance the largest value that can be recorded in 8 bits is 127.
- scale=<expr>
Optionally gives a weight for each entry contributing to
histogram bins. The value of this expression is accumulated, in accordance
with the
combine parameter, into the bin defined by its coordinates. If
no expression is given, the value 1 is assumed.