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.\" RCSid "$Id$"
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.TH RPIECE 1 5/25/05 RADIANCE
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.SH NAME
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rtcontrib - compute contributions in a RADIANCE scene
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.SH SYNOPSIS
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.B rtcontrib
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[
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.B "\-n nprocs"
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][
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.B "\-e expr"
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][
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.B "\-f source"
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][
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.B "\-o fspec"
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][
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.B "\-b binv"
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]
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[
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.B $EVAR
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]
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[
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.B @file
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]
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[
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rtrace options
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]
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.B "\-m mod .."
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.B octree
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.SH DESCRIPTION
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.I Rtcontrib
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computes ray contributions (i.e., color coefficients)
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for objects whose modifiers are named in one or more
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.I \-m
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settings.
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These modifiers are usually materials associated with specific
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light sources, though they could correspond to intermediate objects as well.
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The resulting contributions may then be used in linear combination to
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reproduce any desired variation, e.g., simulating lighting controls or
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changing sky conditions via daylight coefficients.
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More generally,
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.I rtcontrib
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can compute general input-output relationships in optical
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systems, such as light pipes and shading devices.
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.PP
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.I Rtrace(1)
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is called to calculate the contributions for each input ray,
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and the output tallies are sent to one or more files according to the
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.I \-o
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specification.
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If the output file specification contains a "%s" format, this will be
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replaced by the modifier name.
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The
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.I \-b
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option may be used to further define
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a "bin number" within each object if finer resolution is desired, and
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will be applied to a "%d" format in the output file
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specification if present.
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(The actual bin number is computed at run time based on ray direction
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and surface intersection, as described below.)\0
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The most recent
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.I \-b
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and
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.I \-o
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options (to the left) of each
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.I \-m
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setting affect only that modifier, and the ordering
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of other options is unimportant.
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.PP
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Input and output format defaults to plain text, where each ray's
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origin and direction (6 real values) must appear together per
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line of input, and one line of output is produced per output file
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file per ray.
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Alternative input and output formats may be specified using the
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.I \-f[io]
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option, which is explained in the
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.I rtrace
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man page along with the associated
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.I \-x
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and
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.I \-y
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resolution settings.
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In particular, the 'c' output setting
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together with positive dimensions for
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.I \-x
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and
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.I \-y
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will produce an uncompressed RADIANCE picture,
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suitable for manipulation with
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.I pcomb(1)
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and related tools.
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.PP
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If no
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.I \-o
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specification is given, results are written on the standard output in order
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of modifier (as given on the command line) then bin number.
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The same format is used for a simple file name specification
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without any embedded "%s" or "%d" formats.
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If a "%s" format appears but no "%d" in the
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.I \-o
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specification, then each modifier will have its own output file, with
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multiple values per record in the case of a non-zero
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.I \-b
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definition.
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If a "%d" format appears but no "%s", then each bin will get its own
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output file, with modifiers output in order in each record.
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For text output, each RGB coefficient triple is separated by a tab,
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with a newline at the end of each ray record.
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For binary output formats, there is no such delimiter to mark
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the end of each record.
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.PP
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If a
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.I \-b
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expression is defined for a particular modifier,
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the bin number will be evaluated at run time for each
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ray contribution from
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.I rtrace.
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Specifically, each ray's world intersection point will be assigned to
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the variables Px, Py, and Pz, and the normalized ray direction
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will be assigned to Dx, Dy, and Dz.
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These ray parameters may be combined with any definitions given in
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.I \-e
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options, or any files read in from
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.I \-f
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options, to compute the bin, which will be
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rounded to the closest whole number.
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This mechanism allows the user to define precise regions (or directions)
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they wish to accumulate, such as the Tregenza sky grid, which would be
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otherwise impossible to specify as a set of RADIANCE primitives.
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.PP
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If the
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.I \-n
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option is specified with a value greater than 1, multiple
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.I rtrace(1)
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processes will be used to accelerate computation on a shared
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memory machine.
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Note that there is no benefit to using more processes
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than there are local CPUs available to do the work.
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.PP
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Options may be given on the command line and/or read from the
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environment and/or read from a file.
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A command argument beginning with a dollar sign ('$') is immediately
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replaced by the contents of the given environment variable.
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A command argument beginning with an at sign ('@') is immediately
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replaced by the contents of the given file.
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.SH EXAMPLE
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First
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.I rpiece
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process is started on the machine "goober":
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.IP "" .2i
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goober% echo 1 8 > syncfile
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.br
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goober% echo -F syncfile -x 1024 -y 1024 -vf view -o picture octree > args
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.br
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goober% rpiece @args &
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.PP
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Second
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.I rpiece
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processes is started on the machine "sucker":
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.IP "" .2i
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sucker% rpiece @args &
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.SH NOTES
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Due to NFS file buffering, the network lock manager is employed to
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guarantee consistency in the output file even though non-overlapping
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writes are used.
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This would tend to slow the process down if
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.I rpiece
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were to wait for this I/O to complete before starting on the next
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piece, so
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.I rpiece
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forks separate processes to hang around waiting for I/O completion.
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The number of processes thus designated is set by the MAXFORK macro
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in the program (compiled in the src/util directory).
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If the fork call is slow on a system, it may actually be better to
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set MAXFORK to zero.
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In other cases, the network lock manager may be so slow that this
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value should be increased to get the best utilization.
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.PP
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The output picture is not run-length encoded, and can be quite
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large.
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The approximate size (in kilobytes) can be computed by the simple
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formula:
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.IP "" .2i
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filesize = xres*yres/256
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.PP
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Make sure that there is enough space on the filesystem to hold the
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entire picture before beginning.
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Once the picture is finished, the
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.I ra_rgbe(1)
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program with the -r option may be used to convert to a run-length
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encoded picture for more efficient storage, although
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.I pfilt(1)
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or any of the other Radiance picture filters will do the same
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thing.
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.PP
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The ALRM signal may be used to gracefully terminate an
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.I rpiece
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process after it finishes the current piece.
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This permits other currently running or subsequently started
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.I rpiece
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process(es) to continue rendering the picture without loss.
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The
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.I \-T
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option will send the ALRM signal to
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.I rpiece
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after the specified number of (decimal) hours.
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This is the best way to force a time limit on the computation,
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since information will not be lost, though the process may continue
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for some time afterwards to finish its current piece.
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.SH BUGS
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This program may not work on some systems whose NFS lock manager is
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unreliable.
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In particular, some System V derivative UNIX systems often have
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problems with the network lock manager.
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If the output is scrambled or rpict aborts with some ambient file
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related problem, you should just remove the ambient file and go
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back to normal rendering.
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.SH AUTHOR
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Greg Ward
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.SH "SEE ALSO"
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cnt(1), getinfo(1), pcomb(1), pfilt(1), ra_rgbe(1),
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rcalc(1), rpict(1), rtrace(1), vwrays(1), ximage(1)
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