--- ray/doc/man/man1/rtpict.1	2019/12/19 04:56:44	1.5
+++ ray/doc/man/man1/rtpict.1	2020/12/17 02:01:23	1.8
@@ -1,7 +1,7 @@
-.\" RCSid "$Id: rtpict.1,v 1.5 2019/12/19 04:56:44 greg Exp $"
+.\" RCSid "$Id: rtpict.1,v 1.8 2020/12/17 02:01:23 greg Exp $"
 .TH RTPICT 1 3/19/2018 RADIANCE
 .SH NAME
-rtpict - generate a RADIANCE picture or layerd image using rtrace
+rtpict - generate a RADIANCE picture or layered image using rtrace
 .SH SYNOPSIS
 .B rtpict
 .B "-n nproc"
@@ -100,7 +100,7 @@ with the 'v' type will be renamed
 .I "irradiance.hdr"
 and some other output types become irrelevant (i.e., 'r', 'x', 'R', and 'X').
 If one or more of the associated output files already exists in the
-destination directory, it will be overwritten with the new data.
+destination directory, an error will be printed and the command will abort.
 .SH EXAMPLES
 To render a scene with four processes:
 .IP "" .2i
@@ -109,6 +109,25 @@ rtpict -n 4 -vf mypers.vf -ab 1 -af scene.amb scene.oc
 To render radiance, first surface distance, and normals in a layered image:
 .IP "" .2i
 rtpict -n 8 -vf fish.vf @render.opt -ovLn fisholay scene.oct
+.SH NOTES
+Users should seriously consider using
+.I rpiece(1)
+when irradiance caching is employed with a shared ambient file.
+If
+.I rtpict
+is used in multiprocessing mode with a shared irradiance cache,
+it is trying to compute almost
+the same part of the same scanline in different processes, which
+results in many redundant calculations.
+The
+.I rpiece
+program avoids this by working on separate tiles in each
+process, with less overlap in the indirect irradiance calculation.
+Either program will benefit from an "overture" run of a
+single-process rpict to create the ambient file using a low-resolution
+rendering that is discarded, but this will not completely solve the
+problem for
+.I rtpict.
 .SH AUTHOR
 Greg Ward
 .SH "SEE ALSO"