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<strong class="NavBar">POV-Ray 3.6 for UNIX documentation</strong><br> <strong>3.6.1
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<a href="s_129.html">3.6.2 Media</a>
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<h3><a name="s03_06_01">3.6.1 </a>Interior</h3>
<a name="s03_06_01_i1"><a name="interior"></a><a name="s03_06_01_i2"><a name="interior, keyword"></a><a name="s03_06_01_i3"><a name="s03_06_01_i4"><a name="ior, interior"></a><a name="s03_06_01_i5"><a name="s03_06_01_i6"><a name="caustics, interior"></a><a name="s03_06_01_i7"><a name="s03_06_01_i8"><a name="dispersion, interior"></a><a name="s03_06_01_i9"><a name="s03_06_01_i10"><a name="dispersion_samples, interior"></a><a name="s03_06_01_i11"><a name="s03_06_01_i12"><a name="fade_distance, interior"></a><a name="s03_06_01_i13"><a name="s03_06_01_i14"><a name="fade_power, interior"></a><a name="s03_06_01_i15"><a name="s03_06_01_i16"><a name="fade_color, interior"></a><a name="s03_06_01_i17">
<p>
Introduced in POV-Ray 3.1 is an object modifier statement called <code> interior</code>. The syntax is:
</p>
<pre>
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...
</pre>
<p>
Interior default values: <a name="s03_06_01_i18">
</p>
<pre>
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>
</pre>
<a name="s03_06_01_i19"><a name="open"></a>
<p>
The <code>interior</code> contains items which describe the properties of the interior of the object. This is in
contrast to the <code> texture</code> and <code>interior_texture</code> 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.
</p>
<p class="Note">
<strong>Note:</strong> the <code>open</code> keyword, or <code>clipped_by</code> modifier also allows
you to see inside but interior features may not render properly. They should be avoided if accurate interiors are
required.
</p>
<p>
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.
</p>
<pre>
INTERIOR_DECLARATION:
#declare IDENTIFIER = INTERIOR |
#local IDENTIFIER = INTERIOR
</pre>
<p>
Where <em>IDENTIFIER</em> is the name of the identifier up to 40 characters long and <em>INTERIOR</em> is any valid <code><a href="s_161.html#s03_08_09_03">interior</a></code>
statement. See "<a href="s_98.html#s03_02_02_02_02">#declare vs. #local</a>" for information on identifier
scope.
</p>
<h4><a name="s03_06_01_01">3.6.1.1 </a>Why are Interior and Media Necessary?</h4>
<p>
In previous versions of POV-Ray, most of the items in the <code>interior</code> statement were previously part of
the <code><a href="#l155">finish</a></code> statement. Also the <code>halo</code> statement which was once part of the <code><a href="#l156">texture</a></code>
statement has been discontinued and has been replaced by the <code><a href="#l157">media</a></code> statement which is
part of <code>interior</code>.
</p>
<p>
You are probably asking <strong>WHY?</strong> As explained earlier, the <code>interior</code> contains items which
describe the properties of the interior of the object. This is in contrast to the <code>texture</code> which describes
the surface properties only. However this is not just a philosophical change. There were serious inconsistencies in
the old model.
</p>
<p>
The main problem arises when a <code><a href="s_119.html#s03_05_05_01">texture_map</a></code> 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.
</p>
<p>
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.
</p>
<p>
Similarly the <code>halo</code> calculations were not performed as the syntax implied. Using a <code>halo</code> in
such multi-textured objects did not vary the <code>halo</code> through the interior of the object. Rather, it computed
two separate halos through the whole object and averaged the results. The new design for <code>media</code> which
replaces <code> halo</code> 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 <code>interior</code> but
the name was changed.
</p>
<p>
During our development, someone asked if we will create patterned interiors or a hypothetical <code>interior_map</code>
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.
</p>
<h4><a name="s03_06_01_02">3.6.1.2 </a>Empty and Solid Objects</h4>
<a name="s03_06_01_02_i1">
<p>
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.
</p>
<p>
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.
</p>
<p>
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 <code> interior</code>. 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.
</p>
<p>
Empty objects are created by adding the <code>hollow</code> 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.
</p>
<p>
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.
</p>
<p>
There is a pitfall in the empty/solid object implementation that you have to be aware of.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<p>
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.
</p>
<h4><a name="s03_06_01_03">3.6.1.3 </a>Scaling objects with an interior</h4>
<p>
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.
</p>
<p>
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 <code>fade_distance, fade_power</code> themselves have not
been changed.
</p>
<p>
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 <code>emission</code>,
<code>absorption</code>, or in the <code>scattering</code> statement
</p>
<pre>
#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>
}
</pre>
<p>
The third object in the scene above, shows what to do, if you want to scale the object <em>and</em> 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.
</p>
<p>
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:
</p>
<pre>
Density*sqrt(3)/vlength(Scale)
</pre>
<p>
where Density is your original media density and Scale is the scaling vector applied to the container.
</p>
<p class="Note">
<strong>Note:</strong> the density modifiers inside the <code>density{}</code> statement are scaled
along with the object.
</p>
<h4><a name="s03_06_01_04">3.6.1.4 </a>Refraction</h4>
<a name="s03_06_01_04_i1"><a name="ior"></a>
<p>
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 <em> refraction</em>. 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 <em>index of
refraction</em> or <em>ior</em> value is used by scientists to describe the relative density of substances. The <code>ior</code>
keyword is used in POV-Ray in the <code>interior</code> to turn on refraction and to specify the ior value. For
example:
</p>
<pre>
object { MyObject pigment {Clear } interior { ior 1.5 } }
</pre>
<p>
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.
</p>
<p>
Normally transparent or semi-transparent surfaces in POV-Ray do not refract light. Earlier versions of POV-Ray
required you to use the <code> refraction</code> keyword in the <code>finish</code> statement to turn on refraction.
This is no longer necessary. Any non-zero <code>ior</code> value now turns refraction on.
</p>
<p>
In addition to turning refraction on or off, the old <code>refraction</code> 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 <code><a href="s_97.html#s03_02_01_05">filter</a></code> or <code><a href="s_97.html#s03_02_01_05">transmit</a></code>
value in the colors specified in the pigment statement or to use the <code><a href="s_128.html#s03_06_01">fade_power</a></code>
and <code><a href="s_128.html#s03_06_01">fade_distance</a></code> keywords. See "<a href="s_128.html#s03_06_01_06">Attenuation</a>".
</p>
<p class="Note">
<strong>Note:</strong> neither the <code>ior</code> nor <code>refraction</code> keywords cause the
object to be transparent. Transparency only occurs if there is a non-zero <code>filter</code> or <code> transmit</code>
value in the color.
</p>
<p>
The <code>refraction</code> and <code>ior</code> keywords were originally specified in <code>finish</code> but are
now properly specified in <code> interior</code>. They are accepted in <code><a href="#l155">finish</a></code> for
backward compatibility and generate a warning message.
</p>
<h4><a name="s03_06_01_05">3.6.1.5 </a>Dispersion</h4>
<a name="s03_06_01_05_i1"><a name="dispersion_samples"></a>
<p>
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.
</p>
<p>
By default POV-Ray does not calculate dispersion as light travels through a transparent object. In order to get a
more realistic effect the <code>dispersion </code> and <code>dispersion_samples</code> keywords can be added to the <code>interior{}</code>
block. They will simulate dispersion by creating a prismatic color effect in the object.<a name="s03_06_01_05_i2"><a name="dispersion"></a>
</p>
<p>
The <code>dispersion</code> 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.
</p>
<p class="Note">
<strong>Note:</strong> there will be no dispersion, unless the <code>ior</code> keyword has been
specified in <code>interior{ }</code>. An ior of 1 is legal. The ior has no influence on the dispersion strength, only
on the angle of refraction.
</p>
<p>
As POV-Ray does not use wavelengths for raytracing, a spectrum is simulated. The <code>dispersion_samples</code>
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.
</p>
<h5><a name="s03_06_01_05_01">3.6.1.5.1 </a>Dispersion & Caustics</h5>
<p>
Dispersion only affects the interior of an object and has no effect on faked caustics (See "<a href="s_128.html#s03_06_01_07">Faked
Caustics</a>"). <br>To see the effects of dispersion in caustics, photon mapping is needed (See the sections
"<a href="#l158">Photons</a>" and "<a href="s_130.html#s03_06_03_05_03">Dispersion & Photons</a>").
</p>
<h4><a name="s03_06_01_06">3.6.1.6 </a>Attenuation</h4>
<a name="s03_06_01_06_i1"><a name="s03_06_01_06_i2"><a name="s03_06_01_06_i3"><a name="fade_color"></a><a name="s03_06_01_06_i4"><a name="fade_colour"></a><a name="s03_06_01_06_i5"><a name="s03_06_01_06_i6">
<p>
Light attenuation is used to model the decrease in light intensity as the light travels through a transparent
object. The keywords <code>fade_power</code>, <code>fade_distance</code> and <code>fade_color</code> are specified in
the <code>interior</code> statement.
</p>
<p>
The <code>fade_distance</code> value determines the distance the light has to travel to reach half intensity while
the <code>fade_power</code> value determines how fast the light will fall off. <code>fade_color</code> colorizes the
attenuation. For realistic effects a fade power of 1 to 2 should be used. Default values for <code>fade_power</code>
and <code>fade_distance</code> is 0.0 which turns this feature off. Default for <code>fade_color</code> is <code><0,0,0></code>,
if <code>fade_color</code> is <code><1,1,1></code> there is no attenuation. The actual colors give colored
attenuation. <code><1,0,0></code> looks red, not cyan as in media.
</p>
<p>
The attenuation is calculated by a formula similar to that used for light source attenuation.
</p>
<p>
<br><center><img alt="" src="images/reference/medatten.png"></center>
</p>
<p>
If you set fade_power in the interior of an object at 1000 or above, a realistic exponential attenuation function
will be used:
</p>
<pre> Attenuation = exp(-depth/fade_dist)</pre>
<p>
The <code>fade_power</code> and <code>fade_distance</code> keywords were originally specified in <code>finish</code>
but are now properly specified in <code>interior</code>. They are accepted in <code>finish</code> for backward
compatibility and generate a warning message.
</p>
<h4><a name="s03_06_01_07">3.6.1.7 </a>Simulated Caustics</h4>
<a name="s03_06_01_07_i1"><a name="caustics"></a><a name="s03_06_01_07_i2">
<p>
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.
</p>
<p>
Since it is a very difficult and time-consuming process to actually calculate those effects (though it is not
impossible, see the sections <a href="#l159">"Photons"</a>) 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.
</p>
<p>
The <code>caustics</code> <em><code>Power</code></em> 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.
</p>
<p>
The <code> caustics</code> keyword was originally specified in <code> finish</code> but is now properly specified
in <code>interior</code>. It is accepted in <code> finish</code> for backward compatibility and generates a warning
message.
</p>
<h4><a name="s03_06_01_08">3.6.1.8 </a>Object-Media</h4>
<a name="s03_06_01_08_i1"><a name="object media"></a><a name="s03_06_01_08_i2"><a name="media, object"></a>
<p>
The <code>interior</code> statement may contain one or more <code>media</code> 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 <code>media</code> is given
in the section "Media".
</p>
<p>
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.
</p>
<p>
In previous versions of POV-Ray, this feature was called <code> halo</code> and was part of the <code> texture</code>
specification along with <code> pigment</code>, <code> normal</code>, and <code>finish</code>. See "<a href="s_128.html#s03_06_01_01">Why
are Interior and Media Necessary?</a>" for an explanation of the reasons for the change.
</p>
<p>
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 "<a href="s_101.html#s03_03_02_01">Atmospheric Media</a>" for details on
unconstrained uses of media.
</p>
<p>
You may specify more than one <code>media</code> statement per <code> interior</code> statement. In that case, all
of the media participate and where they overlap, they add together.
</p>
<p>
Any object which is supposed to have media effects inside it, whether those effects are object media or atmospheric
media, must have the <code>hollow on</code> keyword applied. Otherwise the media is blocked. See "<a href="s_128.html#s03_06_01_02">Empty
and Solid Objects</a>" for details.
</p>
<p>
<a name="l155">
<small><strong>More about "finish"</strong></small>
</a>
<ul>
<li><small>
<a href="s_117.html#s03_05_03">3.5.3 Finish</a> in 3.5 Textures
</small>
<li><small>
<a href="s_162.html#s03_08_10_06">3.8.10.6 Finish</a> in 3.8.10 Texture
</small>
</ul>
</p>
<p>
<a name="l156">
<small><strong>More about "texture"</strong></small>
</a>
<ul>
<li><small>
<a href="s_114.html#s03_05">3.5 Textures</a> in 3 POV-Ray Reference
</small>
<li><small>
<a href="s_162.html#s03_08_10">3.8.10 Texture</a> in 3.8 Quick Reference
</small>
</ul>
</p>
<p>
<a name="l157">
<small><strong>More about "media"</strong></small>
</a>
<ul>
<li><small>
<a href="s_129.html#s03_06_02">3.6.2 Media</a> in 3.6 Interior & Media & Photons
</small>
<li><small>
<a href="s_163.html#s03_08_11">3.8.11 Media</a> in 3.8 Quick Reference
</small>
</ul>
</p>
<p>
<a name="l158">
<small><strong>More about "Photons"</strong></small>
</a>
<ul>
<li><small>
<a href="s_130.html#s03_06_03">3.6.3 Photons</a> in 3.6 Interior & Media & Photons
</small>
<li><small>
<a href="s_130.html#s03_06_03_02_01">3.6.3.2.1 Photon Global Settings</a> in 3.6.3.2 Using Photon Mapping
in Your Scene
</small>
<li><small>
<a href="s_165.html#s03_08_13_02">3.8.13.2 Photons</a> in 3.8.13 Global Settings
</small>
</ul>
</p>
<p>
<a name="l159">
<small><strong>More about ""Photons""</strong></small>
</a>
<ul>
<li><small>
<a href="s_130.html#s03_06_03">3.6.3 Photons</a> in 3.6 Interior & Media & Photons
</small>
<li><small>
<a href="s_130.html#s03_06_03_02_01">3.6.3.2.1 Photon Global Settings</a> in 3.6.3.2 Using Photon Mapping
in Your Scene
</small>
<li><small>
<a href="s_165.html#s03_08_13_02">3.8.13.2 Photons</a> in 3.8.13 Global Settings
</small>
</ul>
</p>
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