man/man1/mkpmap.1

.\" RCSid "$Id: mkpmap.1,v 1.11 2018/12/06 20:13:13 rschregle Exp $"
.TH MKPMAP 1 "$Date: 2018/12/06 20:13:13 $ $Revision: 1.11 $" RADIANCE

.SH NAME
mkpmap - generate RADIANCE photon map

.SH SYNOPSIS
mkpmap \fB\-apg\fR|\fB\-apc\fR|\fB\-apv\fR|\fB\-apd\fR|\fB\-app\fR|\fB\-apC\fR
\fIfile nphotons\fR [\fIbwidth\fR] ...
       [options] \fIoctree\fR

.SH DESCRIPTION
\fIMkpmap\fR takes a RADIANCE scene description as an octree and
performs Monte Carlo forward path tracing from the light sources,
depositing indirect ray hitpoints along with their energy (flux) as
"photons". The resulting localised energy distribution represents a
global illumination solution which is written to a file for subsequent
evaluation by \fIrpict(1), rtrace(1)\fR and \fIrvu(1)\fR in a backward
raytracing pass. The photon map(s) can be reused for multiple viewpoints
and sensor locations as long as the geometry remains unchanged.

.SH OPTIONS
\fIMkpmap\fR can generate different types of photon maps depending on
the materials present in the scene. In most cases, these can be
specified independently or in combination on the command line. If
multiple photon maps of the same type are specified, the last instance
takes precedence.

.IP "\fB\-apg \fIfile nphotons\fR"
Generate a global photon map containing approximately \fInphotons\fR
photons, and output to \fIfile\fR. This accounts for all
indirect illumination, from both specular and diffuse scattering, on
surfaces with a diffuse component. This is the most general type of
photon map and replaces the ambient calculation in \fIrpict(1),
rtrace(1)\fR and \fIrvu(1)\fR.

.IP "\fB\-apc \fIfile nphotons\fR"
Generate a separate caustic photon map containing approximately
\fInphotons\fR photons, and output to file \fIfile\fR. This is a
subset of the global photon map intended for direct visualisation at
primary rays,  This accounts for all indirect illumination on diffuse
surfaces from specular scattering, which usually exhibits a large
gradient and requires a higher resolution than the global photon map,
typically containing the tenfold number of photons.

.IP "\fB\-apv \fIfile nphotons\fR"
Generate a volume photon map containing approximately \fInphotons\fR
photons, and output to file \fIfile\fR. These account for indirect
inscattering in participating media such as \fBmist\fR and complement
the direct inscattering computed by \fIrpict(1), rtrace(1)\fR and
\fIrvu(1)\fR. See also the \fB\-me\fR, \fB\-ma\fR and \fB\-mg\fR options
below.

.IP "\fB\-apd \fIfile nphotons\fR"
Generate a direct photon map containing approximately \fInphotons\fR
photons, and output to file \fIfile\fR. This only accounts for direct
illumination and is intended for debugging and validation of photon emission
from the light sources, as the quality is too low for actual rendering.

.IP "\fB\-apC \fIfile nphotons\fR" 
Generate a contribution photon map containing approximately
\fInphotons\fR photons, and output to file \fIfile\fR. This may then be
used by \fIrcontrib(1)\fR to compute light source contributions. When used
with \fIrtrace(1)\fR or \fIrpict(1)\fR, contribution photon maps behave as
regular global photon maps and yield cumulative contributions from all light
sources.
.IP
With this option, \fImkpmap\fR uses a modified photon distribution
algorithm that ensures all light sources contribute approximately the
same number of photons. Each photon indexes a primary hitpoint, incident
direction, and emitting light source which can be used to bin
contributions per light source and direction.
.IP
\fIMkpmap\fR cannot generate a contribution photon map in combination with
others in a single run, as it uses a different distribution algorithm. Other
photon maps specified on the command line will be ignored.

.IP "\fB\-app \fIfile nphotons bwidth\fR"
Generate a precomputed global photon map containing a fraction of
\fInphotons\fR photons (specified with the \fB\-apP\fR option, see
below), and output to file \fIfile\fR. This is a special case of the
global photon map where the irradiance is evaluated for a fraction of
the photon positions using \fIbwidth\fR nearest photons, and stored as
photon flux; the remaining photons are discarded as their contributions
have been accounted for. 
.IP 
This obviates the explicit irradiance evaluation by \fIrpict(1),
rtrace(1)\fR and \fIrvu(1)\fR, thus providing a speedup at the expense of
accuracy.  The resulting error is tolerable if the indirect illumination has
a low gradient, as is usually the case with diffuse illumination.

.IP "\fB\-apD \fIpredistrib\fR"
Photon predistribution factor; this is the fraction of \fInphotons\fR
which are emitted in a distribution prepass in order to estimate the
remaining number of photons to emit in the main pass to approximately
yield a photon map of size \fInphotons\fR. 
.IP
Setting this too high may yield more than \fInphotons\fR in the initial pass
with highly reflective geometry.  Note that this value may exceed 1, which
may be useful if the resulting photon map size greatly deviates from
\fInphotons\fR with a very low average reflectance.

.IP "\fB\-api \fIxmin ymin zmin xmax ymax zmax\fR"
Define a region of interest within which to store photons exclusively;
photons will only be stored within the volume bounded by the given minimum
and maximum coordinates.  Multiple instances of this option may be specified
with cumulative effect to define compound regions of interest.  This is
useful for constraining photons to only the relevant regions of a scene, but
may increase the photon distribution time.
.IP
\fBWARNING: this is an optimisation option for advanced users (an elite
group collectively known as \fIZe Ekspertz\fB) and may yield biased results. 
Use with caution!\fR

.IP "\fB\-apm \fImaxbounce\fR"
Synonymous with \fB\-lr\fR for backwards compatibility. May be removed in
future releases.

.IP "\fB\-apM \fImaxprepass\fR"
Maximum number of iterations of the distribution prepass before terminating
if some photon maps are still empty. This option is rarely needed as
an aborted prepass may indicate an anomaly in the geometry or an
incompatibility with the specified photon map types (see \fBNOTES\fR below).

.IP "\fB\-apo \fImod\fR"
Specifies a modifier \fImod\fR to act as a \fIphoton port\fR. All
objects using this modifier will emit photons directly in lieu of any
light sources defined with the \fIsource\fR material. This greatly
accelerates photon distribution in scenes where photons have to enter a
space which separates them from the emitting light source via an
opening, or port. 
.IP
A typical application is daylight simulation, where a fenestration acts as
port to admit photons into an interior after emission from an external light
source.  Multiple instances of this option may be specified.
.IP
Note that port objects must be defined with their surface normals
pointing \fIinside\fR as per \fImkillum\fR convention.

.IP "\fB\-apO \fImodfile\fR"
Read photon port modifiers from the file \fImodfile\fR as a more convenient
alternative to multiple instances of \fB\-apo\fR.

.IP "\fB\-apP \fIprecomp\fR"
Fraction of global photons to precompute in the range ]0,1] when using the
\fB\-app\fR option.

.IP "\fB\-apr \fIseed\fR"
Seed for the random number generator. This is useful for generating 
different photon distributions for the same octree and photon map size,
notably in progressive applications.

.IP "\fB\-aps \fImod\fR"
Specifies a modifier \fImod\fR defined as \fIantimatter\fR material to act
as a virtual (i.e.  invisible) receiver surface.  Photons will be deposited on
all surfaces using this modifier, just like regular materials, but will then
be transferred through the surface without undergoing scattering; the
surface therefore does not affect the light transport and simply acts as an
invisible photon receiver.  This is useful when photon irradiance is to be
evaluated at points which do not lie on regular geometry, e.g.  at workplane
height with \fIrtrace\fR's \fB-I\fR option.  Without this workaround,
photons would be collected from parallel but distant planes, leading to
underestimation.  Note that photons are only deposited when incident from
the front side of the sensor surface, i.e.  when entering the
\fIantimatter\fR, thus the surface normal is relevant.  \fIMkpmap\fR reports
an error if the specified modifier is not an \fIantimatter\fR material.

.IP "\fB\-apS \fImodfile\fR"
Read virtual receiver surface modifiers from the file \fImodfile\fR as a more
convenient alternative to multiple instances of \fB\-aps\fR.

.IP "\fB\-ae \fImod\fR"
Add \fImod\fR to the ambient exclude list, so that it will be ignored by the
photon map.  Objects having \fImod\fR as their modifier will not have
photons deposited on them.  Multiple modifiers may be given, each as separate
instances of this option.
.IP
\fBWARNING: this is an optimisation option for advanced users and may yield
biased results. It may also significantly increase photon distribution
times. Use with caution!\fR

.IP "\fB\-aE \fIfile\fR"
Same as \fI-ae\fR, except modifiers to be exluded are read from \fIfile\fR,
separated by whitespace.  The RAYPATH environment variable determines which
directories are searched for this file.

.IP "\fB\-ai \fImod\fR"
Add \fImod\fR to the ambient include list, so that it will contribute to the
photon map. Only objects having \fImod\fR as their modifier will have
photons deposited on them. Multiple modifiers may be given, each as separate
instances of this option. Note that the ambient include and exclude options
are mutually exclusive. 
.IP
\fBWARNING: this is an optimisation option for advanced users and may yield
biased results. It may also significantly increase photon distribution
times. Use with caution!\fR

.IP "\fB\-aI \fIfile\fR"
Same as \fI-ai\fR, except modifiers to be included are read from \fIfile\fR,
separated by whitespace. The RAYPATH environment variable determines which
directories are searched for this file.

.IP "\fB\-bv\fR[\fB+\fR|\fB-\fR]"
Toggles backface visibility; enabling this causes photons to be stored and
possibly scattered if they strike the back of a surface, otherwise they
are unconditionally absorbed and discarded.

.IP "\fB\-dp \fIsampleres\fR"
Resolution for sampling the spatial emission distribution of a modified
light source (e.g. via \fIbrightfunc\fR), in samples per steradian. This
is required for numerically integrating the flux emitted by the light
source and for constructing a probability density function for photon
emission. The accuracy of photon emission from modified sources
therefore depends on this parameter. This parameter may need increasing
with complex emission distributions in combination with caustics.

.IP "\fB\-ds \fIpartsize\fR"
Light source partition size ratio; a light source object is spatially 
partitioned to distribute the photon emission over its surface. This
parameter specifies the ratio of the size (per dimension) of each
partition to the scene cube, and may need increasing for modified light 
sources (e.g. via \fIbrightfunc\fR) with high spatial variation.

.IP "\fB\-e \fIfile\fR"
Redirect diagnostics and progress reports to \fIfile\fR instead of the
console.

.IP "\fB\-fo\fR[\fB+\fR|\fB-\fR]"
Toggles overwriting of output files. By default, \fImkpmap\fR will not
overwrite an already existing photon map file. This is to prevent
inadvertently destroying the results of potentially lengthy photon
mapping runs.

.IP "\fB\-ld \fImaxdist\fR"
Limit cumulative distance travelled by a photon along its path to
\fImaxdist\fR.  Photon hits within this distance will be stored, and the
photon is terminated once its path length exceeds this limit.  This is
useful for setting radial regions of interest around emitting/reflecting
geometry, but may increase the photon distribution time.  
.IP
\fBWARNING: this is an optimisation option for advanced users (an elite
group collectively known as \fIZe Ekspertz\fB) and may yield biased results. 
Use with caution!\fR

.IP "\fB\-lr \fImaxbounce\fR"
Limit number of bounces (scattering events) along a photon path to
\fImaxbounce\fR before being considered "runaway" and terminated.  Photons
paths are normally terminated via \fIRussian Roulette\fR, depending on their
albedo.  With unrealistically high albedos, this is not guaranteed, and this
option imposes a hard limit to avoid an infinite loop.
.IP
\fBWARNING: this is an optimisation option for advanced users (an elite
group collectively known as \fIZe Ekspertz\fB) and may yield biased results. 
Use with caution!\fR

.IP "\fB\-ma \fIralb galb balb\fR"
Set the global scattering albedo for participating media in conjunction
with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.

.IP "\fB\-me \fIrext gext bext\fR"
Set the global extinction coefficient for participating media in conjunction
with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.

.IP "\fB\-mg \fIgecc\fR"
Set the global scattering eccentricity for participating media in conjunction
with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.

.IP "\fB\-n \fInproc\fR"
Use \fInproc\fR processes for parallel photon distribution. There is no
benefit in specifying more than the number of physical CPU cores available.
This option is currently not available on Windows.

.IP "\fB\-t \fIinterval\fR"
Output a progress report every \fIinterval\fR seconds. This includes 
statistics about the currently emitting light source (including number of
partitions), the total number of photons emitted, the number of each type 
stored, the percentage of the completed pass (pre or main), and the elapsed
time.

.SH NOTES

.SS Parametrisation
\fIMkpmap\fR recognises multiplier suffixes (k = 1000, m = 1000000) to 
facilitate the specification of \fInphotons\fR, both in upper and lower
case.
.PP

.SS Distribution Algorithm
The photon distribution algorithm estimates the number of required
photons to emit to arrive at the specified target count \fInphotons\fR
per photon map using a distribution prepass followed by a main pass.
As a result, \fImkpmap\fR generates the \fBapproximate\fR number of photons
specified, which can vary by up to 10% for typical scenes, but can be
higher for scenes with unusually high or low reflectance. In this case,
the predistribution factor \fB\-apD\fR should be increased for scenes
with low reflectance, and reduced for those with high reflectance.
.PP
There are situations which may prevent certain (or any)
photon types from being generated, depending on the light source and material
configuration. This typically occurs when attempting to generate a caustic
photon map without specular materials present in the scene, or a volume 
photon map without participating media. Ill-configured light sources may also
prevent indirect rays from reaching a surface, and thus no photons being 
deposited. In these cases, \fImkpmap\fR will make a number of distribution
attempts before terminating with an error. This can be adjusted with the 
\fB\-apM\fR option.

.SS Material Support
Not all materials are fully supported by the photon map extension.  The
\fIplasfunc\fR, \fImetfunc\fR, \fItransfunc\fR, \fIplasdata\fR,
\fImetdata\fR and \fItransdata\fR materials currently only scatter photons
diffusely, and will not produce caustics.  The \fIbrtdfunc\fR material only
produces caustics via ideal (mirror) specular reflection and transmission. 
For more realistic scattering behaviour, use the newer \fIbsdf\fR material
instead.
.PP
Virtual light sources (normally enabled with the \fImirror\fR material) are
disabled with the photon map, as the resulting caustics are already accounted
for.

.SS Virtual Receiver Surfaces
Since photons are surface bound, the density estimate is only asymptotically
correct when performed at points which lie on the scene geometry.  The
irradiance is underestimated for arbitrarily placed points when photons are
collected from distant surfaces.  \fIMkpmap\fR offers a workaround with a
virtual receiver surface using the \fIantimatter\fR material; see the \fB-aps\fR
and \fB-apS\fR options for details.

.SH EXAMPLES
The following command generates a global photon map \fIbonzo.gpm\fR and a 
caustic photon map \fIbonzo.cpm\fR containing approximately 10000 and 100000 
photons, respectively, with progress report every 5 seconds:
.IP
mkpmap \-apg bonzo.gpm 10k \-apc bonzo.cpm 100k -t 5 bonzo.oct
.PP
Generate a global photon map containing 80000 photons, then precompute the
diffuse irradiance for 1/4 of these with a bandwidth of 40 photons:
.IP
mkpmap \-app bonzo-precomp.gpm 80k 40 \-apP 0.25 bonzo.oct
.PP
Generate 1 million global photons by emitting them from external light 
sources of type \fIsource\fR into a reference room via a fenestration 
with modifier \fIglazingMat\fR:
.IP
mkpmap \-apg refRoom.gpm 1m \-apo glazingMat refRoom.oct
.PP
Generate a contribution photon map containing 200000 photons suitable for 
obtaining light source contributions with \fIrcontrib(1)\fR:
.IP
mkpmap \-apC bonzo-contrib.gpm 200k bonzo.oct

.SH BUGS
The focus of a spotlight source, as defined by the length of its direction
vector, is ignored by the photon map; photons are unconditionally emitted
from the light source surface, which can lead to deviations from standard
RADIANCE.
.PP
Light sources simply absorb incoming photons.

.SH AUTHOR
Roland Schregle (roland.schregle@{hslu.ch,gmail.com})

.SH COPYRIGHT
(c) Fraunhofer Institute for Solar Energy Systems, Lucerne University of 
Applied Sciences and Arts.

.SH ACKNOWLEDGEMENT
Development of the RADIANCE photon mapping extension was sponsored by the 
German Research Foundation (DFG) and the Swiss National Science Foundation 
(SNF). 

.SH "SEE ALSO"
rpict(1), rtrace(1), rvu(1), rcontrib(1), \fIThe RADIANCE Photon Map
Manual\fR, \fIDevelopment and Integration of the RADIANCE Photon Map
Extension: Technical Report\fR
Revision:	1.12
Committed:	Mon Jun 3 17:08:23 2019 UTC (5 years, 11 months ago) by rschregle
Branch:	MAIN
Changes since 1.11:	+4 -4 lines
Log Message:	Minor typo fix
#	Content
1	.\" RCSid "$Id: mkpmap.1,v 1.11 2018/12/06 20:13:13 rschregle Exp $"
2	.TH MKPMAP 1 "$Date: 2018/12/06 20:13:13 $ $Revision: 1.11 $" RADIANCE
3
4	.SH NAME
5	mkpmap - generate RADIANCE photon map
6
7	.SH SYNOPSIS
8	mkpmap \fB\-apg\fR\|\fB\-apc\fR\|\fB\-apv\fR\|\fB\-apd\fR\|\fB\-app\fR\|\fB\-apC\fR
9	\fIfile nphotons\fR [\fIbwidth\fR] ...
10	[options] \fIoctree\fR
11
12	.SH DESCRIPTION
13	\fIMkpmap\fR takes a RADIANCE scene description as an octree and
14	performs Monte Carlo forward path tracing from the light sources,
15	depositing indirect ray hitpoints along with their energy (flux) as
16	"photons". The resulting localised energy distribution represents a
17	global illumination solution which is written to a file for subsequent
18	evaluation by \fIrpict(1), rtrace(1)\fR and \fIrvu(1)\fR in a backward
19	raytracing pass. The photon map(s) can be reused for multiple viewpoints
20	and sensor locations as long as the geometry remains unchanged.
21
22	.SH OPTIONS
23	\fIMkpmap\fR can generate different types of photon maps depending on
24	the materials present in the scene. In most cases, these can be
25	specified independently or in combination on the command line. If
26	multiple photon maps of the same type are specified, the last instance
27	takes precedence.
28
29	.IP "\fB\-apg \fIfile nphotons\fR"
30	Generate a global photon map containing approximately \fInphotons\fR
31	photons, and output to \fIfile\fR. This accounts for all
32	indirect illumination, from both specular and diffuse scattering, on
33	surfaces with a diffuse component. This is the most general type of
34	photon map and replaces the ambient calculation in \fIrpict(1),
35	rtrace(1)\fR and \fIrvu(1)\fR.
36
37	.IP "\fB\-apc \fIfile nphotons\fR"
38	Generate a separate caustic photon map containing approximately
39	\fInphotons\fR photons, and output to file \fIfile\fR. This is a
40	subset of the global photon map intended for direct visualisation at
41	primary rays, This accounts for all indirect illumination on diffuse
42	surfaces from specular scattering, which usually exhibits a large
43	gradient and requires a higher resolution than the global photon map,
44	typically containing the tenfold number of photons.
45
46	.IP "\fB\-apv \fIfile nphotons\fR"
47	Generate a volume photon map containing approximately \fInphotons\fR
48	photons, and output to file \fIfile\fR. These account for indirect
49	inscattering in participating media such as \fBmist\fR and complement
50	the direct inscattering computed by \fIrpict(1), rtrace(1)\fR and
51	\fIrvu(1)\fR. See also the \fB\-me\fR, \fB\-ma\fR and \fB\-mg\fR options
52	below.
53
54	.IP "\fB\-apd \fIfile nphotons\fR"
55	Generate a direct photon map containing approximately \fInphotons\fR
56	photons, and output to file \fIfile\fR. This only accounts for direct
57	illumination and is intended for debugging and validation of photon emission
58	from the light sources, as the quality is too low for actual rendering.
59
60	.IP "\fB\-apC \fIfile nphotons\fR"
61	Generate a contribution photon map containing approximately
62	\fInphotons\fR photons, and output to file \fIfile\fR. This may then be
63	used by \fIrcontrib(1)\fR to compute light source contributions. When used
64	with \fIrtrace(1)\fR or \fIrpict(1)\fR, contribution photon maps behave as
65	regular global photon maps and yield cumulative contributions from all light
66	sources.
67	.IP
68	With this option, \fImkpmap\fR uses a modified photon distribution
69	algorithm that ensures all light sources contribute approximately the
70	same number of photons. Each photon indexes a primary hitpoint, incident
71	direction, and emitting light source which can be used to bin
72	contributions per light source and direction.
73	.IP
74	\fIMkpmap\fR cannot generate a contribution photon map in combination with
75	others in a single run, as it uses a different distribution algorithm. Other
76	photon maps specified on the command line will be ignored.
77
78	.IP "\fB\-app \fIfile nphotons bwidth\fR"
79	Generate a precomputed global photon map containing a fraction of
80	\fInphotons\fR photons (specified with the \fB\-apP\fR option, see
81	below), and output to file \fIfile\fR. This is a special case of the
82	global photon map where the irradiance is evaluated for a fraction of
83	the photon positions using \fIbwidth\fR nearest photons, and stored as
84	photon flux; the remaining photons are discarded as their contributions
85	have been accounted for.
86	.IP
87	This obviates the explicit irradiance evaluation by \fIrpict(1),
88	rtrace(1)\fR and \fIrvu(1)\fR, thus providing a speedup at the expense of
89	accuracy. The resulting error is tolerable if the indirect illumination has
90	a low gradient, as is usually the case with diffuse illumination.
91
92	.IP "\fB\-apD \fIpredistrib\fR"
93	Photon predistribution factor; this is the fraction of \fInphotons\fR
94	which are emitted in a distribution prepass in order to estimate the
95	remaining number of photons to emit in the main pass to approximately
96	yield a photon map of size \fInphotons\fR.
97	.IP
98	Setting this too high may yield more than \fInphotons\fR in the initial pass
99	with highly reflective geometry. Note that this value may exceed 1, which
100	may be useful if the resulting photon map size greatly deviates from
101	\fInphotons\fR with a very low average reflectance.
102
103	.IP "\fB\-api \fIxmin ymin zmin xmax ymax zmax\fR"
104	Define a region of interest within which to store photons exclusively;
105	photons will only be stored within the volume bounded by the given minimum
106	and maximum coordinates. Multiple instances of this option may be specified
107	with cumulative effect to define compound regions of interest. This is
108	useful for constraining photons to only the relevant regions of a scene, but
109	may increase the photon distribution time.
110	.IP
111	\fBWARNING: this is an optimisation option for advanced users (an elite
112	group collectively known as \fIZe Ekspertz\fB) and may yield biased results.
113	Use with caution!\fR
114
115	.IP "\fB\-apm \fImaxbounce\fR"
116	Synonymous with \fB\-lr\fR for backwards compatibility. May be removed in
117	future releases.
118
119	.IP "\fB\-apM \fImaxprepass\fR"
120	Maximum number of iterations of the distribution prepass before terminating
121	if some photon maps are still empty. This option is rarely needed as
122	an aborted prepass may indicate an anomaly in the geometry or an
123	incompatibility with the specified photon map types (see \fBNOTES\fR below).
124
125	.IP "\fB\-apo \fImod\fR"
126	Specifies a modifier \fImod\fR to act as a \fIphoton port\fR. All
127	objects using this modifier will emit photons directly in lieu of any
128	light sources defined with the \fIsource\fR material. This greatly
129	accelerates photon distribution in scenes where photons have to enter a
130	space which separates them from the emitting light source via an
131	opening, or port.
132	.IP
133	A typical application is daylight simulation, where a fenestration acts as
134	port to admit photons into an interior after emission from an external light
135	source. Multiple instances of this option may be specified.
136	.IP
137	Note that port objects must be defined with their surface normals
138	pointing \fIinside\fR as per \fImkillum\fR convention.
139
140	.IP "\fB\-apO \fImodfile\fR"
141	Read photon port modifiers from the file \fImodfile\fR as a more convenient
142	alternative to multiple instances of \fB\-apo\fR.
143
144	.IP "\fB\-apP \fIprecomp\fR"
145	Fraction of global photons to precompute in the range ]0,1] when using the
146	\fB\-app\fR option.
147
148	.IP "\fB\-apr \fIseed\fR"
149	Seed for the random number generator. This is useful for generating
150	different photon distributions for the same octree and photon map size,
151	notably in progressive applications.
152
153	.IP "\fB\-aps \fImod\fR"
154	Specifies a modifier \fImod\fR defined as \fIantimatter\fR material to act
155	as a virtual (i.e. invisible) receiver surface. Photons will be deposited on
156	all surfaces using this modifier, just like regular materials, but will then
157	be transferred through the surface without undergoing scattering; the
158	surface therefore does not affect the light transport and simply acts as an
159	invisible photon receiver. This is useful when photon irradiance is to be
160	evaluated at points which do not lie on regular geometry, e.g. at workplane
161	height with \fIrtrace\fR's \fB-I\fR option. Without this workaround,
162	photons would be collected from parallel but distant planes, leading to
163	underestimation. Note that photons are only deposited when incident from
164	the front side of the sensor surface, i.e. when entering the
165	\fIantimatter\fR, thus the surface normal is relevant. \fIMkpmap\fR reports
166	an error if the specified modifier is not an \fIantimatter\fR material.
167
168	.IP "\fB\-apS \fImodfile\fR"
169	Read virtual receiver surface modifiers from the file \fImodfile\fR as a more
170	convenient alternative to multiple instances of \fB\-aps\fR.
171
172	.IP "\fB\-ae \fImod\fR"
173	Add \fImod\fR to the ambient exclude list, so that it will be ignored by the
174	photon map. Objects having \fImod\fR as their modifier will not have
175	photons deposited on them. Multiple modifiers may be given, each as separate
176	instances of this option.
177	.IP
178	\fBWARNING: this is an optimisation option for advanced users and may yield
179	biased results. It may also significantly increase photon distribution
180	times. Use with caution!\fR
181
182	.IP "\fB\-aE \fIfile\fR"
183	Same as \fI-ae\fR, except modifiers to be exluded are read from \fIfile\fR,
184	separated by whitespace. The RAYPATH environment variable determines which
185	directories are searched for this file.
186
187	.IP "\fB\-ai \fImod\fR"
188	Add \fImod\fR to the ambient include list, so that it will contribute to the
189	photon map. Only objects having \fImod\fR as their modifier will have
190	photons deposited on them. Multiple modifiers may be given, each as separate
191	instances of this option. Note that the ambient include and exclude options
192	are mutually exclusive.
193	.IP
194	\fBWARNING: this is an optimisation option for advanced users and may yield
195	biased results. It may also significantly increase photon distribution
196	times. Use with caution!\fR
197
198	.IP "\fB\-aI \fIfile\fR"
199	Same as \fI-ai\fR, except modifiers to be included are read from \fIfile\fR,
200	separated by whitespace. The RAYPATH environment variable determines which
201	directories are searched for this file.
202
203	.IP "\fB\-bv\fR[\fB+\fR\|\fB-\fR]"
204	Toggles backface visibility; enabling this causes photons to be stored and
205	possibly scattered if they strike the back of a surface, otherwise they
206	are unconditionally absorbed and discarded.
207
208	.IP "\fB\-dp \fIsampleres\fR"
209	Resolution for sampling the spatial emission distribution of a modified
210	light source (e.g. via \fIbrightfunc\fR), in samples per steradian. This
211	is required for numerically integrating the flux emitted by the light
212	source and for constructing a probability density function for photon
213	emission. The accuracy of photon emission from modified sources
214	therefore depends on this parameter. This parameter may need increasing
215	with complex emission distributions in combination with caustics.
216
217	.IP "\fB\-ds \fIpartsize\fR"
218	Light source partition size ratio; a light source object is spatially
219	partitioned to distribute the photon emission over its surface. This
220	parameter specifies the ratio of the size (per dimension) of each
221	partition to the scene cube, and may need increasing for modified light
222	sources (e.g. via \fIbrightfunc\fR) with high spatial variation.
223
224	.IP "\fB\-e \fIfile\fR"
225	Redirect diagnostics and progress reports to \fIfile\fR instead of the
226	console.
227
228	.IP "\fB\-fo\fR[\fB+\fR\|\fB-\fR]"
229	Toggles overwriting of output files. By default, \fImkpmap\fR will not
230	overwrite an already existing photon map file. This is to prevent
231	inadvertently destroying the results of potentially lengthy photon
232	mapping runs.
233
234	.IP "\fB\-ld \fImaxdist\fR"
235	Limit cumulative distance travelled by a photon along its path to
236	\fImaxdist\fR. Photon hits within this distance will be stored, and the
237	photon is terminated once its path length exceeds this limit. This is
238	useful for setting radial regions of interest around emitting/reflecting
239	geometry, but may increase the photon distribution time.
240	.IP
241	\fBWARNING: this is an optimisation option for advanced users (an elite
242	group collectively known as \fIZe Ekspertz\fB) and may yield biased results.
243	Use with caution!\fR
244
245	.IP "\fB\-lr \fImaxbounce\fR"
246	Limit number of bounces (scattering events) along a photon path to
247	\fImaxbounce\fR before being considered "runaway" and terminated. Photons
248	paths are normally terminated via \fIRussian Roulette\fR, depending on their
249	albedo. With unrealistically high albedos, this is not guaranteed, and this
250	option imposes a hard limit to avoid an infinite loop.
251	.IP
252	\fBWARNING: this is an optimisation option for advanced users (an elite
253	group collectively known as \fIZe Ekspertz\fB) and may yield biased results.
254	Use with caution!\fR
255
256	.IP "\fB\-ma \fIralb galb balb\fR"
257	Set the global scattering albedo for participating media in conjunction
258	with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.
259
260	.IP "\fB\-me \fIrext gext bext\fR"
261	Set the global extinction coefficient for participating media in conjunction
262	with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.
263
264	.IP "\fB\-mg \fIgecc\fR"
265	Set the global scattering eccentricity for participating media in conjunction
266	with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.
267
268	.IP "\fB\-n \fInproc\fR"
269	Use \fInproc\fR processes for parallel photon distribution. There is no
270	benefit in specifying more than the number of physical CPU cores available.
271	This option is currently not available on Windows.
272
273	.IP "\fB\-t \fIinterval\fR"
274	Output a progress report every \fIinterval\fR seconds. This includes
275	statistics about the currently emitting light source (including number of
276	partitions), the total number of photons emitted, the number of each type
277	stored, the percentage of the completed pass (pre or main), and the elapsed
278	time.
279
280	.SH NOTES
281
282	.SS Parametrisation
283	\fIMkpmap\fR recognises multiplier suffixes (k = 1000, m = 1000000) to
284	facilitate the specification of \fInphotons\fR, both in upper and lower
285	case.
286	.PP
287
288	.SS Distribution Algorithm
289	The photon distribution algorithm estimates the number of required
290	photons to emit to arrive at the specified target count \fInphotons\fR
291	per photon map using a distribution prepass followed by a main pass.
292	As a result, \fImkpmap\fR generates the \fBapproximate\fR number of photons
293	specified, which can vary by up to 10% for typical scenes, but can be
294	higher for scenes with unusually high or low reflectance. In this case,
295	the predistribution factor \fB\-apD\fR should be increased for scenes
296	with low reflectance, and reduced for those with high reflectance.
297	.PP
298	There are situations which may prevent certain (or any)
299	photon types from being generated, depending on the light source and material
300	configuration. This typically occurs when attempting to generate a caustic
301	photon map without specular materials present in the scene, or a volume
302	photon map without participating media. Ill-configured light sources may also
303	prevent indirect rays from reaching a surface, and thus no photons being
304	deposited. In these cases, \fImkpmap\fR will make a number of distribution
305	attempts before terminating with an error. This can be adjusted with the
306	\fB\-apM\fR option.
307
308	.SS Material Support
309	Not all materials are fully supported by the photon map extension. The
310	\fIplasfunc\fR, \fImetfunc\fR, \fItransfunc\fR, \fIplasdata\fR,
311	\fImetdata\fR and \fItransdata\fR materials currently only scatter photons
312	diffusely, and will not produce caustics. The \fIbrtdfunc\fR material only
313	produces caustics via ideal (mirror) specular reflection and transmission.
314	For more realistic scattering behaviour, use the newer \fIbsdf\fR material
315	instead.
316	.PP
317	Virtual light sources (normally enabled with the \fImirror\fR material) are
318	disabled with the photon map, as the resulting caustics are already accounted
319	for.
320
321	.SS Virtual Receiver Surfaces
322	Since photons are surface bound, the density estimate is only asymptotically
323	correct when performed at points which lie on the scene geometry. The
324	irradiance is underestimated for arbitrarily placed points when photons are
325	collected from distant surfaces. \fIMkpmap\fR offers a workaround with a
326	virtual receiver surface using the \fIantimatter\fR material; see the \fB-aps\fR
327	and \fB-apS\fR options for details.
328
329	.SH EXAMPLES
330	The following command generates a global photon map \fIbonzo.gpm\fR and a
331	caustic photon map \fIbonzo.cpm\fR containing approximately 10000 and 100000
332	photons, respectively, with progress report every 5 seconds:
333	.IP
334	mkpmap \-apg bonzo.gpm 10k \-apc bonzo.cpm 100k -t 5 bonzo.oct
335	.PP
336	Generate a global photon map containing 80000 photons, then precompute the
337	diffuse irradiance for 1/4 of these with a bandwidth of 40 photons:
338	.IP
339	mkpmap \-app bonzo-precomp.gpm 80k 40 \-apP 0.25 bonzo.oct
340	.PP
341	Generate 1 million global photons by emitting them from external light
342	sources of type \fIsource\fR into a reference room via a fenestration
343	with modifier \fIglazingMat\fR:
344	.IP
345	mkpmap \-apg refRoom.gpm 1m \-apo glazingMat refRoom.oct
346	.PP
347	Generate a contribution photon map containing 200000 photons suitable for
348	obtaining light source contributions with \fIrcontrib(1)\fR:
349	.IP
350	mkpmap \-apC bonzo-contrib.gpm 200k bonzo.oct
351
352	.SH BUGS
353	The focus of a spotlight source, as defined by the length of its direction
354	vector, is ignored by the photon map; photons are unconditionally emitted
355	from the light source surface, which can lead to deviations from standard
356	RADIANCE.
357	.PP
358	Light sources simply absorb incoming photons.
359
360	.SH AUTHOR
361	Roland Schregle (roland.schregle@{hslu.ch,gmail.com})
362
363	.SH COPYRIGHT
364	(c) Fraunhofer Institute for Solar Energy Systems, Lucerne University of
365	Applied Sciences and Arts.
366
367	.SH ACKNOWLEDGEMENT
368	Development of the RADIANCE photon mapping extension was sponsored by the
369	German Research Foundation (DFG) and the Swiss National Science Foundation
370	(SNF).
371
372	.SH "SEE ALSO"
373	rpict(1), rtrace(1), rvu(1), rcontrib(1), \fIThe RADIANCE Photon Map
374	Manual\fR, \fIDevelopment and Integration of the RADIANCE Photon Map
375	Extension: Technical Report\fR