.\" RCSid "$Id: mkpmap.1,v 1.13 2020/08/07 01:22:59 rschregle Exp $"
.TH MKPMAP 1 "$Date: 2020/08/07 01:22:59 $ $Revision: 1.13 $" 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\fR[\fB+\fR|\fB-\fR|\fB0\fR] \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
aperture (e.g. fenestration, skylight) acting as a port. 
.IP
In a typical daylight simulation scenario, a fenestration acts as a port to
admit photons into an interior after emission from sky and solar sources. 
Multiple instances of this option may be specified.
.IP
By default, ports are oriented to emit in the halfspace defined
by their associated surface normal. This can be overridden by
specifying a trivalent suffix as follows:
.RS
.IP \fB+\fR: 
Forward emission; this is equivalent to the abovementioned default behaviour.
.IP \fB-\fR: 
Backward emission; the port is reversed and photons are emitted into the 
halfspace facing away from the surface normal.
.IP \fB0\fR: 
Bidirectional emission; photons are emitted from both sides of the port.
.RE
.IP
Situations that call for a reversed photon port include, for
example, using fenestrations as ports that were (for whatever
reason) defined with outward facing normals, or using a \fBmist\fR 
primitive as a port, since this requires outward facing normals in order to 
register the photons as having entered the volume.
   
.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 acting as photon port, with inward-facing 
normal:
.IP
mkpmap \-apg refRoom.gpm 1m \-apo glazingMat refRoom.oct
.PP
Generate a contribution photon map containing 10 million photons to bin 
light source contributions with \fIrcontrib(1)\fR:
.IP
mkpmap \-apC bonzo-contrib.gpm 10m 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,
.br
(c) Lucerne University of Applied Sciences and Arts,
.br
(c) Tokyo University of Science.

.SH ACKNOWLEDGEMENTS
Development of the RADIANCE photon mapping extension was supported by:

.RS
\fIFraunhofer Institute for Solar Energy Systems\fR funded by
the German Research Foundation (\fIDFG LU204/10-2\fR, "Fassadenintegrierte 
Regelsysteme (FARESYS)"), 

\fILucerne University of Applied Sciences and Arts\fR funded by
the Swiss National Science Foundation (\fISNSF 147053\fR, "Daylight redirecting components"),

\fITokyo University of Science\fR funded by the JSPS Grants-in-Aid for Scientific 
Research Programme (\fIKAKENHI JP19KK0115\fR, "Three-dimensional light flow"). 
.RE

Many thanks also to the many individuals who tested the code and provided
valuable feedback. Special greetz to Don Gregorio, PAB and Capt.\~B!

.SH "SEE ALSO"
rpict(1), rtrace(1), rvu(1), rcontrib(1), 
.br
\fIThe RADIANCE Photon Map Manual\fR,
.br
\fIDevelopment and Integration of the RADIANCE Photon Map Extension: 
Technical Report\fR,
.br
\fIThe RADIANCE Out-of-Core Photon Map: Technical Report\fR,
.br
\fIBonzo Daylighting Tool a.k.a. EvilDRC [TM]\fR