3.3.2 Atmospheric Effects

POV-Ray 3.6 for UNIX documentation 3.3.3 Global Settings

3.3.4 Radiosity

3.3.3 Global Settings

GLOBAL_SETTINGS:
    global_settings { [GLOBAL_SETTINGS_ITEMS...] }
GLOBAL_SETTINGS_ITEM:
    adc_bailout Value | ambient_light COLOR | assumed_gamma Value | 
    hf_gray_16 [Bool] | irid_wavelength COLOR |
    charset GLOBAL_CHARSET | max_intersections Number |
    max_trace_level Number | number_of_waves Number |
    noise_generator Number | radiosity { RADIOSITY_ITEMS... } |
    photon { PHOTON_ITEMS... }
GLOBAL_CHARSET:
    ascii | utf8 | sys

Global setting default values:

charset            : ascii
adc_bailout        : 1/255
ambient_light      : <1,1,1>
assumed_gamma      : No gamma correction
hf_gray_16         : off
irid_wavelength    : <0.25,0.18,0.14>
max_trace_level    : 5
max_intersections  : 64
number_of_waves    : 10
noise_generator    : 2

Radiosity:
  adc_bailout      : 0.01
  always_sample    : on
  brightness       : 1.0
  count            : 35  (max = 1600)
  error_bound      : 1.8
  gray_threshold   : 0.0
  low_error_factor : 0.5
  max_sample       : non-positive value
  minimum_reuse    : 0.015
  nearest_count    : 5   (max = 20)
  normal           : off 
  pretrace_start   : 0.08
  pretrace_end     : 0.04
  recursion_limit  : 3

3.3.3.1 ADC_Bailout

In scenes with many reflective and transparent surfaces, POV-Ray can get bogged down tracing multiple reflections and refractions that contribute very little to the color of a particular pixel. The program uses a system called Adaptive Depth Control (ADC) to stop computing additional reflected or refracted rays when their contribution is insignificant.

You may use the global setting adc_bailout keyword followed by float value to specify the point at which a ray's contribution is considered insignificant. For example:

  global_settings { adc_bailout 0.01 }

The default value is 1/255, or approximately 0.0039, since a change smaller than that could not be visible in a 24 bit image. Generally this setting is perfectly adequate and should be left alone. Setting adc_bailout to 0 will disable ADC, relying completely on max_trace_level to set an upper limit on the number of rays spawned.

See section "Max_Trace_Level" for details on how ADC and max_trace_level interact.

3.3.3.2 Ambient_Light

Ambient light is used to simulate the effect of inter-diffuse reflection that is responsible for lighting areas that partially or completely lie in shadow. POV-Ray provides the ambient_light keyword to let you easily change the brightness of the ambient lighting without changing every ambient value in all finish statements. It also lets you create interesting effects by changing the color of the ambient light source. The syntax is:

 global_settings { ambient_light COLOR }

The default is a white ambient light source set at rgb <1,1,1>. Only the rgb components are used. The actual ambient used is: Ambient = Finish_Ambient * Global_Ambient.

See section "Ambient" for more information.

3.3.3.3 Assumed_Gamma

Many people may have noticed at one time or another that some images are too bright or dim when displayed on their system. As a rule, Macintosh users find that images created on a PC are too bright, while PC users find that images created on a Macintosh are too dim.

The assumed_gamma global setting works in conjunction with the Display_Gamma INI setting (see section "Display Hardware Settings") to ensure that scene files render the same way across the wide variety of hardware platforms that POV-Ray is used on. The assumed gamma setting is used in a scene file by adding

 global_settings { assumed_gamma Value }

where the assumed gamma value is the correction factor to be applied before the pixels are displayed and/or saved to disk. For scenes created in older versions of POV-Ray, the assumed gamma value will be the same as the display gamma value of the system the scene was created on. For PC systems, the most common display gamma is 2.2, while for scenes created on Macintosh systems should use a scene gamma of 1.8. Another gamma value that sometimes occurs in scenes is 1.0.

Scenes that do not have an assumed_gamma global setting will not have any gamma correction performed on them, for compatibility reasons. If you are creating new scenes or rendering old scenes, it is strongly recommended that you put in an appropriate assumed_gamma global setting. For new scenes, you should use an assumed gamma value of 1.0 as this models how light appears in the real world more realistically.

Before we go to the following sections, that explain more thoroughly what gamma is and why it is important, a short overview of how gamma works in POV-Ray:

no assumed_gamma in scene :

No gamma correction is applied to output file.

assumed_gamma 1 :

Gamma Display_Gamma is applied to output file.

If Display_Gamma is not specified, 2.2 is used.

assumed_gamma G :

Gamma Display_Gamma/G is applied to output file.

If Display_Gamma is not specified, 2.2/G is used.

Recommended value for assumed_gamma is 1.

3.3.3.3.1 Monitor Gamma

3.3.3.3.2 Image File Gamma

3.3.3.3.3 Scene File Gamma

3.3.3.4 HF_Gray_16

The hf_gray_16 setting is useful when using POV-Ray to generate heightfields for use in other POV-Ray scenes. The syntax is... global_settings { hf_gray_16 [Bool] }

The boolean value turns the option on or off. If the keyword is specified without the boolean value then the option is turned on. If hf_gray_16 is not specified in any global_settings statement in the entire scene then the default is off.

When hf_gray_16 is on, the output file will be in the form of a heightfield, with the height at any point being dependent on the brightness of the pixel. The brightness of a pixel is calculated in the same way that color images are converted to grayscale images: height = 0.3 * red + 0.59 * green + 0.11 * blue.

Setting the hf_gray_16 option will cause the preview display, if used, to be grayscale rather than color. This is to allow you to see how the heightfield will look because some file formats store heightfields in a way that is difficult to understand afterwards. See section "Height Field" for a description of how POV-Ray heightfields are stored for each file type.

3.3.3.5 Irid_Wavelength

Iridescence calculations depend upon the dominant wavelengths of the primary colors of red, green and blue light. You may adjust the values using the global setting irid_wavelength as follows...

 global_settings { irid_wavelength COLOR }

The default value is rgb <0.25,0.18,0.14> and any filter or transmit values are ignored. These values are proportional to the wavelength of light but they represent no real world units.

In general, the default values should prove adequate but we provide this option as a means to experiment with other values.

3.3.3.6 Charset

This allows you to specify the assumed character set of all text strings. If you specify ascii only standard ASCII character codes in the range from 0 to 127 are valid. You can easily find a table of ASCII characters on the internet. The option utf8 is a special Unicode text encoding and it allows you to specify characters of nearly all languages in use today. We suggest you use a text editor with the capability to export text to UTF8 to generate input files. You can find more information, including tables with codes of valid characters on the Unicode website The last possible option is to use a system specific character set. For details about the sys character set option refer to the platform specific documentation.

3.3.3.7 Max_Trace_Level

In scenes with many reflective and transparent surfaces POV-Ray can get bogged down tracing multiple reflections and refractions that contribute very little to the color of a particular pixel. The global setting max_trace_level defines the integer maximum number of recursive levels that POV-Ray will trace a ray.

 global_settings { max_trace_level Level }

This is used when a ray is reflected or is passing through a transparent object and when shadow rays are cast. When a ray hits a reflective surface, it spawns another ray to see what that point reflects. That is trace level one. If it hits another reflective surface another ray is spawned and it goes to trace level two. The maximum level by default is five.

One speed enhancement added to POV-Ray in version 3.0 is Adaptive Depth Control (ADC). Each time a new ray is spawned as a result of reflection or refraction its contribution to the overall color of the pixel is reduced by the amount of reflection or the filter value of the refractive surface. At some point this contribution can be considered to be insignificant and there is no point in tracing any more rays. Adaptive depth control is what tracks this contribution and makes the decision of when to bail out. On scenes that use a lot of partially reflective or refractive surfaces this can result in a considerable reduction in the number of rays fired and makes it safer to use much higher max_trace_level values.

This reduction in color contribution is a result of scaling by the reflection amount and/or the filter values of each surface, so a perfect mirror or perfectly clear surface will not be optimizable by ADC. You can see the results of ADC by watching the Rays Saved and Highest Trace Level displays on the statistics screen.

The point at which a ray's contribution is considered insignificant is controlled by the adc_bailout value. The default is 1/255 or approximately 0.0039 since a change smaller than that could not be visible in a 24 bit image. Generally this setting is perfectly adequate and should be left alone. Setting adc_bailout to 0 will disable ADC, relying completely on max_trace_level to set an upper limit on the number of rays spawned.

If max_trace_level is reached before a non-reflecting surface is found and if ADC has not allowed an early exit from the ray tree the color is returned as black. Raise max_trace_level if you see black areas in a reflective surface where there should be a color.

The other symptom you could see is with transparent objects. For instance, try making a union of concentric spheres with a clear texture on them. Make ten of them in the union with radius's from 1 to 10 and render the scene. The image will show the first few spheres correctly, then black. This is because a new level is used every time you pass through a transparent surface. Raise max_trace_level to fix this problem.

Note: that raising max_trace_level will use more memory and time and it could cause the program to crash with a stack overflow error, although ADC will alleviate this to a large extent.

Values for max_trace_level can be set up to a maximum of 256. If there is no max_trace_level set and during rendering the default value is reached, a warning is issued.

3.3.3.8 Max_Intersections

POV-Ray uses a set of internal stacks to collect ray/object intersection points. The usual maximum number of entries in these I-Stacks is 64. Complex scenes may cause these stacks to overflow. POV-Ray does not stop but it may incorrectly render your scene. When POV-Ray finishes rendering, a number of statistics are displayed. If you see I-Stack Overflows reported in the statistics you should increase the stack size. Add a global setting to your scene as follows:

 global_settings { max_intersections Integer }

If the I-Stack Overflows remain increase this value until they stop.

3.3.3.9 Number_Of_Waves

The waves and ripples patterns are generated by summing a series of waves, each with a slightly different center and size. By default, ten waves are summed but this amount can be globally controlled by changing the number_of_waves setting.

 global_settings { number_of_waves Integer }

Changing this value affects both waves and ripples alike on all patterns in the scene.

3.3.3.10 Noise_generator

3.3.3.11 Radiosity Basics

Important notice: The radiosity features in POV-Ray are somewhat experimental. There is a high probability that the design and implementation of these features will be changed in future versions. 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.

Radiosity is an extra calculation that more realistically computes the diffuse interreflection of light. This diffuse interreflection can be seen if you place a white chair in a room full of blue carpet, blue walls and blue curtains. The chair will pick up a blue tint from light reflecting off of other parts of the room. Also notice that the shadowed areas of your surroundings are not totally dark even if no light source shines directly on the surface. Diffuse light reflecting off of other objects fills in the shadows. Typically ray-tracing uses a trick called ambient light to simulate such effects but it is not very accurate.

Radiosity calculations are only made when a radiosity{} block is used inside the global_settings{} block.

The following sections describes how radiosity works, how to control it with various global settings and tips on trading quality vs. speed.

More about "Ambient"

• 3.5.3 Finish in 3.5 Textures
• 3.5.3.1 Ambient in 3.5.3 Finish

3.3.2 Atmospheric Effects

3.3.3 Global Settings

3.3.4 Radiosity