--- ray/doc/man/man1/rtcontrib.1	2007/09/04 17:36:41	1.16
+++ ray/doc/man/man1/rtcontrib.1	2008/10/06 23:00:53	1.19
@@ -1,4 +1,4 @@
-.\" RCSid "$Id: rtcontrib.1,v 1.16 2007/09/04 17:36:41 greg Exp $"
+.\" RCSid "$Id: rtcontrib.1,v 1.19 2008/10/06 23:00:53 greg Exp $"
 .TH RTCONTRIB 1 5/25/05 RADIANCE
 .SH NAME
 rtcontrib - compute contribution coefficients in a RADIANCE scene
@@ -9,6 +9,8 @@ rtcontrib - compute contribution coefficients in a RAD
 ][
 .B \-V
 ][
+.B "\-c count"
+][
 .B \-fo
 |
 .B \-r
@@ -56,6 +58,17 @@ this file.
 (No search takes place if a file name begins with a '.', '/' or '~'
 character.)\0
 .PP
+If the
+.I \-n
+option is specified with a value greater than 1, multiple
+.I rtrace
+processes will be used to accelerate computation on a shared
+memory machine.
+Note that there is no benefit to using more processes
+than there are local CPUs available to do the work, and the
+.I rtcontrib
+process itself may use a considerable amount of CPU time.
+.PP
 By setting the boolean
 .I \-V
 option, you may instruct
@@ -84,6 +97,26 @@ rather than radiance, and 
 .I \-V-
 coefficients contain an additonal factor of PI.
 .PP
+The
+.I \-c
+option tells
+.I rtcontrib
+how many rays to accumulate for each record.
+The default value is 1, meaning a full record will be produced for
+each input ray.
+For values greater than 1, contributions will be averaged together
+over the given number of input rays.
+If set to zero, only a single record will be produced at the very
+end, corresponding to the sum of all rays given on the input
+(rather than the average).
+This is equivalent to passing all the output records through a program like
+.I total(1)
+to sum RGB values together, but is much more efficient.
+Using this option, it is possible to reverse sampling, sending rays from
+a parallel source such as the sun to a diffuse surface, for example.
+Note that output flushing via zero-direction rays is disabled
+for accumulated evaluations.
+.PP
 The output of
 .I rtcontrib
 has many potential uses.
@@ -112,6 +145,13 @@ option is given.
 If instead the
 .I \-r
 option is specified, data recovery is attempted on existing files.
+(If 
+.I "\-c 0"
+is used together with the
+.I \-r
+option, existing files are read in and new ray evaluations are added
+to the previous results, providing a convenient means for
+progressive simulation.)\0
 If an output specification contains a "%s" format, this will be
 replaced by the modifier name.
 The
@@ -137,7 +177,14 @@ and
 options to the left of each
 .I \-m
 setting affect only that modifier.
-(The ordering of other options is unimportant.)\0
+The ordering of other options is unimportant, except for
+.I \-x
+and
+.I \-y
+if the
+.I \-c
+is 0, when they control the resolution string
+produced in the corresponding output.
 .PP
 If a
 .I \-b
@@ -212,17 +259,6 @@ suitable for manipulation with
 .I pcomb(1)
 and related tools.
 .PP
-If the
-.I \-n
-option is specified with a value greater than 1, multiple
-.I rtrace
-processes will be used to accelerate computation on a shared
-memory machine.
-Note that there is no benefit to using more processes
-than there are local CPUs available to do the work, and the
-.I rtcontrib
-process itself may use a considerable amount of CPU time.
-.PP
 Options may be given on the command line and/or read from the
 environment and/or read from a file.
 A command argument beginning with a dollar sign ('$') is immediately
@@ -257,4 +293,4 @@ RAYPATH		path to search for \-f and \-M files
 Greg Ward
 .SH "SEE ALSO"
 cnt(1), getinfo(1), pcomb(1), pfilt(1), ra_rgbe(1),
-rcalc(1), rpict(1), rtrace(1), vwrays(1), ximage(1)
+rcalc(1), rpict(1), rtrace(1), total(1), vwrays(1), ximage(1)