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When designing a scene with photons, it helps to think of the scene objects in two categories. Objects in the first category will show photon caustics when hit by photons. Objects in the second category cause photon caustics by reflecting or refracting photons. Some objects may be in both categories, and some objects may be in neither category.
Category 1 - Objects that show photon caustics
By default, all objects are in the first category. Whenever a photon hits
an object, the photon is stored and will later be used to render caustics on
that object. This means that, by default, caustics from photons can appear
on any surface. To speed up rendering, you can take objects out of this
category. You do this with the line: photons{collect off}. If you use
this syntax, caustics from photons will not appear on the object. This will
save both memory and computational time during rendering.
Category 2 - Objects that cause photon caustics
By default, there are no objects in the second category. If you want your
object to cause caustics, you need to do two things. First, make your object into
a "target." You do this with the target keyword. This enables light
sources to shoot photons at your object. Second, you need to specify if
your object reflects photons, refracts photons, or both. This is done with
the reflection on and refraction on keywords. To allow an object to
reflect and refract photons, you would use the following lines of code
inside the object:
photons{
target
reflection on
refraction on
}
Generally speaking, you don't want an object to be in both categories. Most objects that cause photon caustics do not themselves have much color or brightness. Usually they simply refract or reflect their surroundings. For this reason, it is usually a waste of time to display photon caustics on such surfaces. Even if computed, the effects from the caustics would be so dim that they would go unnoticed.
Sometimes, you may also wish to add photons{collect off} to other clear or
reflective objects, even if they are not photon targets. Again, this is
done to prevent unnecessary computation of caustic lighting.
Finally, you may wish to enable photon reflection and refraction for a surface, even if it is not a target. This allows indirect photons (photons that have already hit a target and been reflected or refracted) to continue their journey after hitting this object.
global_photon_block:
photons {
spacing <photon_spacing> | count <photons_to_shoot>
[gather <min_gather>, <max_gather>]
[media <max_steps> [,<factor>]]
[jitter <jitter_amount>]
[max_trace_level <photon_trace_level>]
[adc_bailout <photon_adc_bailout>]
[save_file "filename" | load_file "filename"]
[autostop <autostop_fraction>]
[expand_thresholds <percent_increase>, <expand_min>]
[radius <gather_radius>,<multiplier>,
<gather_radius_media>,<multiplier>]
}
Global : expand_min : 40 gather : 20, 100 jitter : 0.4 media : 0 Object : collect : on refraction : off reflection : off split_union : on target : 1.0 Light_source: area_light : off refraction : off reflection : off
To specify photon gathering and storage options you need to add a photons block to the global_settings section of your scene.
For example:
global_settings {
photons {
count 20000
autostop 0
jitter .4
}
}
The number of photons generated can be set using either the spacing or count keywords:
The keyword gather allows you to specify how many photons are
gathered at each point during the regular rendering step. The first number
(default 20) is the minimum number to gather, while the second number (default
100) is the maximum number to gather. These are good values and you should only
use different ones if you know what you’re doing.
The keyword media turns on media photons. The parameter
max_steps specifies the maximum number of photons to deposit
over an interval. The optional parameter factor specifies the difference in
media spacing compared to surface spacing. You can increase factor and
decrease max_steps if too many photons are being deposited in media.
The keyword jitter specifies the amount of jitter used in the
sampling of light rays in the pre-processing step. The default value is good and
usually does not need to be changed.
The keywords max_trace_level and adc_bailout allow
you to specify these attributes for the photon-tracing step. If you do not
specify these, the values for the primary ray-tracing step will be used.
The keywords save_file and load_file allow you to
save and load photon maps. If you load a photon map, no photons will be shot.
The photon map file contains all surface (caustic) and media
photons.
radius is used for gathering photons. The larger the radius, the longer it
takes to gather photons. But if you use too small of a radius, you might
not get enough photons to get a good estimate. Therefore, choosing a good
radius is important. Normally POV-Ray looks through the photon map and uses some ad-hoc statistical analysis to determine a reasonable radius. Sometimes it does a good job, sometimes it does not. The radius keyword lets you override or adjust POV-Ray's guess.
radius parameters (all are optional):
radius ,0.5" to lower the radius (multiply by 0.5) and speed up the render at the cost of quality.The keywords autostop
and expand_thresholds will be explained later.
object_photon_block:
photons {
[target [<spacing_multiplier>]]
[refraction on|off]
[reflection on|off]
[collect on|off]
[pass_through]
}
To shoot photons at an object, you need to tell POV that the object receives
photons. To do this, create a photons { } block within the object. For example:
object {
MyObject
photons {
target
refraction on
reflection on
collect off
}
}
In this example, the object both reflects and refracts photons. Either of these options could be turned off (by specifying reflection off, for example). By using this, you can have an object with a reflective finish which does not reflect photons for speed and memory reasons.
The keyword target makes this object a target.
The density of the photons can be adjusted by specifying the
spacing_multiplier. If, for example, you specify a
spacing_multiplier of 0.5, then the spacing for photons hitting
this object will be 1/2 of the distance of the spacing for other objects.
Note: This means four times as many surface photons, and eight times as many media photons.
The keyword collect off causes the object to ignore photons.
Photons are neither deposited nor gathered on that object.
The keyword pass_through causes photons to pass through the
object unaffected on their way to a target object. Once a
photon hits the target object, it will ignore the pass_through
flag. This is basically a photon version of the no_shadow
keyword, with the exception that media within the object will still be
affected by the photons (unless that media specifies collect off). If you
use the no_shadow keyword, the object will be tagged as
pass_through automatically. You can then turn off
pass_through if necessary by simply using photons {
pass_through off }.
Note: Photons will not be shot at an object unless you
specify the target keyword. Simply turning refraction on will not
suffice.
When shooting photons at a CSG-union, it may sometimes be of advantage to use
split_union off inside the union.
POV-Ray will be forced to shoot at the whole object, instead of splitting it up
and shooting photons at its compound parts.
light_photon_block:
photons {
[refraction on | off]
[reflection on | off]
[area_light]
}
Example:
light_source {
MyLight
photons {
refraction on
reflection on
}
}
Sometimes, you want photons to be shot from one light source and not another.
In that case, you can turn photons on for an object, but specify photons {
reflection off refraction off } in the light source’s definition.
You can also turn off only reflection or only refraction for any light
source.
global_settings {
photons {
count 10000
media 100
}
}
Photons also interact fully with media. This means that volumetric photons are stored in scattering media. This is enabled by using the keyword media within the photons block.
To store photons in media, POV deposits photons as it steps through the media during the photon-tracing phase of the render. It will deposit these photons as it traces caustic photons, so the number of media photons is dependant on the number of caustic photons. As a light ray passes through a section of media, the photons are deposited, separated by approximately the same distance that separates surface photons.
You can specify a factor as a second optional parameter to the media keyword. If, for example, factor is set to 2.0, then photons will be spaced twice as far apart as they would otherwise have been spaced.
Sometimes, however, if a section of media is very large, using these settings could create a large number of photons very fast and overload memory. Therefore, following the media keyword, you must specify the maximum number of photons that are deposited for each ray that travels through each section of media. A setting of 100 should probably work in most cases.
You can put collect off into media to make that media ignore
photons. Photons will neither be deposited nor gathered in a media that is
ignoring them. Photons will also not be gathered nor deposited in non-scattering
media. However, if multiple medias exist in the same space, and at least
one does not ignore photons and is scattering, then photons will be deposited in
that interval and will be gathered for use with all media in that interval.
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