## 3.6 Interior & Media & Photons

### 3.6.1 Interior

Introduced in POV-Ray 3.1 is an object modifier statement called `interior`. The syntax is:

    INTERIOR:
      interior { [INTERIOR_IDENTIFIER] [INTERIOR_ITEMS...] }
    INTERIOR_ITEM:
      ior Value | caustics Value | dispersion Value |
      dispersion_samples Samples | fade_distance Distance |
      fade_power Power | fade_color <Color>
      MEDIA...

Interior default values:

    ior                : 1.0
    caustics           : 0.0
    dispersion         : 1.0
    dispersion_samples : 7
    fade_distance      : 0.0
    fade_power         : 0.0
    fade_color         : <0,0,0>

The `interior` contains items which describe the properties of the interior of the object. This is in contrast to the `texture` and `interior_texture` which describe the surface properties only. The interior of an object is only of interest if it has a transparent texture which allows you to see inside the object. It also applies only to solid objects which have a well-defined inside/outside distinction.

Note: The `open` keyword, or `clipped_by` modifier also allows you to see inside but interior features may not render properly. They should be avoided if accurate interiors are required.

Interior identifiers may be declared to make scene files more readable and to parameterize scenes so that changing a single declaration changes many values. An identifier is declared as follows.

    INTERIOR_DECLARATION:
      #declare IDENTIFIER = INTERIOR |
      #local IDENTIFIER = INTERIOR

Where IDENTIFIER is the name of the identifier up to 40 characters long and INTERIOR is any valid `interior` statement. See #declare vs. #local for information on identifier scope.

#### 3.6.1.1 Why are Interior and Media Necessary?

In previous versions of POV-Ray, most of the items in the `interior` statement were previously part of the `finish` statement. Also the `halo` statement which was once part of the `texture` statement has been discontinued and has been replaced by the `media` statement which is part of `interior`.

You are probably asking WHY? As explained earlier, the `interior` contains items which describe the properties of the interior of the object. This is in contrast to the `texture` which describes the surface properties only. However this is not just a philosophical change. There were serious inconsistencies in the old model.

The main problem arises when a `texture_map` or other patterned texture is used. These features allow you to create textures that are a blend of two textures and which vary the entire texture from one point to another. It does its blending by fully evaluating the apparent color as though only one texture was applied and then fully reevaluating it with the other texture. The two final results are blended.

It is totally illogical to have a ray enter an object with one index or refraction and then recalculate with another index. The result is not an average of the two ior values. Similarly it makes no sense to have a ray enter at one ior and exit at a different ior without transitioning between them along the way. POV-Ray only calculates refraction as the ray enters or leaves. It cannot incrementally compute a changing ior through the interior of an object. Real world objects such as optical fibers or no-line bifocal eyeglasses can have variable iors but POV-Ray cannot simulate them.

Similarly the `halo` calculations were not performed as the syntax implied. Using a `halo` in such multi-textured objects did not vary the `halo` through the interior of the object. Rather, it computed two separate halos through the whole object and averaged the results. The new design for `media` which replaces `halo` makes it possible to have media that varies throughout the interior of the object according to a pattern but it does so independently of the surface texture. Because there are other changes in the design of this feature which make it significantly different, it was not only moved to the `interior` but the name was changed.

During our development, someone asked if we will create patterned interiors or a hypothetical `interior_map` feature. We will not. That would defeat the whole purpose of moving these features in the first place. They cannot be patterned and have logical or self-consistent results.

#### 3.6.1.2 Empty and Solid Objects

It is very important that you know the basic concept behind empty and solid objects in POV-Ray to fully understand how features like interior and translucency are used. Objects in POV-Ray can either be solid, empty or filled with (small) particles.

A solid object is made from the material specified by its pigment and finish statements (and to some degree its normal statement). By default all objects are assumed to be solid. If you assign a stone texture to a sphere you will get a ball made completely of stone. It is like you had cut this ball from a block of stone. A glass ball is a massive sphere made of glass. You should be aware that solid objects are conceptual things. If you clip away parts of the sphere you will clearly see that the interior is empty and it just has a very thin surface.

This is not contrary to the concept of a solid object used in POV-Ray. It is assumed that all space inside the sphere is covered by the sphere's `interior`. Light passing through the object is affected by attenuation and refraction properties. However there is no room for any other particles like those used by fog or interior media.

Empty objects are created by adding the `hollow` keyword (see Hollow) to the object statement. An empty (or hollow) object is assumed to be made of a very thin surface which is of the material specified by the pigment, finish and normal statements. The object's interior is empty, it normally contains air molecules.

An empty object can be filled with particles by adding fog or atmospheric media to the scene or by adding an interior media to the object. It is very important to understand that in order to fill an object with any kind of particles it first has to be made hollow.

There is a pitfall in the empty/solid object implementation that you have to be aware of.

In order to be able to put solid objects inside a media or fog, a test has to be made for every ray that passes through the media. If this ray travels through a solid object the media will not be calculated. This is what anyone will expect. A solid glass sphere in a fog bank does not contain fog.

The problem arises when the camera ray is inside any non-hollow object. In this case the ray is already traveling through a solid object and even if the media's container object is hit and it is hollow, the media will not be calculated. There is no way of telling between these two cases.

POV-Ray has to determine whether the camera is inside any object prior to tracing a camera ray in order to be able to correctly render medias when the camera is inside the container object. There is no way around doing this.

The solution to this problem (that will often happen with infinite objects like planes) is to make those objects hollow too. Thus the ray will travel through a hollow object, will hit the container object and the media will be calculated.

#### 3.6.1.3 Scaling objects with an interior

All the statements that can be put in an interior represent aspects of the matter that an object is made of. Scaling an object, changing its size, does not change its matter. Two pieces of the same quality steel, one twice as big as the other, both have the same density. The bigger piece is quite a bit heavier though.

So, in POV-Ray, if you design a lens from a glass with an ior of 1.5 and you scale it bigger, the focal distance of the lens will get longer as the ior stays the same. For light attenuation it means that an object will be darker after being scaled up. The light intensity decreases a certain amount per pov-unit. The object has become bigger, more pov-units, so more light is faded. The `fade_distance, fade_power` themselves have not been changed.

The same applies to media. Imagine media as a density of particles, you specify 100 particles per cubic pov-unit. If we scale a 1 cubic pov-unit object to be twice as big in every direction, we will have a total of 800 particles in the object. The object will look different, as we have more particles to look through. Yet the objects density is still 100 particles per cubic pov-unit. In media this particle density is set by the color after `emission`, `absorption`, or in the `scattering` statement

    #version 3.5;
    global_settings {
      assumed_gamma 1.0
      }

    camera {location <0, 0,-12.0> look_at 0 angle 30 }

    #declare Container_T =
      texture {
        pigment {rgbt <1,1,1,1>}
      finish {ambient 0 diffuse 0}
      }

    #declare Scale=2;

    box {                             //The reference
      <-1,-1,0>,<1,1,.3>
      hollow
      texture {Container_T}
      interior {
        media {
          intervals 1
          samples 1,1
          emission 1
          }
        }
      translate <-2.1,0,0>
      }

    box {                             //Object scaled twice as big
      <-1,-1,0>,<1,1,.3>  //looks different but same
      hollow                          //particle density
      texture {Container_T}
        interior {
          media {
            intervals 1
            samples 1,1
            emission 1
            }
          }
      scale Scale
      translate<0,0,12>
      }

    box {                             //Object scaled twice as big
      <-1,-1,0>,<1,1,.3>  //looks the same but particle
      hollow                          //density scaled down
      texture {Container_T}
        interior {
          media {
            intervals 1
            samples 1,1
            emission 1/Scale
            }
          }
      scale Scale
      translate<0,0,12>
      translate<4.2,0,0>
      }

The third object in the scene above, shows what to do, if you want to scale the object and want it to keep the same look as before. The interior feature has to be divided by the same amount, that the object was scaled by. This is only possible when the object is scaled uniform.

In general, the correct approach is to scale the media density proportionally to the change in container volume. For non-uniform scaling to get an unambiguous result, that can be explained in physical terms, we need to do:

    Density*sqrt(3)/vlength(Scale)

where Density is your original media density and Scale is the scaling vector applied to the container.

Note: The density modifiers inside the `density{}` statement are scaled along with the object.

#### 3.6.1.4 Refraction

When light passes through a surface either into or out of a dense medium the path of the ray of light is bent. Such bending is called refraction. The amount of bending or refracting of light depends upon the density of the material. Air, water, crystal and diamonds all have different densities and thus refract differently. The index of refraction or ior value is used by scientists to describe the relative density of substances. The `ior` keyword is used in POV-Ray in the `interior` to turn on refraction and to specify the ior value. For example:

    object { MyObject pigment {Clear } interior { ior 1.5 } }

The default ior value of 1.0 will give no refraction. The index of refraction for air is 1.0, water is 1.33, glass is 1.5 and diamond is 2.4.

Normally transparent or semi-transparent surfaces in POV-Ray do not refract light. Earlier versions of POV-Ray required you to use the `refraction` keyword in the `finish` statement to turn on refraction. This is no longer necessary. Any non-zero `ior` value now turns refraction on.

In addition to turning refraction on or off, the old `refraction` keyword was followed by a float value from 0.0 to 1.0. Values in between 0.0 and 1.0 would darken the refracted light in ways that do not correspond to any physical property. Many POV-Ray scenes were created with intermediate refraction values before this bug was discovered so the feature has been maintained. A more appropriate way to reduce the brightness of refracted light is to change the `filter` or `transmit` value in the colors specified in the pigment statement or to use the `fade_power` and `fade_distance` keywords. See Attenuation.

Note: Neither the `ior` nor `refraction` keywords cause the object to be transparent. Transparency only occurs if there is a non-zero `filter` or `transmit` value in the color.

The `refraction` and `ior` keywords were originally specified in `finish` but are now properly specified in `interior`. They are accepted in `finish` for backward compatibility and generate a warning message.

#### 3.6.1.5 Dispersion

For all materials with a ior different from 1.0 the refractive index is not constant throughout the spectrum. It changes as a function of wavelength. Generally the refractive index decreases as the wavelength increases. Therefore light passing through a material will be separated according to wavelength. This is known as chromatic dispersion.

By default POV-Ray does not calculate dispersion as light travels through a transparent object. In order to get a more realistic effect the `dispersion` and `dispersion_samples` keywords can be added to the `interior{}` block. They will simulate dispersion by creating a prismatic color effect in the object.

The `dispersion` value is the ratio of refractive indices for violet to red. It controls the strength of dispersion (how much the colors are spread out) used. A DISPERSION_VALUE of 1 will give no dispersion, good values are 1.01 to 1.1.

Note: There will be no dispersion, unless the `ior` keyword has been specified in `interior{ }`. An ior of 1 is legal. The ior has no influence on the dispersion strength, only on the angle of refraction.

As POV-Ray does not use wavelengths for raytracing, a spectrum is simulated. The `dispersion_samples` value controls the amount of color-steps and smoothness in the spectrum. The default value is 7, the minimum is 2. Values up to 100 or higher may be needed to get a very smooth result.

##### 3.6.1.5.1 Dispersion & Caustics

Dispersion only affects the interior of an object and has no effect on faked caustics (See Faked Caustics).
To see the effects of dispersion in caustics, photon mapping is needed (See the sections Photons and Photons & Dispersion).

#### 3.6.1.6 Attenuation

Light attenuation is used to model the decrease in light intensity as the light travels through a transparent object. The keywords `fade_power`, `fade_distance` and `fade_color` are specified in the `interior` statement.

The `fade_distance` value determines the distance the light has to travel to reach half intensity while the `fade_power` value determines how fast the light will fall off. `fade_color` colorizes the attenuation. For realistic effects a fade power of 1 to 2 should be used. Default values for `fade_power` and `fade_distance` is 0.0 which turns this feature off. Default for `fade_color` is `<0,0,0>`, if `fade_color` is `<1,1,1>` there is no attenuation. The actual colors give colored attenuation. `<1,0,0>` looks red, not cyan as in media.

The attenuation is calculated by a formula similar to that used for light source attenuation.

Media Attenuation

If you set fade_power in the interior of an object at 1000 or above, a realistic exponential attenuation function will be used:

       Attenuation = exp(-depth/fade_dist)

The `fade_power` and `fade_distance` keywords were originally specified in `finish` but are now properly specified in `interior`. They are accepted in `finish` for backward compatibility and generate a warning message.

#### 3.6.1.7 Simulated Caustics

Caustics are light effects that occur if light is reflected or refracted by specular reflective or refractive surfaces. Imagine a glass of water standing on a table. If sunlight falls onto the glass you will see spots of light on the table. Some of the spots are caused by light being reflected by the glass while some of them are caused by light being refracted by the water in the glass.

Since it is a very difficult and time-consuming process to actually calculate those effects (though it is not impossible, see the sections Photons) POV-Ray uses a quite simple method to simulate caustics caused by refraction. The method calculates the angle between the incoming light ray and the surface normal. Where they are nearly parallel it makes the shadow brighter. Where the angle is greater, the effect is diminished. Unlike real-world caustics, the effect does not vary based on distance. This caustic effect is limited to areas that are shaded by the transparent object. You will get no caustic effects from reflective surfaces nor in parts that are not shaded by the object.

The `caustics` `Power` keyword controls the effect. Values typically range from 0.0 to 1.0 or higher. Zero is the default which is no caustics. Low, non-zero values give broad hot-spots while higher values give tighter, smaller simulated focal points.

The `caustics` keyword was originally specified in `finish` but is now properly specified in `interior`. It is accepted in `finish` for backward compatibility and generates a warning message.

#### 3.6.1.8 Object-Media

The `interior` statement may contain one or more `media` statements. Media is used to simulate suspended particles such as smoke, haze, or dust. Or visible gasses such as steam or fire and explosions. When used with an object interior, the effect is constrained by the object's shape. The calculations begin when the ray enters an object and ends when it leaves the object. This section only discusses media when used with object interior. The complete syntax and an explanation of all of the parameters and options for `media` is given in the section Media.

Typically the object itself is given a fully transparent texture however media also works in partially transparent objects. The texture pattern itself does not effect the interior media except perhaps to create shadows on it. The texture pattern of an object applies only to the surface shell. Any interior media patterns are totally independent of the texture.

In previous versions of POV-Ray, this feature was called `halo` and was part of the `texture` specification along with `pigment`, `normal`, and `finish`. See Why are Interior and Media Necessary? for an explanation of the reasons for the change.

Media may also be specified outside an object to simulate atmospheric media. There is no constraining object in this case. If you only want media effects in a particular area, you should use object media rather than only relying upon the media pattern. In general it will be faster and more accurate because it only calculates inside the constraining object. See Atmospheric Media for details on unconstrained uses of media.

You may specify more than one `media` statement per `interior` statement. In that case, all of the media participate and where they overlap, they add together.

Any object which is supposed to have media effects inside it, whether those effects are object media or atmospheric media, must have the `hollow on` keyword applied. Otherwise the media is blocked. See Empty and Solid Objects for details.

### 3.6.2 Media

The `media` statement is used to specify particulate matter suspended in a medium such air or water. It can be used to specify smoke, haze, fog, gas, fire, dust etc. Previous versions of POV-Ray had two incompatible systems for generating such effects. One was `halo` for effects enclosed in a transparent or semi-transparent object. The other was `atmosphere` for effects that permeated the entire scene. This duplication of systems was complex and unnecessary. Both `halo` and `atmosphere` have been eliminated. See Why are Interior and Media Necessary? for further details on this change. See Object Media for details on how to use `media` with objects. See Atmospheric Media for details on using `media` for atmospheric effects outside of objects. This section and the sub-sections which follow explains the details of the various `media` options which are useful for either object media or atmospheric media.

Media works by sampling the density of particles at some specified number of points along the ray's path. Sub-samples are also taken until the results reach a specified confidence level. POV-Ray provides three methods of sampling. When used in an object's `interior` statement, sampling only occurs inside the object. When used for atmospheric media, the samples run from the camera location until the ray strikes an object. Therefore for localized effects, it is best to use an enclosing object even though the density pattern might only produce results in a small area whether the media was enclosed or not.

The complete syntax for a `media` statement is as follows:

    MEDIA:
      media { [MEDIA_IDENTIFIER] [MEDIA_ITEMS...] }
    MEDIA_ITEMS:
      method Number | intervals Number | samples Min, Max |
      confidence Value  | variance Value | ratio Value | jitter Value
      absorption COLOR | emission COLOR | aa_threshold Value |
      aa_level Value |
      scattering {
        Type, COLOR [ eccentricity Value ] [ extinction Value ]
        }  |
      density {
        [DENSITY_IDENTIFIER] [PATTERN_TYPE] [DENSITY_MODIFIER...]
        }   |
      TRANSFORMATIONS
    DENSITY_MODIFIER:
      PATTERN_MODIFIER | DENSITY_LIST | COLOR_LIST |
      color_map { COLOR_MAP_BODY } | colour_map { COLOR_MAP_BODY } |
      density_map { DENSITY_MAP_BODY }

Media default values:

    aa_level     : 3
    aa_threshold : 0.1
    absorption   : <0,0,0>
    confidence   : 0.9
    emission     : <0,0,0>
    intervals    : 1
    jitter       : 0.0
    method       : 3
    ratio        : 0.9
    samples      : Min 1, Max 1
    variance     : 1/128
    SCATTERING
    COLOR        : <0,0,0>
    eccentricity : 0.0
    extinction   : 1.0

If a media identifier is specified, it must be the first item. All other media items may be specified in any order. All are optional. You may have multiple `density` statements in a single `media` statement. See Multiple Density vs. Multiple Media for details. Transformations apply only to the `density` statements which have been already specified. Any `density` after a transformation is not affected. If the `media` has no `density` statements and none was specified in any media identifier, then the transformation has no effect. All other media items except for `density` and transformations override default values or any previously set values for this `media` statement.

Note: Some media effects depend upon light sources. However the participation of a light source depends upon the `media_interaction` and `media_attenuation` keywords. See Atmospheric Media Interaction and Atmospheric Attenuation for details.

Note: If you specify `transmit` or `filter` to create a transparent container object, `absorption` media will always cast a shadow. The same applies to `scattering` media unless `extinction` is set to zero, so if a shadow is not desired, use the `no_shadow` keyword for the container object. This does not apply to `emission` media as it never casts a shadow.

#### 3.6.2.1 Media Types

There are three types of particle interaction in `media`: absorbing, emitting, and scattering. All three activities may occur in a single media. Each of these three specifications requires a color. Only the red, green, and blue components of the color are used. The filter and transmit values are ignored. For this reason it is permissible to use one float value to specify an intensity of white color. For example, the following two lines are legal and produce the same results:

    emission 0.75
    emission rgb <0.75,0.75,0.75>

##### 3.6.2.1.1 Absorption

The `absorption` keyword specifies a color of light which is absorbed when looking through the media. For example, `absorption rgb<0,1,0>` blocks the green light but permits red and blue to get through. Therefore a white object behind the media will appear magenta.

The default value is `rgb<0,0,0>` which means no light is absorbed -- all light passes through normally.

##### 3.6.2.1.2 Emission

The `emission` keyword specifies the color of the light emitted from the particles. Particles which emit light are visible without requiring additional illumination. However, they will only illuminate other objects if radiosity is used with media on. This is similar to an object with high `ambient` values. The default value is `rgb<0,0,0>` which means no light is emitted.

##### 3.6.2.1.3 Scattering

The syntax of a `scattering` statement is:

    SCATTERING:
      scattering {
        Type, COLOR [ eccentricity Value ] [ extinction Value ]
        }

The first float value specifies the type of scattering. This is followed by the color of the scattered light. The default value if no `scattering` statement is given is `rgb <0,0,0>` which means no scattering occurs.

The scattering effect is only visible when light is shining on the media from a light source. This is similar to `diffuse` reflection off of an object. In addition to reflecting light, scattering media also absorbs light like an `absorption` media. The balance between how much absorption occurs for a given amount of scattering is controlled by the optional `extinction` keyword and a single float value. The default value of 1.0 gives an extinction effect that matches the scattering. Values such as `extinction 0.25` give 25% the normal amount. Using `extinction 0.0` turns it off completely. Any value other than the 1.0 default is contrary to the real physical model but decreasing extinction can give you more artistic flexibility.

The integer value `Type` specifies one of five different scattering phase functions representing the different models: isotropic, Mie (haze and murky atmosphere), Rayleigh, and Henyey-Greenstein.

Type 1, isotropic scattering is the simplest form of scattering because it is independent of direction. The amount of light scattered by particles in the atmosphere does not depend on the angle between the viewing direction and the incoming light.

Types 2 and 3 are Mie haze and Mie murky scattering which are used for relatively small particles such as minuscule water droplets of fog, cloud particles, and particles responsible for the polluted sky. In this model the scattering is extremely directional in the forward direction, i.e. the amount of scattered light is largest when the incident light is anti-parallel to the viewing direction (the light goes directly to the viewer). It is smallest when the incident light is parallel to the viewing direction. The haze and murky atmosphere models differ in their scattering characteristics. The murky model is much more directional than the haze model.

The Mie haze scattering function

The Mie murky scattering function

Type 4 Rayleigh scattering models the scattering for extremely small particles such as molecules of the air. The amount of scattered light depends on the incident light angle. It is largest when the incident light is parallel or anti-parallel to the viewing direction and smallest when the incident light is perpendicular to the viewing direction. You should note that the Rayleigh model used in POV-Ray does not take the dependency of scattering on the wavelength into account.

The Rayleigh scattering function

Type 5 is the Henyey-Greenstein scattering model. It is based on an analytical function and can be used to model a large variety of different scattering types. The function models an ellipse with a given eccentricity e. This eccentricity is specified by the optional keyword `eccentricity` which is only used for scattering type five. The default eccentricity value of zero defines isotropic scattering while positive values lead to scattering in the direction of the light and negative values lead to scattering in the opposite direction of the light. Larger values of e (or smaller values in the negative case) increase the directional property of the scattering.

The Henyey-Greenstein scattering function for different eccentricity values

Note: See the section on Light Groups for additional information when using scattering media in a light group.

#### 3.6.2.2 Sampling Parameters & Methods

Media effects are calculated by sampling the media along the path of the ray. It uses a process called Monte Carlo integration. POV-Ray provides three different types of media sampling. The `method` keyword lets you specify what sampling type is used.

Sample `method 3` uses adaptive sampling (similar to adaptive anti-aliasing) which is very much like the sampling method used in POV-Ray 3.0 atmosphere. This code was written from the ground-up to work with media. However, adaptive sampling works by taking another sample between two existing samples if there is too much variance in the original two samples. This leads to fewer samples being taken in areas where the effect from the media remains constant. The adaptive sampling is only performed if the minimum samples are set to 3 or more.

You can specify the anti-aliasing recursion depth using the `aa_level` keyword followed by an integer. You can specify the anti-aliasing threshold by using the `aa_threshold` followed by a float. The default for `aa_level` is 4 and the default `aa_threshold` is 0.1. `jitter` also works with method 3.

Note: It is usually best to only use one interval with method 3. Too many intervals can lead to artifacts, and POV will create more intervals if it needs them.

Be Aware: As of version 3.5 the default sampling `method` is 3, and it's default for `intervals` is 1. Sampling methods 1 and 2 have been retained for legacy purposes.

Sample `method 1` used the `intervals` keyword to specify the integer number of intervals used to sample the ray. For object media, the intervals are spread between the entry and exit points as the ray passes through the container object. For atmospheric media, the intervals spans the entire length of the ray from its start until it hits an object. For media types which interact with spotlights or cylinder lights, the intervals which are not illuminated by these light types are weighted differently than the illuminated intervals when distributing samples.

The `ratio` keyword distributes intervals differently between lit and unlit areas. The default value of `ratio 0.9` means that lit intervals get more samples than unlit intervals. Note that the total number of intervals must exceed the number of illuminated intervals. If a ray passes in and out of 8 spotlights but you have only specified 5 intervals then an error occurs.

The `samples` `Min`, `Max` keyword specifies the minimum and maximum number of samples taken per interval. The default values are `samples 1,1`. The value for Max may be omitted, in which case the range Min = Max will be used.

As each interval is sampled, the variance is computed. If the variance is below a threshold value, then no more samples are needed. The `variance` and `confidence` keywords specify the permitted variance allowed and the confidence that you are within that variance. The exact calculations are quite complex and involve chi-squared tests and other statistical principles too messy to describe here. The default values are `variance 1.0/128` and `confidence 0.9`. For slower more accurate results, decrease the variance and increase the confidence.

Note: The maximum number of samples limits the calculations even if the proper variance and confidence are never reached.

Sample `method 2` distributed samples evenly along the viewing ray or light ray. The latter can make things look smoother sometimes. If you specify a maximum number of samples higher than the minimum number of samples, POV will take additional samples, but they will be random, just like in method 1. Therefore, it is suggested you set the max samples equal to the minimum samples. `jitter` will cause method 2 to look similar to method 1. It should be followed by a float, and a value of 1 will stagger the samples in the full range between samples.

#### 3.6.2.3 Density

Particles of media are normally distributed in constant density throughout the media. However, the `density` statement allows you to vary the density across space using any of POV-Ray's pattern functions such as those used in textures. If no `density` statement is given then the density remains a constant value of 1.0 throughout the media. More than one `density` may be specified per `media` statement. See Multiple Density vs. Multiple Media. The syntax for `density` is:

    DENSITY:
      density {
        [DENSITY_IDENTIFIER]
        [DENSITY_TYPE]
        [DENSITY_MODIFIER...]
        }

    DENSITY_TYPE:
      PATTERN_TYPE | COLOR
      DENSITY_MODIFIER:
      PATTERN_MODIFIER | DENSITY_LIST | color_map { COLOR_MAP_BODY } |
      colour_map { COLOR_MAP_BODY } | density_map { DENSITY_MAP_BODY }

The `density` statement may begin with an optional density identifier. All subsequent values modify the defaults or the values in the identifier. The next item is a pattern type. This is any one of POV-Ray's pattern functions such as `bozo`, `wood`, `gradient`, `waves`, etc. Of particular usefulness are the `spherical`, `planar`, `cylindrical`, and `boxed` patterns which were previously available only for use with our discontinued `halo` feature. All patterns return a value from 0.0 to 1.0. This value is interpreted as the density of the media at that particular point. See Patterns for details on particular pattern types. Although a solid COLOR pattern is legal, in general it is used only when the `density` statement is inside a `density_map`.

##### 3.6.2.3.1 General Density Modifiers

A `density` statement may be modified by any of the general pattern modifiers such as transformations, `turbulence` and `warp`. See Pattern Modifiers for details. In addition, there are several density-specific modifiers which can be used.

##### 3.6.2.3.2 Density with color_map

Typically, a `media` uses just one constant color throughout. Even if you vary the density, it is usually just one color which is specified by the `absorption`, `emission`, or `scattering` keywords. However, when using `emission` to simulate fire or explosions, the center of the flame (high density area) is typically brighter and white or yellow. The outer edge of the flame (less density) fades to orange, red, or in some cases deep blue. To model the density-dependent change in color which is visible, you may specify a `color_map`. The pattern function returns a value from 0.0 to 1.0 and the value is passed to the color map to compute what color or blend of colors is used. See Color Maps for details on how pattern values work with `color_map`. This resulting color is multiplied by the `absorption`, `emission` and `scattering` color. Currently there is no way to specify different color maps for each media type within the same `media` statement.

Consider this example:

    media {
      emission 0.75
      scattering {1, 0.5}
      density {
        spherical
        color_map {
          [0.0 rgb <0,0,0.5>]
          [0.5 rgb <0.8, 0.8, 0.4>]
          [1.0 rgb <1,1,1>]
          }
        }
      }

The color map ranges from white at density 1.0 to bright yellow at density 0.5 to deep blue at density 0. Assume we sample a point at density 0.5. The emission is 0.75*<0.8,0.8,0.4> or <0.6,0.6,0.3>. Similarly the scattering color is 0.5*<0.8,0.8,0.4> or <0.4,0.4,0.2>.

For block pattern types `checker`, `hexagon`, and `brick` you may specify a color list such as this:

    density {
     checker
       density {rgb<1,0,0>}
       density {rgb<0,0,0>}
       }

See Color List Pigments which describes how `pigment` uses a color list. The same principles apply when using them with `density`.

##### 3.6.2.3.3 Density Maps and Density Lists

In addition to specifying blended colors with a color map you may create a blend of densities using a `density_map`. The syntax for a density map is identical to a color map except you specify a density in each map entry (and not a color).

The syntax for `density_map` is as follows:

    DENSITY_MAP:
      density_map { DENSITY_MAP_BODY }
    DENSITY_MAP_BODY:
      DENSITY_MAP_IDENTIFIER | DENSITY_MAP_ENTRY...
    DENSITY_MAP_ENTRY:
      [ Value DENSITY_BODY ]

Where `Value` is a float value between 0.0 and 1.0 inclusive and each DENSITY_BODY is anything which can be inside a `density{...}` statement. The `density` keyword and `{}` braces need not be specified.

Note: The `[]` brackets are part of the actual DENSITY_MAP_ENTRY. They are not notational symbols denoting optional parts. The brackets surround each entry in the density map.

There may be from 2 to 256 entries in the map.

Density maps may be nested to any level of complexity you desire. The densities in a map may have color maps or density maps or any type of density you want.

Entire densities may also be used with the block patterns such as `checker`, `hexagon` and `brick`. For example:

    density {
      checker
        density { Flame scale .8 }
        density { Fire scale .5 }
        }

Note: In the case of block patterns the `density` wrapping is required around the density information.

A density map is also used with the `average` density type. See Average for details.

You may declare and use density map identifiers but the only way to declare a density block pattern list is to declare a density identifier for the entire density.

##### 3.6.2.3.4 Multiple Density vs. Multiple Media

It is possible to have more than one `media` specified per object and it is legal to have more than one `density` per `media`. The effects are quite different. Consider this example:

    object {
      MyObject
      pigment { rgbf 1 }
      interior {
        media {
          density { Some_Density }
          density { Another_Density }
          }
        }
      }

As the media is sampled, calculations are performed for each density pattern at each sample point. The resulting samples are multiplied together. Suppose one density returned `rgb<.8,.8,.4>` and the other returned `rgb<.25,.25,0>`. The resulting color is `rgb<.2,.2,0>`.

Note: In areas where one density returns zero, it will wipe out the other density. The end result is that only density areas which overlap will be visible. This is similar to a CSG intersection operation. Now consider

    object {
      MyObject
      pigment { rgbf 1 }
      interior {
        media {
          density { Some_Density }
          }
        media {
          density { Another_Density }
          }
        }
      }

In this case each media is computed independently. The resulting colors are added together. Suppose one density and media returned `rgb<.8,.8,.4>` and the other returned `rgb<.25,.25,0>`. The resulting color is `rgb<1.05,1.05,.4>`. The end result is that density areas which overlap will be especially bright and all areas will be visible. This is similar to a CSG union operation. See the sample scene `~scenes\interior\media\media4.pov` for an example which illustrates this.

### 3.6.3 Photons

#### 3.6.3.1 Overview

The basic goal of this implementation of the photon map is to render true reflective and refractive caustics. The photon map was first introduced by Henrik Wann Jensen (see Suggested Reading).

Photon mapping is a technique which uses a forward ray-tracing pre-processing step to render refractive and reflective caustics realistically. This means that mirrors can reflect light rays and lenses can focus light.

Photon mapping works by shooting packets of light (photons) from light sources into the scene. The photons are directed towards specific objects. When a photon hits an object after passing through (or bouncing off of) the target object, the ray intersection is stored in memory. This data is later used to estimate the amount of light contributed by reflective and refractive caustics.

##### 3.6.3.1.1 Examples

This image shows refractive caustics from a sphere and a cylinder. Both use an index of refraction of `1.2`. Also visible is a small amount of reflective caustics from the metal sphere, and also from the clear cylinder and sphere. |
| Reflective caustics

| Here we have three lenses and three light sources. The middle lens has photon mapping turned off. You can also see some reflective caustics from the brass box (some light reflects and hits the blue box, other light bounces through the nearest lens and is focused in the lower left corner of the image).
Photons used for lenses and caustics |

#### 3.6.3.2 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.

##### 3.6.3.2.1 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):

1. 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.
2. 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.
3. Manually set the gather radius for media photons.
4. Adjust the radius for media photons by setting a multiplier.

The keywords `autostop` and `expand_thresholds` will be explained later.

##### 3.6.3.2.2 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.

##### 3.6.3.2.3 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.

Note: The photon shooting performance has been improved with the addition of multiple-thread support. To take advantage of this at the moment, your scene will need multiple light sources.

##### 3.6.3.2.4 Photons and Media

    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.

#### 3.6.3.3 Photons FAQ

I made an object with IOR 1.0 and the shadows look weird.

If the borders of your shadows look odd when using photon mapping, do not be alarmed. This is an unfortunate side-effect of the method. If you increase the density of photons (by decreasing spacing and gather radius) you will notice the problem diminish. We suggest not using photons if your object does not cause much refraction (such as with a window pane or other flat piece of glass or any objects with an IOR very close to 1.0).

My scene takes forever to render.

When POV-Ray builds the photon maps, it continually displays in the status bar the number of photons that have been shot. Is POV-Ray stuck in this step and does it keep shooting lots and lots of photons?

yes

If you are shooting photons at an infinite object (like a plane), then you should expect this. Either be patient or do not shoot photons at infinite objects.

Are you shooting photons at a CSG difference? Sometimes POV-Ray does a bad job creating bounding boxes for these objects. And since photons are shot at the bounding box, you could get bad results. Try manually bounding the object. You can also try the autostop feature (try `autostop 0`). See the docs for more info on autostop.

no

Does your scene have lots of glass (or other clear objects)? Glass is slow and you need to be patient.

My scene has polka dots but renders really quickly. Why?

You should increase the number of photons (or decrease the spacing).

The photons in my scene show up only as small, bright dots. How can I fix this?

The automatic calculation of the gather radius is probably not working correctly, most likely because there are many photons not visible in your scene which are affecting the statistical analysis.

You can fix this by either reducing the number of photons that are in your scene but not visible to the camera (which confuse the auto-computation), or by specifying the initial gather radius manually by using the keyword radius. If you must manually specify a gather radius, it is usually best to also use spacing instead of count, and then set radius and spacing to a 5:1 (radius:spacing) ratio.

Adding photons slowed down my scene a lot, and I see polka dots.

This is usually caused by having both high- and low-density photons in the same scene. The low density ones cause polka dots, while the high density ones slow down the scene. It is usually best if the all photons are on the same order of magnitude for spacing and brightness. Be careful if you are shooting photons objects close to and far from a light source. There is an optional parameter to the target keyword which allows you to adjust the spacing of photons at the target object. You may need to adjust this factor for objects very close to or surrounding the light source.

I added photons, but I do not see any caustics.

When POV-Ray builds the photon maps, it continually displays in the status bar the number of photons that have been shot. Did it show any photons being shot?

no

Try avoiding `autostop`, or you might want to bound your object manually.

Try increasing the number of photons (or decreasing the spacing).

yes

Were any photons stored (the number after `total` in the rendering message as POV-Ray shoots photons)?

no

It is possible that the photons are not hitting the target object (because another object is between the light source and the other object).

yes

The photons may be diverging more than you expect. They are probably there, but you cannot see them since they are spread out too much

The base of my glass object is really bright.

Use `collect off` with that object.

Will area lights work with photon mapping?

Photons do work with area lights. However, normally photon mapping ignores all area light options and treats all light sources as point lights. If you would like photon mapping to use your area light options, you must specify the "area_light" keyword within the `photons { }` block in your light source's code. Doing this will not increase the number of photons shot by the light source, but it might cause regular patterns to show up in the rendered caustics (possibly splotchy).

What do the stats mean?

In the stats, `photons shot` means how many light rays were shot from the light sources. `photons stored` means how many photons are deposited on surfaces in the scene. If you turn on reflection and refraction, you could get more photons stored than photons shot, since the each ray can get split into two.

#### 3.6.3.4 Photon Tips

- Use `collect off` in objects that photons do not hit. Just put `photons { collect off }` in the object's definition.
- Use `collect off` in glass objects.
- Use `autostop` unless it causes problems.
- A big tip is to make sure that all of the final densities of photons are of the same general magnitude. You do not want spots with really high density photons and another area with really low density photons. You will always have some variation (which is a good thing), but having really big differences in photon density is what causes some scenes to take many hours to render.

#### 3.6.3.5 Advanced Techniques

##### 3.6.3.5.1 Autostop

To understand the `autostop` option, you need to understand the way photons are shot from light sources. Photons are shot in a spiral pattern with uniform angular density. Imagine a sphere with a spiral starting at one of the poles and spiraling out in ever-increasing circles to the equator. Two angles are involved here. The first, phi, is the how far progress has been made in the current circle of the spiral. The second, theta, is how far we are from the pole to the equator. Now, imagine this sphere centered at the light source with the pole where the spiral starts pointed towards the center of the object receiving photons. Now, photons are shot out of the light in this spiral pattern.

| Example of the photon autostop option

Normally, POV does not stop shooting photons until the target object's entire bounding box has been thoroughly covered. Sometimes, however, an object is much smaller than its bounding box. At these times, we want to stop shooting if we do a complete circle in the spiral without hitting the object. Unfortunately, some objects (such as copper rings), have holes in the middle. Since we start shooting at the middle of the object, the photons just go through the hole in the middle, thus fooling the system into thinking that it is done. To avoid this, the `autostop` keyword lets you specify how far the system must go before this auto-stopping feature kicks in. The value specified is a fraction of the object's bounding box. Valid values are 0.0 through 1.0 (0% through 100%). POV will continue to shoot photons until the spiral has exceeded this value or the bounding box is completely covered. If a complete circle of photons fails to hit the target object after the spiral has passed the autostop threshold, POV will then stop shooting photons.

The `autostop` feature will also not kick in until at least one photon has hit the object. This allows you to use `autostop 0` even with objects that have holes in the middle.

Note: If the light source is within the object's bounding box, the photons are shot in all directions from the light source.

##### 3.6.3.5.2 Adaptive Search Radius

Unless photons are interacting with media, POV-Ray uses an adaptive search radius while gathering photons. If the minimum number of photons is not found in the original search radius, the radius is expanded and searched again. Using this adaptive search radius can both decrease the amount of time it takes to render the image, and sharpen the borders in the caustic patterns.

Sometimes this adaptive search technique can create unwanted artifacts at borders. To remove these artifacts, a few thresholds are used, which can be specified by `expand_thresholds`. For example, if expanding the radius increases the estimated density of photons by too much (threshold is percent_increase, default is 20%, or 0.2), the expanded search is discarded and the old search is used instead. However, if too few photons are gathered in the expanded search (`expand_min`, default is 40), the new search will be used always, even if it means more than a 20% increase in photon density.

##### 3.6.3.5.3 Photons and Dispersion

When dispersion is specified for interior of a transparent object, photons will make use of that and show "colored" caustics.

##### 3.6.3.5.4 Saving and Loading Photon Maps

It is possible to save and load photon maps to speed up rendering. The photon map itself is view-independent, so if you want to animate a scene that contains photons and you know the photon map will not change during the animation, you can save it on the first frame and then load it for all subsequent frames.

To save the photon map, put the line

    save_file "myfile.ph"

into the `photons { }` block inside the `global_settings` section.

Loading the photon map is the same, but with `load_file` instead of `save_file`. You cannot both load and save a photon map in the POV file. If you load the photon map, it will load all of the photons. No photons will be shot if the map is loaded from a file. All other options (such as gather radius) must still be specified in the POV scene file and are not loaded with the photon map.

When can you safely re-use a saved photon map?

- Moving the camera is always safe.
- Moving lights that do not cast photons is always safe.
- Moving objects that do not have photons shot at them, that do not receive photons, and would not receive photons in the new location is always safe.
- Moving an object that receives photons to a new location where it does not receive photons is sometimes safe.
- Moving an object to a location where it receives photons is not safe
- Moving an object that has photons shot at it is not safe
- Moving a light that casts photons is not safe.
- Changing the texture of an object that receives photons is safe.
- Changing the texture of an object that has photons shot at it produces results that are not realistic, but can be useful sometimes.

In general, changes to the scene geometry require photons to be re-shot. Changing the camera parameters or changing the image resolution does not.