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<h3><a name="s03_03_04">3.3.4 </a>Radiosity</h3>
<a name="s03_03_04_i1"><a name="Radiosity, reference"></a><a name="s03_03_04_i2"><a name="radiosity"></a>
<h4><a name="s03_03_04_01">3.3.4.1 </a>How Radiosity Works</h4>
<a name="s03_03_04_01_i1">
<p>
The problem of ray-tracing is to figure out what the light level is at each point that you can see in a scene.
Traditionally, in ray tracing, this is broken into the sum of these components:
</p>
<dl>
<dt>
Diffuse
<dd>
the effect that makes the side of things facing the light brighter;
<dt>
Specular
<dd>
the effect that makes shiny things have dings or sparkles on them;
<dt>
Reflection
<dd>
the effect that mirrors give; and
<dt>
Ambient
<dd>
the general all-over light level that any scene has, which keeps things in shadow from being pure black.
</dl>
<p>
POV-Ray's radiosity system, based on a method by Greg Ward, provides a way to replace the last term - the constant
ambient light value - with a light level which is based on what surfaces are nearby and how bright in turn they are.
</p>
<p>
The first thing you might notice about this definition is that it is circular: the brightness and color of
everything is dependent on everything else and vice versa. This is true in real life but in the world of ray-tracing,
we can make an approximation. The approximation that is used is: the objects you are looking at have their <code>ambient</code>
values calculated for you by checking the other objects nearby. When those objects are checked during this process,
however, their <code>diffuse</code> term is used. The brightness of radiosity in POV-Ray is based on two things:
</p>
<ol>
<li>
the amount of light "gathered"
</li>
<li>
the 'diffuse' property of the surface finish
</li>
</ol>
<p>
An object can have both radiosity and an ambient term. However, it is suggested that if you use radiosity in a
scene, you either set <code>ambient_light</code> to 0 in <code>global_settings</code>, or use <code>ambient 0</code>
in each object's finish. This lighting model is much more realistic, and POV-Ray will not try to adjust the overall
brightness of the radiosity to match the ambient level specified by the user.
</p>
<p>
How does POV-Ray calculate the ambient term for each point? By sending out more rays, in many different directions,
and averaging the results. A typical point might use 200 or more rays to calculate its ambient light level correctly.
</p>
<p>
Now this sounds like it would make the ray-tracer 200 times slower. This is true, except that the software takes
advantage of the fact that ambient light levels change quite slowly (remember, shadows are calculated separately, so
sharp shadow edges are not a problem). Therefore, these extra rays are sent out only <em>once in a while</em> (about 1
time in 50), then these calculated values are saved and reused for nearby pixels in the image when possible.
</p>
<p>
This process of saving and reusing values is what causes the need for a variety of tuning parameters, so you can
get the scene to look just the way you want.
</p>
<h4><a name="s03_03_04_02">3.3.4.2 </a>Adjusting Radiosity</h4>
<a name="s03_03_04_02_i1">
<p>
As described earlier, radiosity is turned on by using the <code>radiosity{}</code> block in <code>global_setting</code>.
Radiosity has many parameters that are specified as follows:
</p>
<pre>
global_settings { radiosity { [RADIOSITY_ITEMS...] } }
RADIOSITY_ITEMS:
adc_bailout Float | always_sample Bool | brightness Float |
count Integer | error_bound Float | gray_threshold Float |
load_file Filename | low_error_factor Float | max_sample Float |
media Bool | minimum_reuse Float | nearest_count Integer |
normal Bool | pretrace_end Float | pretrace_start Float |
recursion_limit Integer | save_file Filename
</pre>
<p>
Each item is optional and may appear in any order. If an item is specified more than once the last setting
overrides previous values. Details on each item is given in the following sections.
</p>
<p class="Note">
<strong>Note:</strong> Considerable changes have been made to the way radiosity works in POV-Ray 3.5
compared to POV-Ray 3.1. Old scene will not render to the same result, if they render at all. It is not possible to
use the <code>#version</code> directive to get backward compatibility for radiosity.
</p>
<h5><a name="s03_03_04_02_01">3.3.4.2.1 </a>radiosity adc_bailout</h5>
<a name="s03_03_04_02_01_i1">
<p>
You can specify an adc_bailout for radiosity rays. Use <code>adc_bailout = 0.01 / brightest_ambient_object</code>
for good results. Default is 0.01.
</p>
<h5><a name="s03_03_04_02_02">3.3.4.2.2 </a>always_sample</h5>
<a name="s03_03_04_02_02_i1"><a name="always_sample"></a>
<p>
You can force POV-Ray to only use the data from the pretrace step and not gather any new samples during the final
radiosity pass. This may reduce splotchiness. To do this, use <code>always_sample off</code>, by default it is <code>on</code>.
It can also be usefully when reusing previously saved radiosity data.
</p>
<h5><a name="s03_03_04_02_03">3.3.4.2.3 </a>brightness</h5>
<a name="s03_03_04_02_03_i1"><a name="brightness"></a>
<p>
The <code>brightness</code> keyword specifies a float value that is the degree to which objects are brightened
before being returned upwards to the rest of the system. The default value is 1.0. In cases where you would raise the <code>global_settings{ambient_light
value}</code> to increase the over all brightness in a non-radiosity scene, you can use <code>brightness</code> in a
radiosity scene.
</p>
<h5><a name="s03_03_04_02_04">3.3.4.2.4 </a>count</h5>
<a name="s03_03_04_02_04_i1"><a name="count"></a>
<p>
The integer number of rays that are sent out whenever a new radiosity value has to be calculated is given by <code>count</code>.
A value of 35 is the default, the maximum is 1600. When this value is too low, the light level will tend to look a
little bit blotchy, as if the surfaces you are looking at were slightly warped. If this is not important to your scene
(as in the case that you have a bump map or if you have a strong texture) then by all means use a lower number.
</p>
<h5><a name="s03_03_04_02_05">3.3.4.2.5 </a>error_bound</h5>
<p>
The <code>error_bound</code> float value is one of the two main speed/quality tuning values (the other is of course
the number of rays shot). In an ideal world, this would be the <code>only</code> value needed. It is intended to mean
the fraction of error tolerated. For example, if it were set to 1 the algorithm would not calculate a new value until
the error on the last one was estimated at as high as 100%. Ignoring the error introduced by rotation for the moment,
on flat surfaces this is equal to the fraction of the reuse distance, which in turn is the distance to the closest
item hit. If you have an old sample on the floor 10 inches from a wall, an error bound of 0.5 will get you a new
sample at a distance of about 5 inches from the wall.
</p>
<p>
The default value of 1.8 is good for a smooth general lighting effect. Using lower values is more accurate, but it
will strongly increase the danger of artifacts and therefore require higher <code>count</code>. You can use values
even lower than 0.1 but both render time and memory use can become extremely high then.
</p>
<h5><a name="s03_03_04_02_06">3.3.4.2.6 </a>gray_threshold</h5>
<a name="s03_03_04_02_06_i1"><a name="gray_threshold"></a>
<p>
Diffusely interreflected light is a function of the objects around the point in question. Since this is recursively
defined to millions of levels of recursion, in any real life scene, every point is illuminated at least in part by
every other part of the scene. Since we cannot afford to compute this, if we only do one bounce, the calculated
ambient light is very strongly affected by the colors of the objects near it. This is known as color bleed and it
really happens but not as much as this calculation method would have you believe. The <code>gray_threshold</code>
float value grays it down a little, to make your scene more believable. A value of .6 means to calculate the ambient
value as 60% of the equivalent gray value calculated, plus 40% of the actual value calculated. At 0%, this feature
does nothing. At 100%, you always get white/gray ambient light, with no hue.
</p>
<p class="Note">
<strong>Note:</strong> this does not change the lightness/darkness, only the strength of hue/grayness
(in HLS terms, it changes S only). The default value is 0.0
</p>
<h5><a name="s03_03_04_02_07">3.3.4.2.7 </a>low_error_factor</h5>
<a name="s03_03_04_02_07_i1"><a name="low_error_factor"></a>
<p>
If you calculate just enough samples, but no more, you will get an image which has slightly blotchy lighting. What
you want is just a few extra interspersed, so that the blending will be nice and smooth. The solution to this is the
mosaic preview, controlled by <code><a href="s_103.html#s03_03_04_02_13">pretrace</a></code>: it goes over the image
one or more times beforehand, calculating radiosity values. To ensure that you get a few extra, the radiosity
algorithm lowers the error bound during the pre-final passes, then sets it back just before the final pass. The <code>
low_error_factor</code> is a float tuning value which sets the amount that the error bound is dropped during the
preliminary image passes. If your low error factor is 0.8 and your error bound is set to 0.4 it will really use an
error bound of 0.32 during the first passes and 0.4 on the final pass. The default value is 0.5.
</p>
<h5><a name="s03_03_04_02_08">3.3.4.2.8 </a>max_sample</h5>
<a name="s03_03_04_02_08_i1"><a name="max_sample"></a>
<p>
Sometimes there can be problems with splotchiness that is caused by objects that are very bright. This can be
sometimes avoided by using the <code>max_sample</code> keyword. <code>max_sample</code> takes a float parameter which
specifies the brightest that any gathered sample is allowed to be. Any samples brighter than this will have their
brightness decreased (without affecting color). Specifying a non-positive value for <code>max_sample</code> will allow
any brightness of samples (which is the default).
</p>
<h5><a name="s03_03_04_02_09">3.3.4.2.9 </a>Media and Radiosity</h5>
<p>
Radiosity estimation can be affected by media. To enable this feature, add <code>media on</code> to the <code>radiosity{}</code>
block. The default is <code>off</code>
</p>
<h5><a name="s03_03_04_02_10">3.3.4.2.10 </a>minimum_reuse</h5>
<a name="s03_03_04_02_10_i1"><a name="minimum_reuse"></a>
<p>
The minimum effective radius ratio is set by <code>minimum_reuse</code> float value. This is the fraction of the
screen width which sets the minimum radius of reuse for each sample point (actually, it is the fraction of the
distance from the eye but the two are roughly equal). For example, if the value is 0.02, the radius of maximum reuse
for every sample is set to whatever ground distance corresponds to 2% of the width of the screen. Imagine you sent a
ray off to the horizon and it hits the ground at a distance of 100 miles from your eye point. The reuse distance for
that sample will be set to 2 miles. At a resolution of 300*400 this will correspond to (very roughly) 8 pixels. The
theory is that you do not want to calculate values for every pixel into every crevice everywhere in the scene, it will
take too long. This sets a minimum bound for the reuse. If this value is too low, (which it should be in theory)
rendering gets slow, and inside corners can get a little grainy. If it is set too high, you do not get the natural
darkening of illumination near inside edges, since it reuses. At values higher than 2% you start getting more just
plain errors, like reusing the illumination of the open table underneath the apple. Remember that this is a unit less
ratio. The default value is 0.015.
</p>
<h5><a name="s03_03_04_02_11">3.3.4.2.11 </a>nearest_count</h5>
<a name="s03_03_04_02_11_i1"><a name="error_bound"></a><a name="s03_03_04_02_11_i2"><a name="nearest_count"></a>
<p>
The <code>nearest_count</code> integer value is the minimum number of old ambient values blended together to create
a new interpolated value. The total number blended will vary depending on <code>error_bound</code>. All previous
values that fit within the specified error_bound will be used in the average.
</p>
<p>
It will always be the n geometrically closest reusable points that get used. If you go lower than 4, things can get
pretty patchy. This can be good for debugging, though. Must be no more than 20, since that is the size of the array
allocated. The default value is 5.
</p>
<h5><a name="s03_03_04_02_12">3.3.4.2.12 </a>Normal and Radiosity</h5>
<p>
Radiosity estimation can be affected by normals. To enable this feature, add <code>normal on</code> to the <code>radiosity{}</code>
block. The default is <code>off</code>
</p>
<h5><a name="s03_03_04_02_13">3.3.4.2.13 </a>Pretrace</h5>
<a name="s03_03_04_02_13_i1"><a name="pretrace_start"></a><a name="s03_03_04_02_13_i2"><a name="pretrace_end"></a>
<p>
To control the radiosity pre-trace gathering step, use the keywords <code>pretrace_start</code> and <code>pretrace_end</code>
within the <code>radiosity{}</code> block. Each of these is followed by a decimal value between 0.0 and 1.0 which
specifies the size of the blocks in the mosaic preview as a percentage of the image size. The defaults are 0.08 for <code>pretrace_start</code>
and 0.04 for <code>pretrace_end</code>
</p>
<h5><a name="s03_03_04_02_14">3.3.4.2.14 </a>recursion_limit</h5>
<a name="s03_03_04_02_14_i1"><a name="recursion_limit"></a>
<p>
The <code>recursion_limit</code> is an integer value which determines how many recursion levels are used to
calculate the diffuse inter-reflection. The default value is 3, the upper limit is 20.
</p>
<h5><a name="s03_03_04_02_15">3.3.4.2.15 </a>Save and load radiosity data</h5>
<a name="s03_03_04_02_15_i1"><a name="load_file"></a><a name="s03_03_04_02_15_i2"><a name="save_file"></a><a name="s03_03_04_02_15_i3"><a name="load_file, radiosity"></a><a name="s03_03_04_02_15_i4"><a name="save_file, radiosity"></a>
<p>
You can save the radiosity data using <code>save_file "file_name"</code> and load the same data later
using <code>load_file "file_name"</code>. In general, it is not a good idea to save and load radiosity data
if scene objects are moving. Even if data are loaded, more samples may be taken during rendering (which produces a
better approximation). You can disable samples from being taken during the final rendering phase by specifying <code>always_sample
off</code>.
</p>
<h4><a name="s03_03_04_03">3.3.4.3 </a>Tips on Radiosity</h4>
<a name="s03_03_04_03_i1">
<p>
Have a look at the <a href="s_72.html#s02_03_07">"Radiosity Tutorial"</a> in the "Advanced
Tutorial" section, to get a feel for what the visual result of changing radiosity parameters is.
</p>
<p>
If you want to see where your values are being calculated set radiosity <code>count</code> down to about 20, set
radiosity <code> nearest_count</code> to 1 and set <code>gray_threshold</code> to 0. This will make everything
maximally patchy, so you will be able to see the borders between patches. There will have been a radiosity calculation
at the center of most patches. As a bonus, this is quick to run. You can then change the <code> error_bound</code> up
and down to see how it changes things. Likewise modify <code>minimum_reuse</code>.
</p>
<p>
One way to get extra smooth results: crank up the sample count (we have gone as high as 1300) and drop the <code>
low_error_factor</code> to something small like 0.6. Bump up the <code> nearest_count</code> to 7 or 8. This will get
better values, and more of them, then interpolate among more of them on the last pass. This is not for people with a
lack of patience since it is like a squared function. If your blotchiness is only in certain corners or near certain
objects try tuning the error bound instead. Never drop it by more than a little at a time, since the run time will get
very long.
</p>
<p>
Sometimes extra samples are taken during the final rendering pass. These newer samples can cause discontinuities in
the radiosity in some scenes. To decrease these artefacts, use a pretrace_end of 0.04 (or even 0.02 if you are really
patient and picky). This will cause the majority of the samples to be taken during the preview passes, and decrease
the artefacts created during the final rendering pass. You can force POV-Ray to only use the data from the pretrace
step and not gather any new samples during the final radiosity pass. To do this, use "always_sample no"
within the radiosity block inside global_settings.
</p>
<p>
If your scene uses ambient objects (especially small ambient objects) as light sources, you should probably use a
higher count (100-150 and higher). For such scenes, an error_bound of 1.0 is usually good. Higher causes too much
error, but lower causes very slow rendering. And it is important to adapt adc_bailout.
</p>
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