2.6.2  Photons, Dispersion and Improved Radiosity

Photons

The basic goal 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 backwards 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. Photons can also interact with media, to show the path the light travels.

A code example:

 global_settings {
   photons {
     count 20000
     autostop 0
     jitter .4
   }
 }

 light_source {
   MyLight
   photons {
     refraction on
     reflection on
   }
 }

 object {
   MyObject
   photons {
     target
     refraction on
     reflection on
     collect off
   }
 }

Dispersion

Since the refractive index of a material changes as a function of the lights wavelength, light passing through a material will be separated according to wavelength. This is known as chromatic dispersion. It creates a prismatic color effect inside the object.

As POV-Ray does not use wavelengths for rendering, the effect is simulated. Dispersion can also be used in combination with photons (to recreate that famous record sleeve)

Improved Radiosity

Many things have changed in radiosity, compared to POV-Ray 3.1. A short overview:

• The brightness of radiosity in POV-Ray 3.5 is based on: The amount of light "gathered" and the diffuse property of the surface finish.
• Use nearest_count to specify the minimum number of old values blended together. The total number blended will vary depending on error_bound.
• Distance_maximum is automatically computed
• Radiosity works with any sized rooms, also "outdoors".
• Radiosity recursion_limit is not limited to 2. You can specify any recursion_limit up to 20.
• You can specify an adc_bailout for radiosity rays.
• Radiosity estimation can be affected by surface normal, add normal on to the radiosity block within global_settings.
• Radiosity estimation can be affected by media, add media on to the radiosity block within global_settings.
• You can save the radiosity data using save_file "file_name" and load the same data later using load_file "file_name".
• Instead of using the INI options "Preview_Start_Size" and "Preview_End_Size" to control the radiosity pre-trace gathering step, use the keywords pretrace_start and pretrace_end within the radiosity block inside global_settings.
• Radiosity is enabled only when the radiosity block exists in POV file.
• Added max_sample. It takes a float parameter which specifies the brightest that any gathered sample is allowed to be.

Radiosity is still an experimental feature. We cannot guarantee that scenes using these features in this version will render identically in future releases or that full backwards compatibility of language syntax can be maintained.