2.6.3.2 Using Photon Mapping in Your Scene
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 do not 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.
2.6.3.2.1 Photon Global Settings
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>]
}
All photons default values:
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:
-
If spacing is used, it specifies approximately the average distance between photons on surfaces. If you cut the
spacing in half, you will get four times as many surface photons, and eight times as many media photons.
-
If count is used, POV-Ray will shoot the approximately number of photons specified. The actual number of photons
that result from this will almost always be at least slightly different from the number specified. Still, if you
double the photons_to_shoot value, then twice as many photons will be shot. If you cut the value in half, then half
the number of photons will be shot.
-
It may be less, because POV shoots photons at a target object's bounding box, which means that some photons
will miss the target object.
-
On the other hand, may be more, because each time one object hits an object that has both reflection and
refraction, two photons are created (one for reflection and one for refraction).
-
POV will attempt to compensate for these two factors, but it can only estimate how many photons will actually
be generated. Sometimes this estimation is rather poor, but the feature is still usable.
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 are 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):
-
Manually set the gather radius for surface photons. If this is either zero or if you leave it out, POV-Ray will
analyze and guess.
-
Adjust the radius for surface photons by setting a multiplier. If POV-Ray, for example, is picking a radius that
you think is too big (render is too slow), you can use "
radius ,0.5 " to lower the radius
(multiply by 0.5) and speed up the render at the cost of quality.
-
Manually set the gather radius for media photons.
-
Adjust the radius for media photons by setting a multiplier.
The keywords autostop and expand_thresholds
will be explained later.
2.6.3.2.2 Shooting Photons at an Object
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.
2.6.3.2.3 Photons and Light Sources
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 dependent 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.
More about "split_union off"
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