man/man1/mkpmap.1

.\" RCSid "$Id: mkpmap.1,v 1.7 2018/01/26 18:36:04 rschregle Exp $"
.TH MKPMAP 1 "$Date: 2018/01/26 18:36:04 $ $Revision: 1.7 $" 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 interested 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 a
an aborted prepass indicates 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 necessary for generating 
different photon distributions for the same octree and photon map size.

.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\-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
The \fIplasfunc\fR, \fImetfunc\fR, \fItransfunc\fR, \fIbrtdfunc\fR,
\fIplasdata\fR, \fImetdata\fR and \fItransdata\fR materials are not
supported by the photon mapping extension. 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:	1.8
Committed:	Fri Feb 2 19:49:02 2018 UTC (7 years, 3 months ago) by rschregle
Branch:	MAIN
Changes since 1.7:	+58 -27 lines
Log Message:	Documented -lr, -ld and -api, added warnings for Ze non-Ekspertz
#	Content
1	.\" RCSid "$Id: mkpmap.1,v 1.7 2018/01/26 18:36:04 rschregle Exp $"
2	.TH MKPMAP 1 "$Date: 2018/01/26 18:36:04 $ $Revision: 1.7 $" 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 interested 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 a
122	an aborted prepass indicates 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 necessary for generating
150	different photon distributions for the same octree and photon map size.
151
152	.IP "\fB\-aps \fImod\fR"
153	Specifies a modifier \fImod\fR defined as \fIantimatter\fR material to act
154	as a virtual (i.e. invisible) receiver surface. Photons will be deposited on
155	all surfaces using this modifier, just like regular materials, but will then
156	be transferred through the surface without undergoing scattering; the
157	surface therefore does not affect the light transport and simply acts as an
158	invisible photon receiver. This is useful when photon irradiance is to be
159	evaluated at points which do not lie on regular geometry, e.g. at workplane
160	height with \fIrtrace\fR's \fB-I\fR option. Without this workaround,
161	photons would be collected from parallel but distant planes, leading to
162	underestimation. Note that photons are only deposited when incident from
163	the front side of the sensor surface, i.e. when entering the
164	\fIantimatter\fR, thus the surface normal is relevant. \fIMkpmap\fR reports
165	an error if the specified modifier is not an \fIantimatter\fR material.
166
167	.IP "\fB\-apS \fImodfile\fR"
168	Read virtual receiver surface modifiers from the file \fImodfile\fR as a more
169	convenient alternative to multiple instances of \fB\-aps\fR.
170
171	.IP "\fB\-bv\fR[\fB+\fR\|\fB-\fR]"
172	Toggles backface visibility; enabling this causes photons to be stored and
173	possibly scattered if they strike the back of a surface, otherwise they
174	are unconditionally absorbed and discarded.
175
176	.IP "\fB\-dp \fIsampleres\fR"
177	Resolution for sampling the spatial emission distribution of a modified
178	light source (e.g. via \fIbrightfunc\fR), in samples per steradian. This
179	is required for numerically integrating the flux emitted by the light
180	source and for constructing a probability density function for photon
181	emission. The accuracy of photon emission from modified sources
182	therefore depends on this parameter. This parameter may need increasing
183	with complex emission distributions in combination with caustics.
184
185	.IP "\fB\-ds \fIpartsize\fR"
186	Light source partition size ratio; a light source object is spatially
187	partitioned to distribute the photon emission over its surface. This
188	parameter specifies the ratio of the size (per dimension) of each
189	partition to the scene cube, and may need increasing for modified light
190	sources (e.g. via \fIbrightfunc\fR) with high spatial variation.
191
192	.IP "\fB\-e \fIfile\fR"
193	Redirect diagnostics and progress reports to \fIfile\fR instead of the
194	console.
195
196	.IP "\fB\-fo\fR[\fB+\fR\|\fB-\fR]"
197	Toggles overwriting of output files. By default, \fImkpmap\fR will not
198	overwrite an already existing photon map file. This is to prevent
199	inadvertently destroying the results of potentially lengthy photon
200	mapping runs.
201
202	.IP "\fB\-ld \fImaxdist\fR"
203	Limit cumulative distance travelled by a photon along its path to
204	\fImaxdist\fR. Photon hits within this distance will be stored, and the
205	photon is terminated once its path length exceeds this limit. This is
206	useful for setting radial regions of interest around emitting/reflecting
207	geometry, but may increase the photon distribution time.
208	.IP
209	\fBWARNING: this is an optimisation option for advanced users (an elite
210	group collectively known as \fIZe Ekspertz\fB) and may yield biased results.
211	Use with caution!\fR
212
213	.IP "\fB\-lr \fImaxbounce\fR"
214	Limit number of bounces (scattering events) along a photon path to
215	\fImaxbounce\fR before being considered "runaway" and terminated. Photons
216	paths are normally terminated via \fIRussian Roulette\fR, depending on their
217	albedo. With unrealistically high albedos, this is not guaranteed, and this
218	option imposes a hard limit to avoid an infinite loop.
219	.IP
220	\fBWARNING: this is an optimisation option for advanced users (an elite
221	group collectively known as \fIZe Ekspertz\fB) and may yield biased results.
222	Use with caution!\fR
223
224	.IP "\fB\-ma \fIralb galb balb\fR"
225	Set the global scattering albedo for participating media in conjunction
226	with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.
227
228	.IP "\fB\-me \fIrext gext bext\fR"
229	Set the global extinction coefficient for participating media in conjunction
230	with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.
231
232	.IP "\fB\-mg \fIgecc\fR"
233	Set the global scattering eccentricity for participating media in conjunction
234	with the \fB\-apv\fR option. See \fIrpict(1)\fR for details.
235
236	.IP "\fB\-n \fInproc\fR"
237	Use \fInproc\fR processes for parallel photon distribution. There is no
238	benefit in specifying more than the number of physical CPU cores available.
239	This option is currently not available on Windows.
240
241	.IP "\fB\-t \fIinterval\fR"
242	Output a progress report every \fIinterval\fR seconds. This includes
243	statistics about the currently emitting light source (including number of
244	partitions), the total number of photons emitted, the number of each type
245	stored, the percentage of the completed pass (pre or main), and the elapsed
246	time.
247
248	.SH NOTES
249
250	.SS Parametrisation
251	\fIMkpmap\fR recognises multiplier suffixes (k = 1000, m = 1000000) to
252	facilitate the specification of \fInphotons\fR, both in upper and lower
253	case.
254	.PP
255
256	.SS Distribution Algorithm
257	The photon distribution algorithm estimates the number of required
258	photons to emit to arrive at the specified target count \fInphotons\fR
259	per photon map using a distribution prepass followed by a main pass.
260	As a result, \fImkpmap\fR generates the \fBapproximate\fR number of photons
261	specified, which can vary by up to 10% for typical scenes, but can be
262	higher for scenes with unusually high or low reflectance. In this case,
263	the predistribution factor \fB\-apD\fR should be increased for scenes
264	with low reflectance, and reduced for those with high reflectance.
265	.PP
266	There are situations which may prevent certain (or any)
267	photon types from being generated, depending on the light source and material
268	configuration. This typically occurs when attempting to generate a caustic
269	photon map without specular materials present in the scene, or a volume
270	photon map without participating media. Ill-configured light sources may also
271	prevent indirect rays from reaching a surface, and thus no photons being
272	deposited. In these cases, \fImkpmap\fR will make a number of distribution
273	attempts before terminating with an error. This can be adjusted with the
274	\fB\-apM\fR option.
275
276	.SS Material Support
277	The \fIplasfunc\fR, \fImetfunc\fR, \fItransfunc\fR, \fIbrtdfunc\fR,
278	\fIplasdata\fR, \fImetdata\fR and \fItransdata\fR materials are not
279	supported by the photon mapping extension. Use the newer \fIbsdf\fR material
280	instead.
281	.PP
282	Virtual light sources (normally enabled with the \fImirror\fR material) are
283	disabled with the photon map, as the resulting caustics are already accounted
284	for.
285
286	.SS Virtual Receiver Surfaces
287	Since photons are surface bound, the density estimate is only asymptotically
288	correct when performed at points which lie on the scene geometry. The
289	irradiance is underestimated for arbitrarily placed points when photons are
290	collected from distant surfaces. \fIMkpmap\fR offers a workaround with a
291	virtual receiver surface using the \fIantimatter\fR material; see the \fB-aps\fR
292	and \fB-apS\fR options for details.
293
294	.SH EXAMPLES
295	The following command generates a global photon map \fIbonzo.gpm\fR and a
296	caustic photon map \fIbonzo.cpm\fR containing approximately 10000 and 100000
297	photons, respectively, with progress report every 5 seconds:
298	.IP
299	mkpmap \-apg bonzo.gpm 10k \-apc bonzo.cpm 100k -t 5 bonzo.oct
300	.PP
301	Generate a global photon map containing 80000 photons, then precompute the
302	diffuse irradiance for 1/4 of these with a bandwidth of 40 photons:
303	.IP
304	mkpmap \-app bonzo-precomp.gpm 80k 40 \-apP 0.25 bonzo.oct
305	.PP
306	Generate 1 million global photons by emitting them from external light
307	sources of type \fIsource\fR into a reference room via a fenestration
308	with modifier \fIglazingMat\fR:
309	.IP
310	mkpmap \-apg refRoom.gpm 1m \-apo glazingMat refRoom.oct
311	.PP
312	Generate a contribution photon map containing 200000 photons suitable for
313	obtaining light source contributions with \fIrcontrib(1)\fR:
314	.IP
315	mkpmap \-apC bonzo-contrib.gpm 200k bonzo.oct
316
317	.SH BUGS
318	The focus of a spotlight source, as defined by the length of its direction
319	vector, is ignored by the photon map; photons are unconditionally emitted
320	from the light source surface, which can lead to deviations from standard
321	RADIANCE.
322	.PP
323	Light sources simply absorb incoming photons.
324
325	.SH AUTHOR
326	Roland Schregle (roland.schregle@{hslu.ch,gmail.com})
327
328	.SH COPYRIGHT
329	(c) Fraunhofer Institute for Solar Energy Systems, Lucerne University of
330	Applied Sciences and Arts.
331
332	.SH ACKNOWLEDGEMENT
333	Development of the RADIANCE photon mapping extension was sponsored by the
334	German Research Foundation (DFG) and the Swiss National Science Foundation
335	(SNF).
336
337	.SH "SEE ALSO"
338	rpict(1), rtrace(1), rvu(1), rcontrib(1), \fIThe RADIANCE Photon Map
339	Manual\fR, \fIDevelopment and Integration of the RADIANCE Photon Map
340	Extension: Technical Report\fR